— l~ - J 1 1 K fl 1 ft — . 5 H BBC Microcomputer System User Guide Original edition written by John Coll, edited by David Allen. Amendments and corrections to this edition by Acorn Computers Limited Part no 0433 000 Issue 1 Date October 1984 WARNING: THE COMPUTER MUST BE EARTHED Important: The wires in the mains lead to the computer are coloured in accordance with the following code: Green and yellow Earth Blue Neutral Brown Live As the colours of the wires may not correspond with the coloured markings identifying the terminals in your plug, proceed as follows: The wire which is coloured green and yellow must be connected to the terminal in the plug which is marked by the letter E, or by the safety earth symbol 4r or coloured green, or green and yellow. The wire which is coloured blue must be connected to the terminal which is marked with the letter N, or coloured black. The wire which is coloured brown must be connected to the terminal which is marked with the letter L, or coloured red. If the socket outlet available is not suitable for the plug supplied, the plug should be cut off and the appropriate plug fitted and wired as previously noted. The moulded plug which was cut off should be disposed of as it could be a potential shock hazard if it were to be plugged in with the cut off end of the mains cord exposed. The moulded plug must be used with the fuse and fuse carrier firmly in place. The fuse carrier is of the same basic colour* as the coloured insert in the base of the plug. Different manufacturers’ plugs and fuse carriers are not interchangeable. In the event of loss of the fuse carrier, the moulded plug MUST NOT be used. Either replace the moulded plug with another conventional plug wired as previously described, or obtain a replacement fuse carrier from an authorised BBC Microcomputer dealer. In the event of the fuse blowing it should be replaced, after clearing any faults, with a 3 amp fuse that is ASTA approved to BS1362. *Not necessarily the same shade of that colour. Exposure Like all electronic equipment, the BBC Microcomputer should not be exposed to direct sunlight or moisture for long periods. Econet and The Tube are trademarks of Acorn Computers Limited ©The author and the British Broadcasting Corporation 1982 Neither the whole nor any part of the information contained in, or the product described in, this manual may be adapted or reproduced in any material form except with the prior written approval of Acorn Computers Limited (Acorn Computers). The product described in this manual and products for use with it are subject to continuous development and improvement. All information of a technical nature and particulars of the product and its use (including the information and particulars in this manual) are given by Acorn Computers in good faith. However, it is acknowledged that there may be errors or omissions in this manual. A list of details of any amendments or revisions to this manual can be obtained upon request from Acorn Computers Technical Enquiries. Acorn Computers welcome comments and suggestions relating to the product and this manual. All correspondence should be addressed to: Technical Enquiries Acorn Computers Limited Newmarket Road Cambridge CB5 8PD All maintenance and service on the product must be carried out by Acorn Computers’ authorised dealers. Acorn Computers can accept no liability whatsoever for any loss or damage whatsoever caused by service or maintenance by unauthorised personnel. This manual is intended only to assist the reader in the use of the product, and therefore Acorn Computers shall not be liable for any loss or damage whatsoever arising from the use of any information or particulars in, or any error or omission in, this manual, or any incorrect use of the product. First published 1984 Published by the British Broadcasting Corporation Typeset by Bateman Typesetters, Cambridge Within this publication the term ‘BBC’ is used as an abbreviation for ‘British Broadcasting Corporation’. This book is part of the BBC Computer Literacy Project, prepared in consultation with the BBC Continuing Education Advisory Council. The editor of the project is David Allen Note: If this manual is to be used in conjunction with a BBC Microcomputer which is fitted with an Operating System with version number lower than 2.00, then the following points should be borne in mind: - Chapter 42 and all other references to the ‘shadow screen’ should be ignored. - Chapter 49 should be ignored (unless the machine with which the manual is to be used is fitted with BASIC II). In all other respects this manual is functionally compatible with earlier versions of the BBC Microcomputer. Contents Introduction 1 Equipment required 1 Text conventions used in this manual 1 What this user guide can and can’t do 2 1 Getting started 3 Experimenting 5 Connecting up the cassette recorder 7 Leads 7 Volume 8 Running the WELCOME programs 8 The keyboard 11 Cursor control keys 13 Giving the computer instructions - Part 1 2 Commands 15 3 An introduction to variables 18 4 A simple program 20 Using the screen editor 22 Deleting part of a program 24 Removing a program 25 5 Recording programs on cassette 26 Saving a program on cassette 26 Checking a recording 27 Loading a program from cassette 27 Cataloguing a tape 28 What the numbers mean 28 6 Sample programs 30 7 AUTO, DELETE, REM, RENUMBER 43 8 Introducing graphics 45 Modes, colours, graphics and windows 45 Graphics 46 Windows 47 Making a graphics window 47 Making a text window 48 Changing the colours of text and graphics 50 9 More on variables 52 Numbers and characters 52 String variables 53 How numbers and letters are stored in the computer’s memory 54 Real and integer variables 55 Summary 56 10 PRINT formatting and cursor control 57 Field widths in different screen modes 57 Altering the width of the field and the way in which numbers are printed 60 For the more technically minded 60 TAB(X) 62 TAB(X,Y) 62 Advanced print positioning 63 Cursor control 65 Cursor on/off 66 11 Input 67 12 GET, INKEY 70 Advanced features 13 TIME, RND 73 Structure in BASIC 14 REPEAT...UNTIL, TRUE, FALSE 74 15 FOR... NEXT 77 A note on listo TTTTTTTTn >80 16 IF...THEN...ELSE. More on TRUE and FALSE 84 17 Procedures 87 Local variables in procedures 18 Functions 19 GOSUB 96 GOTO 20 ON GOTO. ON GOSUB 1M Giving the computer instructions - 21 Even more on variables Part 2 102 Arrays 102 22 READ, DATA, RESTORE 107 23 Integer handling 110 24 String handling 114 25 Programming the red user-defined keys 119 The BREAK key 120 Other keys 120 26 Operator priority 122 27 Error handling 125 28 Teletext control codes and MODE 7 128 To change the colour of the text 129 To make characters flash 129 To produce double-height characters 130 Graphics 132 Graphics codes 133 Making a large shape 133 Teletext graphics codes for the more adventurous 134 29 Advanced graphics 137 How to change the screen display modes 137 How to draw lines 138 How to draw a square in the centre of the screen 138 Changing the colour of the square 138 How to fill in with colour 138 How to change colours 139 How to plot a point on the screen 144 How to remove a point selectively 144 Animation 145 How to make a ball and move it on the screen 145 How to create your own ‘graphics’ characters 146 How to make a character (eg a man) 146 How to make him move 147 How to make a larger character 148 How to make the movement smoother 149 Making a complete lunar landing game 151 Running the program 154 30 Sound 155 The pitch envelope 158 The amplitude envelope 159 Note synchronisation and other effects 161 31 File handling 163 32 Speeding up programs and saving memory space!68 Reference section 33 BASIC keywords alphabetical summary 170 34 VDU drivers 347 VDU code summary 348 Detailed description 349 35 Cassette files 360 Cassette motor control 360 Recording levels 360 Playback volume and tone 360 Keeping an index of programs 360 Saving a BASIC program 361 Saving a section of memory 362 Loading a BASIC program 362 Loading a machine code program 363 Loading and running a BASIC program 363 Loading and running a machine code program 364 Using a cassette file to provide keyboard input 364 Reading cassette data files 365 Testing for end of file 365 Storing data on tape 366 Recording single characters on tape 366 File names 366 Responses to errors 367 Changing responses to errors 368 Cassette tape format 369 36 Changing filing systems 370 37 How to merge two BASIC programs 371 38 Using printers 373 Connecting the printer to the computer 373 A parallel printer cable 374 Parallel printer connections 375 Telling the computer whether you are using a parallel or serial printer 376 Telling the computer to copy everything to the printer 377 Characters not sent to the printer 377 39 Indirection operators 378 40 HIMEM, LOMEM, TOP and PAGE 383 41 Operating system statements 385 42 The shadow screen 387 Other shadow mode-related commands 388 43 The operating system and how to make use of it 389 What is the operating system? 389 The *FX commands 390 OSBYTE calls from BASIC 391 OSBYTE calls from assembly language 393 The *FX commands and OSBYTE calls 395 Functional summary (alphabetical) 396 Numerical summary 398 44 An introduction to assembly language 428 Machine code and the assembler 428 Uses of assembly language 429 The main features of 6502 assembly language 429 The 6512 registers 430 Program counter 430 Accumulator 430 X register 430 Y register 430 Program status register 431 Stack pointer 431 The assembler delimiters T and and general assembly language syntax rules 431 Addressing modes 432 Implicit addressing 432 Immediate addressing and zero page addressing 432 Absolute addressing 433 Indirect addressing 433 Indexed addressing 433 Relative addressing 435 Accumulator addressing 436 Placing machine code programs in memory 436 OPT, forward referencing and two-pass assembly 438 The EQUate facility 439 Machine code entry points 441 442 45 The operating system calls Files 442 OSWRSC 443 OSRDSC 443 OSFIND 443 OSGBPB 445 OSBPUT 445 OSBGET 445 OSARGS 445 OSFILE 446 OSRDCH 448 OSASCI 448 OSNEWL 449 OSWRCH 449 OSWORD 449 Command line interpreter (&FFF7) 455 Faults, events and BRK handling 456 Accumulator description 456 Interrupt handling 457 NMI - non-maskable interrupt 457 IRQ - interrupt request 457 46 Analogue input 459 Digital input/output using the eight bit user port 460 47 Error messages 462 48 Minimum abbreviations 473 49 BASIC II 475 ABS 475 COUNT 475 ELSE 475 EVAL 475 INPUT 476 INSTR 476 ON ERROR 476 OPENIN and OPENUP 476 ASC 477 EQUB, EQUD, EQUS, EQUW 477 OPT 477 Appendix A 478 Teletext (MODE 7) displayed alphanumeric characters Appendix B 480 Teletext (MODE 7) displayed graphic characters Appendix C 482 ASCII (MODES 0 to 6) displayed character set and control codes Appendix D 484 Hexadecimal ASCII codes Appendix E 485 Text and graphics planning sheets Appendix F 489 Keyboard codes Appendix G 490 Printed circuit board layout for the BBC Microcomputer Appendix H 491 External connections at the rear of the BBC Microcomputer Appendix I 492 External connections underneath the BBC Microcomputer Appendix J 493 Memory map amd memory map assignments Appendix K 496 Circuit layouts Appendix L 501 VDU code summary Appendix M 502 6502 instruction set Appendix N 504 *FX and OSBYTE call summary Appendix O 506 Operating system calls Index 507 Introduction l Equipment required Before you start using your computer check that you have received the following in addition to this User Guide: - A BBC Microcomputer. - A guarantee registration card. - An aerial lead about two metres long which connects the computer to your television. - The Welcome Package - containing a cassette and an introductory booklet. If you are short of any of these items then write immediately to your supplier quoting the number given to you when you placed your order. The number also appears on the dispatch label. You will also require a lead to connect your computer to an ordinary cassette tape recorder. If you ordered the appropriate lead when you placed your order, check that it has arrived. If you didn’t, take your cassette recorder, the computer and this book to a dealer and ask if he can supply a lead or make one up for you. In many cases a standard audio lead will be suitable. The most common, useful type is a 5-pin DIN to 5-pin DIN (see below). Alternatively, order the appropriate lead from the supplier of your BBC Microcomputer. Unfortunately, as there are a large number of different kinds of connections, it has not been possible to supply a lead to fit every machine. Text conventions used in this manual You will notice that the style of printing used to present the text in this manual varies. This is to help you tell the difference between explanatory text, words which appear on your monitor screen (including BASIC keywords) and certain keys on the computer keyboard. - Ordinary text appears like this, or like this for emphasis. - Text displayed on the screen (including BASIC keywords) appears like this. - Words like RETURN mean that you should press the key marked RETURN rather than type the letters RETURN. 2 What this User Guide can and can’t do The BBC Microcomputer is a very versatile machine. On its own, connected to your television set, the computer can respond to programs which you yourself type in, to produce numbers, words, lines and movement on the screen and sound. Connect a suitable cassette tape recorder and you can then save your own programs for future use or run programs which have been written by other people. The WELCOME cassette which comes with the computer contains a number of programs specially written for the machine. Other programs are available in large numbers, including programs linked to hobbies and games, and programs for use in the home, in business and in education. Languages other than BASIC (such as LISP, FORTH, BCPL and PASCAL) are available. These languages are stored in an integrated circuit which has to be plugged into your BBC Microcomputer. This must be done by your dealer. The early chapters of this book will show you how to load and save programs from cassette, how to write simple programs and how to create certain graphics effects on the screen. There are also some complete programs to type in yourself. However, this is not a step-by-step course in BASIC programming. Most of what follows in the later chapters forms a reference guide on how to use the various commands and keywords of the BBC BASIC language. If you are an absolute beginner then much of this will not be very easy to understand. However, as you get more experience of programming, this material will prove invaluable. 1 Getting started 3 To get your computer working you will need a television set for a screen. Most people at home will use their ordinary colour or black and white television to show the pictures that the BBC Microcomputer produces. You will also need a cassette recorder if you wish to save and load programs. If you have a high quality monitor (for example in a school) then it can be connected directly to one of the sockets at the back of the computer. To connect the monitor to the computer you will need a special monitor lead. Assuming that you want to use your normal television set, then you can connect it to the computer using the aerial lead that is supplied with the computer. One of the plugs on this lead has a long central prong which fits into the socket on the back of the computer marked UHF out. The other end of the lead goes into the back of your television set in place of the normal aerial lead (see figure 1). Don’t worry about the cassette recorder for the moment. Next, plug your computer into the mains and switch it on (the On/Off switch is at the back). It should make a short ‘beep’ and the red light marked caps lock should come on. Turn the television on too and let it warm up for a moment. Probably all you will see on the TV screen at this stage is a ‘snow storm’. You will have to tune the TV so that it can receive the transmissions from the BBC Microcomputer. When your television is tuned correctly words will appear on the screen. Your television probably has some push-buttons which can be used to select different channels. Often button number 1 is tuned to BBC 1, button number 2 to BBC2, button number 3 to ITV and so on. It is best to tune a spare channel for the computer, for example channel 8. You can then use this for the computer without interfering with the tuning of the normal channels. Different televisions tune channels in different ways. For some of them, you turn the same knob that you use to select the channel. For others, there are separate controls. In either case, you should depress a spare channel button and then adjust it, or the associated control, until you get a good picture on the screen. A message similar to Acorn OS 64K BASIC > 4 5 should appear, which should be clear and sharp. Many types of tuning control indicate the channel number that you are tuning to. The BBC Microcomputer transmits on channel 36. It will not be too difficult to find the right channel but you will have to tune the TV carefully to get a really clear picture. When you have a clear picture, do by all means press every button in sight on the computer - you can’t do it any harm at all. Usually it just keeps on saying Mistake > whenever you press the large key marked RETURN. Mistake just means that the computer does not understand your commands. Its fault - not yours! You will see that if you hold any key down for more than a short time the character on the key appears on the screen, then there is a short pause, then the character repeats until you take your finger off again. On the whole, when pressing keys on the keyboard you should press them briefly - unless you want this repetition. Experimenting Now you are ready to experiment. You might like to try some of the following to see what the computer can do, but first be sure to press the key marked BREAK. This will clear the screen and get the computer ready for you. Type in the following exactly as shown: MODE 5 and then press the RETURN key. As you will see the command MODE 5 clears the screen and just leaves the > mark on the screen. > is known as the ‘prompt’ and it means that the computer is ready for your next command. Pressing the RETURN key tells the computer that you have finished the line you are typing and that you want it to obey your command. Before you press the RETURN key you can correct errors by pressing the key marked DELETE . If the computer says Mistake then press the BREAK key and try again, starting with MODE 5 . Then type in each of the following lines - but don’t forget to press the RETURN key at the end of every line. Don’t worry if you make a mistake - it really doesn’t matter! DRAW 1000 , 100 DRAW 0,750 6 GCOL 0,1 PLOT 85,0,0 If the computer says No such variable then you are probably pressing the letter O instead of the number 0. PLOT 86,1000,750 VDU 19,1,4,0,0,0 VDU 19,3,2,0,0,0 VDU 19,0,1,0,0,0 DRAW 200,0 DRAW 0,200 As you can see, the DRAW command is used to draw lines while PLOT 8 5 and PLOT 8 6 are used to plot and fill in triangles on the screen. When using the graphics the points on the screen are numbered from 0 to 1279 (left to right) and from 0 to 1023 (bottom to top). They are like positions on a piece of graph paper. Words can also be plotted in colours, as you will have seen. Clear the screen by typing MODE 5 and then type the following: COLOUR 1 This selects a red foreground. COLOUR 2 This selects a yellow foreground. COLOUR 3 This selects a white foreground. COLOUR 129 This selects a red background. COLOUR 0 This selects a black foreground. COLOUR 130 This selects a yellow background. The computer can create sound as well. Try typing this in: SOUND 1,-15,100,200 and then press RETURN. That gives a simple, crude sound. It is also possible to alter the quality of the sound. Try this: ENVELOPE 2,3,2,-4,4,50,50,50,127,0,0,0,126,0 7 (This should be typed in as one line even though it may spill over to the next line on the screen just as it has on this page. The computer will treat it as being ‘one line’ when you press RETURN.) Now carry on with: SOUND 1,2,1,10 SOUND 2,2,100,1 SOUND 3,2,200,1 You will have to press ESCAPE to stop the sound. Here’s another one: ENVELOPE 1,1, -26, -36, -45, 255, 255, 255, 127, 0,0,0, 126, 0 SOUND 1 , 1 , 1 , 1 There is a whole chapter on sound later on. Connecting up the cassette recorder Now get a cassette recorder connected so that you can load the demonstration programs into the computer from the cassette tape supplied in the WELCOME pack. For the moment just follow the instructions - we can sort out the ‘whys and wherefores’ later. You have to do two things before you can load the programs from the WELCOME tape: first get the right lead to connect your cassette recorder to the computer and secondly set the volume control on the cassette recorder to the correct position. Leads There are a number of different kinds of leads (figure 2). The connection to the computer is through a 7-pin DIN connector; a lead has not been supplied with the machine because there are so many connections to the many different cassette recorders in use. In many cases a standard 5-pin DIN to 5-pin DIN lead will be suitable, provided you do not want to use the motor control. If you want full motor control, take your cassette recorder to your nearest BBC Microcomputer dealer who will be able to supply a lead or make one up for you. Alternatively, take your cassette recorder and this book to a local hi-fi dealer. Note: Although you may find the ideal cassette lead difficult to buy locally, many cassette recorders do have a standard 5-pin DIN socket and a standard 5-pin DIN to 5-pin DIN hi-fi lead will work with the BBC Microcomputer in many cases. 8 Volume Having got the cassette recorder connected to the computer the only remaining thing to do is to set the playback volume on the cassette recorder to the correct level. With the BBC Microcomputer the cassette volume control setting is not critical. However, a special procedure for setting the volume control correctly is incorporated into the first program on the tape. Running the WELCOME programs Note: If your machine is fitted with a Disc, Econet, Teletext or IEEE interface and you wish to use a cassette, you must first select the Cassette Filing System by typing *TAPE RETURN This command should also be typed in (if your machine has one or more of the above interfaces) directly after use of BREAK or CTRL BREAK. In some cases, very long cassette programs may not run because of the small amount of extra memory used by the Disc and Net filing systems. To overcome this, follow the *TAPE RETURN command by: PAGE = &E0 0 RETURN NEW RETURN (See chapter 33 for explanations of PAGE and NEW .) If at any time you wish to return to the Disc or Econet filing systems, press BREAK or CTRL BREAK, or type: * D I S C RETURN or *NET RETURN Bearing in mind the above note, the WELCOME programs can be run by typing in CHAIN "WELCOME 9 Figure 2 A range of possible cassette leads You will need to select a lead with a 7-pin or 5-pin DIN lead at one end. This plugs into the computer. The other end of the lead must have suitable plugs for your particular recorder. Note: a standard 5-pin DIN lead will work with many recorders but will not enable you to make use of the computer’s ability to start and stop the cassette recorder automatically. 10 and then press the RETURN key. Next insert the WELCOME cassette into your recorder. If your cassette recorder has a tone control then set it to maximum ‘treble’ and leave it there. Now start the cassette recorder playing by pressing the PLAY button on the recorder. Then adjust the cassette recorder volume control slowly, until you get the message: Your volume control is now properly set. Please wait while the first program is loaded on the screen. This will give the minimum volume level. You should then increase the setting a little more. If you need to, you can rewind the tape at any time. If no message appears rewind the tape and play it again, increasing the volume control setting in larger steps, or check the cassette leads are correctly plugged in. The system is very reliable, so if you have problems it may be that your tape recorder is at fault or that you have a fault in the computer. You are advised to contact your dealer. Note: Each computer program is recorded on the tape as a kind of screeching noise. It’s not meant to be listened to, but some cassette recorders have the annoying habit of playing the tape through the loudspeaker while the tape is loading into the computer. Everything depends on what plugs and sockets are being used. It is possible to stop this on most recorders by inserting a small (3.5mm) jack plug into the socket on the recorder marked EAR. You could insert the ear piece supplied with the recorder if that is more convenient. On other recorders you may have to insert a DIN loudspeaker plug, with no wire connections, into the socket marked LS to turn off the noise. Don’t try turning the volume control down because then the computer will not be able to ‘hear’ the tape either. The important thing to do is to try to disable the loudspeaker as described above. Make a note of the volume setting on your cassette recorder and always use that setting when playing back the WELCOME cassette. You may need to use a different setting with other tapes that you have purchased or recorded yourself. On the WELCOME cassette the volume control setting is repeated many times at the beginning of the tape. With practice it is possible to save time by running the tape forward by about two minutes (once the volume control is set) and then begin playing the tape from this point, having first entered the command CHAIN "WELCOME" RETURN When the first WELCOME program has loaded into the computer it will clear the screen and give you instructions. The WELCOME pack includes a booklet which describes not only how to get the programs running but also what each of the programs does. 11 The keyboard Anyone who has used a standard typewriter will be familiar with the positions of most of the symbols on the keyboard of the BBC Microcomputer. However, there are a number of special keys which need to be mastered (see figure 3) and these are described below. If you are a keyboard novice you may find the layout confusing. Don’t worry - first of all it is not necessary to be a touch typist to work the computer; secondly, there is a program on the WELCOME cassette which will help you to practice finding the various keys, and most people find that with a little practice they become familiar with them fairly quickly. Some keys have two symbols engraved on them - we’ll call those on the top ‘upper case’ and those below ‘lower case’ symbols. CAPS LOCK When the machine is switched on, the middle light should be on, telling you that the CAPS LOCK key is on. This gives capital letters and lower case symbols and is the most useful state for programming because the computer only recognises commands typed in using capital letters. By pressing the CAPS LOCK key once you can switch the light off. Now you get lower case letters and lower case symbols. Press it again and it will be on again. SHIFT Whether CAPS LOCK is on or off, if you press either of the SHIFT keys and hold it down while typing in a character you will get a capital letter or upper case symbol. Holding down CTRL and SHIFT together stops the computer ‘writing’ to the screen. This can be useful if it is writing faster than you can read. SHIFT LOCK Pressing this key once gives capital letters and upper case symbols until it is pressed again. It has its own on/off light. Practice in the use of these keys is given in one of the first programs in the introductory pack - the one called KEYBOARD. SHIFT CAPS LOCK Depressing and releasing SHIFT and CAPS LOCK in unison reverses the effect of the SHIFT key, causing a lower case letter to be printed when pressing a letter key with SHIFT held down. Pressing CAPS LOCK again returns the keyboard to normal. Ten user defined function keys Editing keys 12 Socket for plug-in ‘ROM’ cartridges 13 RETURN This key is the most commonly used key on the keyboard. When a command or anything else is typed in, it is not usually acted upon until the RETURN key is pressed. In other words, this key informs the computer that you have finished entering a line or a reply. Until you press RETURN, you can add to or delete what you have typed in. CURSOR control keys 3EEQI These enable you to move the flashing cursor around the screen when editing a program. Pressing any of them makes the computer automatically enter the ‘editing mode’ during which two cursors are shown on the screen (see chapter 4). DELETE Pressing this key will cause the last character typed in to be erased from the screen. If held down, it will then erase further characters until released. COPY This key, used in conjunction with the cursor control keys, enables anything on the screen to be copied - a useful feature when editing a line in a program. ESCAPE This key is usually used to stop a program which is running. However, it can be programmed to do other things when pressed - such as moving you from one part of a program to another. BREAK This key stops the computer no matter what it is doing. The computer forgets almost everything that it has been set to do. Pressing BREAK also resets the screen to MODE 7. Do not get into the habit of using BREAK. The ESCAPE key provides a much less violent way of escaping from a program! (See chapter 25 for more details on BREAK). 14 CTRL This key behaves similarly to the SHIFT key in that it can be used to change the character generated by other keys. For example, pressing CTRL and G (called Control G) makes the internal speaker make a short noise. CTRL B is used to turn a printer on and CTRL C turns it off. CTRL N makes the computer stop at the bottom of each page, etc, etc. More information on control codes is given in chapter 34. TAB Another key that is useful in special circumstances - like word processing. ~ ; ■c > £ A \ [ 1 — These keys can be somewhat confusing because they seem to generate the wrong characters sometimes. The problem is that there are two international standards for displayed characters (Teletext and ASCII) and the BBC Microcomputer can display either. MODE 7 generates the Teletext display characters and MODES 0 to 6 show the ASCII characters. But don’t worry, the computer recognises the key correctly regardless of what is displayed on the screen. Here is a table showing all these characters: On the key Displayed on the screen in MODE 7 in MODES 0 to 6 A t A 1 1 II 1 1 \ y 2 \ ( V4 { I <- I 1 % 1 1 1 Note that in MODE 7 a zero is shown as a rather pointed O whereas in all other modes, zeros have a slash - 0 - to help to differentiate them from the letter O . The keyboard is also marked in this way. 2 Commands 15 There are two ways of getting the computer to do something: 1. Give it commands which it can act on immediately. This is what happened when you typed in the lines in chapter 1. 2. Give it a series of numbered instructions, often called statements, which it can store in its memory and carry out in sequence when told to do so. A stored series of instructions is called a program. Many of the keywords in BASIC can be used both as commands and as statements in a program. The rest of this chapter is concerned with ‘command mode’. PRINT is used to make the computer print something on the screen. Try these two examples: PRINT "HELLO" Don’t forget to press RETURN at the end of each line. PRINT 3+4 In the second example you have given the computer a command to print the sum of 3 and 4. The computer can very easily do addition, subtraction, multiplication and division. The addition, subtraction, multiplication and division signs areall on the right side of the keyboard. If you are interested in doing mathematical or financial work then you will need to know the symbols that the computer uses for various mathematical operations. They are: + Addition Subtraction * Multiplication \ Division A Exponentiation Decimal point If you want to get the + or * then you will have to press the SHIFT key as well as the key you want. It’s rather like a typewriter: while holding the SHIFT key down, press the + sign once. 16 Try typing in the following and check that they work - in other words see that they produce the expected answers. PRINT 4+8 PRINT 18 - 2 * 4 PRINT 131/4 PRINT SQR ( 2 ) The last one will print the square root of 2 which is 1.41421356. Then try MODE 5 which will make the computer clear the screen and get it ready to draw lines as well as text. In this mode COLOUR 129 will select a red background, and CLS will clear the screen to the background colour. In each case you have given the computer a command and it has obeyed it immediately. Working like this is called ‘working in command mode’. While in this mode you might like to learn how to use the bright red user defined function keys. Each of these keys can be used to store a word or several words. For example they could be programmed so that each one selects a different colour. Try this: *KE Y 2 COLOUR 2 |M The | shown above is produced by a special key. On the keyboard this key is the third key from the right on the row below the red keys. In MODE 7 this key produces || on the screen. Once you have typed that in then every time you press the key marked f2, the computer will change to COLOUR 2 which gives yellow lettering. In a similar way you could program some of the other keys like this: *KE Y 0 COLOUR 0 |M *KE Y 1 COLOUR 1 |M *KE Y 3 COLOUR 3 |M Note the exact position of spaces when you type in a command. 17 Of course red letters don’t show up very well on a red background! You will have noticed the | M at the end of each line above. That is the code used to get a RETURN into the user defined function keys. If the picture on your television screen is either too far up or too far down the screen, you can move the whole display with the command *TV. * TV 2 5 5 will move down one line * TV 2 5 4 will move down two lines * TV 1 will move up one line * TV 2 will move up two lines The movements come into effect next time you press BREAK or change mode. * TV also controls the interlace of the television display. See chapter 43. 18 3 An introduction to variables In the last chapter we made the computer do a number of calculations but it was never expected to remember any of the results after it had printed them out. Suppose that you have to calculate the wages for everyone in a company. After you have worked out each person’s wage, it would be useful to be able to add them all together, so that in the end you would know the total wage bill. Keeping track of things that vary during a long calculation is done by using variables. Try typing this line into the computer: LET Z=5 And now try typing in each of the following lines: PRINT Z+6 PRINT Z * 12 As you will have seen, once we have told the computer that ‘Z is 5’ it understands that every time we use the letter Z in a sum it has to go and have a look to find out what the value of Z is (5 in this case) and use that number in the arithmetic that we set it to do. Now type in LET Z = 7 and then try these two lines: PRINT Z+12 PRINT Z/3 As you will gather the value of Z has changed from 5 to 7. In computer jargon Z is called a numeric variable. That means that Z can be used to store any number, and you can change the value of Z at any time you want to. The computer is able to store hundreds of different variables and the variables don’t have to be called something as simple as Z; you can call a variable by as long a name as you want. For example you could type MYAGE=30 Notice that MY AGE was written without any spaces between the word 19 MY and AGE. There are only four restrictions about the names that we give to variables: 1. There must be no spaces in the middle of a variable name. 2. All variable names must start with a letter - however you can add in as many numbers as you want to later on. 3. You must not use punctuation marks (such as exclamation marks and question marks) in the variable name but you can use an underline character. 4. Variable names should not begin with BASIC keywords like PRINT and LET. One that is particularly easy to use by mistake is the keyword TO. However it is permissible to start a variable name with a lower case ‘to’ because upper and lower case names are different. There is a full list of keywords in chapter 48 and they are described in detail in chapter 33. To get lower case characters on the screen, make sure that the CAPS LOCK is off by depressing it to turn off its light. Now you will get small letters and numbers. Hold the SHIFT key down if you want to use capital with lower case letters. Any of the following variable names are acceptable. LET AGE = 3 8 LET this year=1984 LET lengthOFr od= 1 8 LET CAR_mi leage = 1 3 2 8 0 LET value5=l 6 . 1 LET we i ght 4 = 0 . 0 0 1 3 5 LET chi cken2 egg3= 5 1 . 6 However the following variable names are illegal. LET Football Result = 3 (There’s a space.) LET Who? = 6 (There’s a question mark.) LET 4thvalue = 16 . 3 (Starts with a number.) LET TODAY = 23 (Starts with TO.) LET PRINT=1234 . 56 (PRINT is a reserved word.) You will notice that in all the examples above we have put the word LET before the variable name. That gives a clear indication of what is actually happening inside the computer, namely that the numeric variable this_year, in one of the examples, is being given a new value ‘1984’. The word LET is optional and the computer will also accept this_year=1984 This shortened version is more frequently used. 20 4 A simple program In the previous chapter we have been giving the computer commands which it obeys immediately. The problem with this technique is that you have to wait until the computer has completed one command before you can give it the next one. If the computer takes a long time to work out one of the problems you have set it, then you may have to waste an awful lot of time just sitting there waiting for it. For example if you want your computer to work out the number of £1, £5 and £10 notes that you will need to pay the wages at the end of the week the computer will take a fair time to calculate all the wages before it can sort out the notes required. The same problem comes up when you take a car into a garage to be serviced. You could for example stand by the mechanic and say ‘First of all I want the oil changed’ and then you could wait for him to change the oil. When that is completed you could then say ‘Now I want you to replace the bulb that has blown in one of the front headlights’ and then you could wait for that job to be done. And thirdly you might say ‘The exhaust is making a noise, so I want you to put the car up on the ramp and check it’. You would spend a great deal of time waiting for the mechanic to complete each job before assigning the next. There is a far more efficient way of doing things; when you go into the garage you give the mechanic a whole set of instructions, for example: - First of all change the oil. - Secondly replace the headlight bulb. - Thirdly stop the noise in the exhaust. Once you have given your set of instructions and checked that the garage understands what has to be done, you can walk off and have a cup of coffee and then go back expecting the job to be finished. Now the same thing applies with a computer. It is far better to give it a whole set of instructions and let it run while you wander off and have a cup of coffee. ‘Writing a computer program’ is nothing more than giving a set of instructions. If you give the computer a command like PRINT "HOW ARE YOU" then the computer will do that immediately. On the other hand, if you give the computer a statement 10 PRINT "HOW ARE YOU" 21 then the computer will regard that as instruction number 10 and it will not do it immediately, but expect other instructions to follow. Instruction number 10 is usually referred to as line 10. Again: if there is a line number then the statement is part of a program; if there is no line number then it is a command which the computer must obey immediately. When you have given the computer a set of instructions and you then want it to carry them out, you type the word RUN on the keyboard. The computer will then carry out the instructions that you asked it to do one at a time and in line- number order. In other words, it will ‘execute’ the program that you have typed in. Just to check that you have got the idea of what is going on, here is a small program that you can type in. 10 REPEAT 20 PRINT "GIVE ME A NUMBER"; 30 INPUT B 40 PRINT "12 TIMES ";B;" IS ";12*B 50 UNTIL B = 0 When you RUN the program line 20 will print the message GIVE ME A NUMBER on the screen. Line 30 will print a question mark on the screen and wait for you to type in a number (followed by RETURN - as usual). The number you type in will become the value of the variable ‘B’. Line 40 will first print the words 12 TIMES followed on the same line by the number you typed in, followed on the same line by the word I S followed by the result of the calculation. The semi-colons tell the computer to print the next item on the same line as the previous one and right up against it. Line 50 sends the computer back to line 10 unless B=0, when the program will stop. Another way of stopping the program is to press the ‘panic button’ which is marked ESCAPE (it’s at the top left of the keyboard). If the computer seems to be ignoring you because it’s too busy running a program. You can nearly always get its attention by pressing the ESCAPE key. When you do that it will stop running your program and print a > prompt to show that it has stopped the program and is waiting for your command. When the computer shows a > it is in command mode. You can change your program, give it commands for immediate execution, or tell it to RUN the program (in its memory) again. It doesn’t forget a program when you press ESCAPE. 22 If the computer is in command mode (in other words if the last thing on the screen is >) then you can command it to print the program in its memory by typing LIST and pressing RETURN. The computer will then give a listing of the program on the screen for you to cheek. If you discover that you have made an error, for example that you have got something wrong in line 20, then it is easy to correct the error. There are two ways of correcting major errors: - Retype the whole line. - Use the screen editor. Using the screen editor There is a group of six keys on the right hand side of the keyboard which can be used to edit, or alter, program lines that are displayed on the screen. Four of the keys have arrows on them and are coloured a lighter brown that most of the other keys. These keys enable you to move a flashing cursor around the screen to a line that you wish to edit. As soon as you press one of these keys the computer enters a special ‘editing mode’ where it displays two cursors. The large white block is called the write cursor and it shows you where anything that you enter will appear. The other small, flashing cursor - the read cursor - is the one that can be moved around by the arrow keys. Try moving the read cursor, by using the arrow keys, until it is under a letter at the start of a word and then press the COPY key several times. As you will see the COPY key copies everything that the read cursor passes under into the new input line. Halfway through copying a line you can always use to move the read cursor to some new place on the screen before using COPY again to copy some other text to your new input line. The DELETE key can always be used to delete characters from the input line. You can also type new characters in at any time instead of using the COPY key. When your new input line is complete just press RETURN in the usual way. 23 Try the following: clear the screen with the command C L S and then LIST the program. It should include the line 20 PRINT "GIVE ME A NUMBER"; If not, then type that line in so that you can edit it. Suppose that you wanted to insert the word BIG so that line 20 reads 20 PRINT "GIVE ME A BIG NUMBER" ; then all you have to do is to press the up-arrow cursor key until the small flashing line is positioned under the 2 of 2 0 . Then press the COPY key to copy the first part of line 20 to a fresh line at the bottom. When the cursor reaches the space after the A where you want to insert the word BIG, just type it in with a space in front - it will appear on the bottom line. Then COPY the rest of the line 20. The space after the A becomes the space after BIG. At the end press RETURN. Now try changing the program already in the computer once again by doing the following: 1. List the program by using the LIST command. 2. Practice using the cursor control and COPY keys to alter line 20 so that it reads: 20 PRINT "NOW GIVE ME A BIG NUMBER"; 3. Now add these new lines. Don’t forget to press RETURN after each one. 5 CLS 25 REPEAT 35 IF B< 1 0 0 0 THEN PRINT "I SAID A BIG NUMBER" 37 UNTIL B> = 1 0 0 0 Note: It doesn’t matter in what order you type new lines. The computer will automatically put them into numerical order. You will see that this is true by typing LIST RETURN These extra lines tell the computer to reject any number smaller than 1000 and to keep on going back to line 30 to ask for a new number until that number is greater than 1000. The symbol < means ‘is smaller than’, and > means ‘is greater than’. I F and THEN are self explanatory. 24 4. Now RUN the program. >RUN NOW GIVE ME A BIG NUMBER? 16 I SAID A BIG NUMBER ? 2 0 I SAID A BIG NUMBER ? 2 0 0 0 12 TIMES 2000 IS 24000 NOW GIVE ME A BIG NUMBER? This program will go on running until you press ESCAPE. If you look you will see that if you give the value 0 for the number, the program never reaches line 50, so it can never end unless you press the panic button! Deleting part of a program Quite often you will want to delete a whole line or group of lines in your program. This is easy to do but don’t forget that if you type in a new line 20 (for example), it will automatically remove the old line 20 and replace it with your new one. If you want to delete a line completely then type in just the line number and press RETURN: 20 RETURN To delete a whole set of line numbers, for example, lines 50 to 70 inclusive, you can type DELETE 50, 70 You cannot get these lines back once they are deleted - unless you can copy them off the screen, so use this with care. After you have deleted several lines - or if you have typed in lots of new lines you often find that you have a very odd set of line numbers. The command RENUMBER will make the computer go through your whole program renumbering all the lines so that they are given line numbers in a numeric sequence. Here is an example of terrible programming style - but it will illustrate the RENUMBER command. Don’t bother to type it in - just look at it. > L I S T 1 REM ** GOTO GOTO GOTO 2 REM WITH ACKNOWLEDGEMENTS TO 3 REM "COMPUTERS IN SCHOOLS" 4 REM THE JOURNAL OF MUSE 15 GOTO 100 16 GOTO 95 40 N=N+ 1 25 4 4 END 57 IF N=1 8 THEN PRINT "GOTO OR NOT TO GOTO" 60 IF N> 3 5 THEN GOTO 110 78 GOTO 40 95 PRINT " * * THE GOTO SHOW**": GOTO 40 100 N=0 : GOTO 16 105 PRINT "GOT TO GOTO GOTO NOW" 110 GOTO 44 115 PRINT "GOTO OR NOT TO GOTO";:GOTO 60 >RENUMBER > L I S T 10 REM ** GOTO GOTO GOTO 20 REM WITH ACKNOWLEDGEMENTS TO 30 REM "COMPUTERS IN SCHOOLS" 40 REM THE JOURNAL OF MUSE 50 GOTO 130 60 GOTO 120 70 N=N+ 1 80 END 90 IF N=1 8 THEN PRINT "GOTO OR NOT TO GOTO" 100 IF N> 3 5 THEN GOTO 150 110 GOTO 70 120 PRINT " * * THE GOTO SHOW**": GOTO 70 130 N=0 : GOTO 60 140 PRINT "GOT TO GOTO GOTO NOW" 150 GOTO 80 160 PRINT "GOTO OR NOT TO GOTO";:GOTO 100 >RUN * * THE GOTO SHOW** As you will see, the RENUMBER command has not only renumbered the references to line numbers which occur within the program itself - namely after the statements containing the keyword GOTO. (This gives the computer the instruction to go to a particular line number and carry out the instruction it finds there.) Removing a program If you want to write a new program you will want to remove the old program from the computer’s memory. This can be done by using the command NEW, or by pressing the BREAK key. In either case, if you regret having lost your program, type OLD and press RETURN and, providing you haven’t begun to type in the new program, the old one should reappear. You can always check what’s in the memory by typing LIST. Try experimenting with these various commands on the program you have typed in. 26 5 Recording programs on cassette The WELCOME cassette supplied with your BBC Microcomputer has a number of programs stored on it. You can store a copy of any program on cassette and then load it back into the machine at some time in the future. It really is just like recording music onto a cassette - you can then play the cassette back a few days later and the music will still be there. If you decide that you don’t want to keep the computer program that you have saved on cassette then you can just record a new program over the old one in the same way that you can re-use a cassette when recording music. And in the same way that it is very easy to forget where a particular piece of music is recorded on a cassette, so it’s very easy to forget where on the cassette you have stored a particular program. It is very strongly suggested that you use the tape counter to keep an index of where programs are on cassette. Also you must leave gaps between programs. It is easy to let one program run over the start of the next one if they are all squashed close together. If programs do overlap then you will definitely lose one of them. Be warned! Most short programs will only move the cassette tape counter on 30 or 40 positions but play safe and spread the programs out over the length of the cassette. If you record the first program at 0000, the second at 0100, the next at 0200 and so on then they will be easy to find and they are unlikely to run over each other. Note: don’t make the mistake of trying to record on the clear plastic tape ‘leader’ - wind the tape on by hand until the brown tape itself is exposed. Saving a program on cassette If you have typed a program into your microcomputer then all you have to do to save it is to 1. Insert the cassette into the recorder. 2. Set the tape counter to 0000 when the tape is fully re-wound. 3. Type SAVE " MYPROG " on the computer and then press the RETURN key. 4. The message RECORD then RETURN will appear. 27 5. Fast forward the cassette to the place where you want to record the program - this will be 100 or 200 or 300 etc, on the tape counter. 6. Press the RECORD button on the cassette and then press the RETURN key. If you want to give up at any time then press the ESCAPE key. Notice that MYPROG is the name that we happened to give to the program. You can call your program by any name you like so long as it has no more than ten characters. For example you could have typed SAVE "FRED" or SAVE "GAME 3" or SAVE "picture" While the program is being saved on cassette the name of the program and some numbers will appear to tell you that things are happening. When the computer has finished, the > prompt will re-appear and the tape will stop automatically. If you don’t have cassette motor control then you will have to stop the recorder manually after the > prompt re-appears. That’s it. Checking a recording If you want to check that you have successfully recorded your program on the tape then you can use the *CAT command (see below). If your recording failed for any reason you can always re-record it. See chapter 35 if you have problems. Loading a program from cassette Loading a program back into the computer is just like playing a particular piece of music which has been recorded on the cassette. 1. Type LOAD "MYPROG" and then press the RETURN key. The message Searching will appear. Of course if your program is called something else then use the right name, for example LOAD "GAME 3" RETURN 2. Rewind the cassette to just before the start of your program (which will be at 100 or 200 etc.) 3. Check that the volume and tone control settings are correct — see chapter 1 if you are not sure how to find the correct settings. 4. Start playing the cassette by pressing the PLAY button on the recorder. 28 When the computer finds any program on the cassette it will show the name of the program on the screen. When it finds the program it is looking for it will print Loading to let you know that it is now loading the right program. When the computer has finished loading the program it will print the > prompt. It will also automatically stop the tape if you have automatic motor control, if not then you will have to stop the tape manually. The program is now in the computer. You can type RUN RETURN to make it work, as usual. There is one more useful feature to do with loading and saving programs. Instead of typing LOAD " MYPROG " RETURN you can type CHAIN "MYPROG" RETURN This not only loads in the program MYPROG but also starts it working as soon as it has loaded. It is normally more convenient to use CHAIN than LOAD . Cataloguing a tape If you forget what programs you have on the tape then you can get a catalogue by typing * CAT and then playing the tape, but you’ll have to wait until the tape has run through the programs. What the numbers mean A typical catalogue looks like this WELCOME 00 0084 INTRO 08 088E INDEX 0A 0 ABA KEYBOARD 25 2545 29 The file-name is followed by two ‘hexadecimal’ numbers which give the ‘block number’. Each program is recorded as a series of ‘blocks’. See chapter 10 for an explanation of hexadecimal numbers. The last number on the line gives the ‘length’ of the file. The action of cataloguing a tape also lets the computer verify the information recorded. If there are errors in any of the data on the tape it will print a message and continue. The ESCAPE key allows you to leave cassette operations whenever you like. If you leave from the middle of a LOAD operation you will probably get a Bad Program error. Type NEW to remove this . More information about cassette formats, loading errors and files is given in Chapter 35. 30 6 Sample programs Most of the rest of this book is concerned with introducing the various parts of the BBC BASIC language which the computer understands and other features of the machine. But first, here are a few complete programs which you can try to type in yourself. They must be typed in accurately and can then be run. If a program fails to run properly, then you probably typed a line in incorrectly - for instance, you may have typed ; when you should have typed : or typed O instead of 0 . Most of the sample programs are too big to fit all of the lines on the screen. If you LIST a program you have typed in, for example to check that you have made no mistakes, you may find that the lines you want to look at disappear off the top of the screen. To prevent this you can specify the range of lines you want to be listed. For example LIST 100,200 will only list those lines numbered between 100 and 200. Alternatively you can enter ‘paged mode’ by pressing CTRL N (hold down CTRL and press N ). In this mode the listing will stop after every ‘page’ and will continue only when you press the SHIFT key. Paged mode is switched off by pressing CTRL O and you should always remember to do this after you have listed the program. Typing in programs will help you to get a feel for the keyboard and, if you save them on cassette after you are satisfied that they do run properly, will enable you to start building up a library of them. Learning to use the computer is a little like learning to drive a car - when you first start you find that there are an enormous number of things to think about all at one time. Many of the things you come across from now on will be bewildering at first, but as you get further into the book and as you gain experience in using BASIC, the various parts of the jigsaw puzzle should begin to fall into place. So don’t worry if, for instance, some of the comments about the following programs are difficult to understand at first. Note: In the program listings which follow, extra spaces have been inserted between the line numbers (10,20, etc) and what follows on each line. This is to improve the readability of the programs. However, although it will do no harm, there is no reason to type in any spaces after the line number. For example in the first program, called POLYGON, when entering line 250, all you need to type is 250MOVE 0 , 0 31 POLYGON This program draws polygons (many sided shapes) in random colours. Lines 120 to 180 move to a random place on the screen which will be the centre (origin) of the next shape. Lines 210 to 290 calculate the X and Y coordinates of each ‘corner’ of the polygon and store the values in two ‘arrays’ for future use. In addition the shape is filled with black triangles (lines 260 and 290) that make it appear that the new polygon is in front of the older ones. Lines 310 to 370 draw all the lines that make up the polygon. Lines 50 to 70 set the actual colour of logical colours 1,2 and 3 to red, blue and yellow. You can change these if you wish to use other colours. 10 REM POLYGON 20 REM JOHN A COLL 30 REM VERSION 1/16 NOV 81 40 MODE 5 50 VDU 19,1,1,0,0,0 60 VDU 19,2,4,0,0,0 70 VDU 19,3,3,0,0,0 80 DIM X (10) 90 DIM Y (10) 100 110 FOR C=1 TO 2500 120 xorigin = RND (1200) 130 y o r igin = RND ( 7 5 0 ) 140 VDU2 9 , xo r igin ; y or igin ; 150 radius=RND (300) +50 160 sides=RND ( 8 ) +2 170 MOVE radius, 0 180 MOVE 10,10 190 200 GCOL 0,0 210 FOR S I DE = 1 TO sides 220 angle= (SIDE-1 ) *2*PI/sides 230 X (SIDE) =radius*COS (angle) 240 Y (SIDE) =radius*SIN (angle) 250 MOVE 0 , 0 260 PLOT 8 5 , X ( S IDE ) , Y(SIDE) 270 NEXT SIDE 2 80 MOVE 0 , 0 290 PLOT 85, radius, 0 300 310 GCOL 0 , RND (3 ) 320 FOR S I DE = 1 TO sides 330 FOR line=SIDE TO sides 32 340 MOVE X(SIDE), Y(SIDE) 350 DRAW X ( line ) , Y(line) 360 NEXT line 370 NEXT SIDE 380 NEXT C You may like to try this alternative for line 200 200 GCOL 0 , RND (4 ) MONTHLY This program plots a set of ‘blocks’ on the screen which might represent prices over a 12-month period. In this example the height of the bars is randomly selected at line 170. Lines 180 to 270 then draw a ‘solid’ bar and the edges are marked in black by lines 290 to 330. Lines 340 and 350 print out one letter representing the month of the year at the bottom of each bar. Notice that lines 60 and 70 set up two of the function keys. Key fO sets the computer to MODE 7 and then lists the program. Key f9 can be used to run the program. 10 REM MONTHLY 20 REM JOHN A COLL 30 REM VERSION 1/16 NOV 81 50 60 *KE Y 0 "MODE7 |M LIST |M" 70 *KE Y 9 "RUN |M" 80 M$= " JFMAM JJASOND " 90 C = 0 100 MODE 2 110 VDU5 120 VDU 29, 0; 100; 130 140 FOR X= 0 TO 1100 STEP 100 150 GCOL 0,C MOD 7+1 160 C=C+1 170 H = RND (400) +200 180 MOVE X, 0 190 MOVE X , H 200 PLOT 85,X + 100, 0 210 PLOT 85,X+100,H 220 MOVE X+70,H+50 230 MOVE X , H 240 PLOT 85 , X+170 , H+50 250 PLOT 85,X+100,H 260 PLOT 85, X+170, 50 33 270 PLOT 85 , X+100 , 0 280 GCOL , 0 290 MOVEX , H 300 DRAW X+100, H 310 DRAW X + 1 7 0 , H+ 5 0 320 MOVE X+100, H 330 DRAW X+100 , 0 340 MOVE X + 10 , 50 350 PRINT MID$ (M$, C, 1) 360 NEXT 370 380 GCOL 4,1 390 MOVE 0,450: PRINT " " 400 VDU 4 410 PRINTTAB (3, 0) "critical level" The height of each bar is set randomly by the variable H. If you want to put in your own values (data), then type the following extra lines. Line 170 must also be deleted by typing 17 0 followed by RETURN. 50 DIM data (12) 82 FOR J=1 TO 12 84 PRINT "Input data for month " MID $ ( M$ , J , 1 ) ; 86 INPUT data ( J) 88 NEXT J 89 INPUT "CRITICAL LEVEL", level 155 H = dat a ( C+ 1 ) 390 MOVE 0 , level : PRINT" " QUADRAT This program can be used to solve equations of the form Y=Ax 2 + Bx + C The ‘roots of the equation’ are printed to two decimal places. The number of decimal places is set by line 90. The main program between lines 110 and 170 uses three procedures - one for each of the three types of result. The main program is surrounded by REPEAT UNTIL FALSE which keeps the program going for ever - or until the ESCAPE key is pressed. 34 Line 170 PRINT 1 ' ' prints three blank lines to separate one set of results from the next. 10 REM QUADRAT 20 REM JOHN A COLL BASED ON A PROGRAM 30 REM BY MAX BRAMER, OPEN UNIVERSITY 40 REM VERSION 1.0 /16 NOV 81 50 REM SOLVES AN EQUATION OF THE FORM 60 REM A*X A 2 + B*X + C 70 ON ERROR GOTO 350 80 MODE 7 90 @ % = 2 0 2 0A 100 REPEAT 110 PRINT "What are the three coefficients 120 INPUT A , B , C : IF A=0 THEN 110 130 DISCRIM = B A 2 — 4*A*C 140 IF D I S CRIM< 0 THEN PROCcomplex 150 IF DISCRIM=0 THEN P ROC c o i n c i de nt 160 IF D I S CRIM> 0 THEN PROCreal 170 PRINT ' ' ' 180 UNTIL FALSE 190 END 200 210 DEF PROCcomplex 220 PRINT "Complex roots X = " ; — B / ( 2 * A ) ; 230 PRINT " +/- AB S ( SQR ( -D I SCRIM) / ( 2 * A) ) "i" 240 ENDPROC 250 260 DEF PROCcoincident 270 PRINT "Co-incident roots X=";B/(2*A) 280 ENDPROC 290 300 DEF PROCreal 310 Xl= (-B+SQR (DISCRIM) )/ (2*A) 320 X=2 (-B-SQR (DISCRIM) )/ (2*A) 330 PRINT "Real distinct roots X=";X1;" and X=";X2 340 ENDPROC 350 @%=&90A:REPORT :PRINT" at line "ERL >RUN What are the three coefficients ?l,-l,-2 Real distinct roots X=2.00 and X=-1.00 What are the three coefficients ?3,3,3 Complex roots X=-0.50 +/- 0.87i 35 What are the three coefficients ?1,2,1 Co-incident roots X=1.00 What are the three coefficients ? Escape at line 120 > FOURPNT This program draws a pattern (lines 80 to 140) and then changes foreground and background colours with a half second pause between each change. 10 REM FOURPNT /DRAWS A PATTERN WITH 4 POINTS 20 REM JOHN A COLL 30 REM VERSION 1 /16 NOV 81 50 MODE 4 60 VDU 29, 640; 512 70 80 FOR A= 0 TO 500 STEP 15 90 MOVE A-500 , 0 100 DRAW 0 , A 110 DRAW 5 0 0 -A , 0 12 0 DRAW 0 , -A 130 DRAW A-500 , 0 140 NEXT A 150 160 FOR B= 0 TO 7 : REM CHANGE THE COLOUR 170 FOR C=1 TO 3 180 T=T IME : REM WAIT A WHILE 190 REPEAT UNTIL TIME-T>50 200 VDU 19, 3, C, 0,0,0 210 VDU 19, 0, B, 0, 0, 0 220 NEXT C 230 NEXT B TARTAN This program builds up a changing pattern by overdrawing lines on the screen. The main program between lines 90 and 140 loops for ever and calls various subroutines as necessary. The use of subroutines with implied GOTO (eg line 170) results in a structure which is not easy to follow! It would be better to use ‘structures’ such as procedures (see chapter 17). 10 REM TARTAN 20 REM BASED ON RESEARCH MACHINES DEMO 30 REM VERSION 1.0/16 NOV 81 36 40 MODE 2: REM ALSO WORKS IN MODE 5 50 R= 1 : D=1 : X=0 60 Y=RND (800) 70 MOVE X , Y 80 90 REPEAT 100 ON D GOSUB 160,260,350,430 110 IF RND (1000) <10 THEN R=D-1 120 GCOL R, ( D * 1 . 7) 130 DRAW X , Y 140 UNTIL FALSE 150 160 X = X + 8 0 0 - Y 170 IF X>1000 THEN 220 180 Y = 8 0 0 190 D=2 200 RETURN 210 220 Y=800/1000-X 230 X = 1 0 0 0 : D = 4 240 RETURN 250 260 Y = Y- 8 0 0 +X 270 IF Y< 0 THEN 310 280 X = 1 0 0 0 : D = 3 290 RETURN 300 310 X=1000+Y 320 Y = 0 : D = 1 330 RETURN 340 350 X = X - Y 360 IF X<0 THEN 400 370 Y = 0 : D = 4 380 RETURN 390 400 Y = -X: X = 0 : D = 2 410 RETURN 420 430 Y= Y+X 440 IF Y> 8 0 0 THEN 480 450 X = 0 : D = 1 460 RETURN 470 37 480 X = Y- 8 0 0 490 Y = 8 0 4 : D = 3 500 RETURN PERSIAN This program produces a pattern by drawing hundreds of lines. Random colours are selected by lines 60 and 70. Line 80 moves the origin (middle) of the picture to the middle of the screen. 10 REM PERSIAN 20 REM ACORN COMPUTERS 30 REM VERSION 2/16 NOV 81 40 MODE 1 50 D % = 4 60 VDU 1 9 , 2 , RND (3 ) +1 , 0 , 0 , 0 70 VDU 1 9 , 3 , RND (3 ) +4 , 0 , 0 , 0 80 VDU 29, 640; 400; 90 J1%=0 100 FOR K% = 4 0 0 TO 280 STEP -40 110 REPEAT J2 % = RND ( 3 ) : UNTIL J2%oJl% 120 J1 % = J2 % 130 GCOL 3 , J1 % 140 FOR I % = — K % TO K % STEP D% 150 MOVE K % , I % 160 DRAW — K% , — I % 170 MOVE I % , — K% 180 DRAW — I % , K% 190 NEXT 200 NEXT SQR ROOT This program calculates the square root of a number by repeating a simple operation (line 90 and 200) until the calculated result stays steady. The program also indicates how long the calculation takes. This program illustrates an important mathematical technique but of course you don’t have to work out square roots this way - the function SQR is provided in BASIC (see chapter 33). 10 REM ROOT 20 REM VERSION 1.0 / 16 NOV 81 30 REM TRADITIONAL ITERATION METHOD 40 REM TO CALCULATE THE SQUARE ROOT 50 REM OF A NUMBER TO 3 DECIMAL PLACES 60 MODE 7 38 70 ON ERROR GOTO 300 80 @%=&2030A 90 REPEAT 100 count=0 110 REPEAT 120 INPUT "What is your number ",N 130 UNTIL N> 0 140 DELTA=N 150 ROOT=N/2 160 T=T IME 170 REPEAT 180 count=count+l 190 DELTA= (N/ROOT-ROOT) /2 200 ROOT=ROOT+DELTA 210 UNTIL ABS (DELTA) <0.001 220 T=T IME-T 230 PRINT 240 PRINT "Number ",N 250 PRINT "Root " , ROOT 260 PRINT " Iterations ", count 270 PRINT "Time" , T/100 ; " seconds" 280 PRINT ' ' 290 UNTIL FALSE 300 @%=& 90A : PRINT : REPORT : PRINT >RUN What is your number?34 Number 34.000 Root 5.831 Interations 5.000 Time 0.070 seconds What is your number?125 Number 125.000 Root 11.180 Interations 6.000 Time 0.080 seconds What is your number? 39 BRIAN This program prints a ‘path in the grass’. It is a fine example of a ‘non-structured’ use of BASIC; you might like to try and ‘structure’ it. 90 REM BRIAN2 100 REM(C) BRIAN R SMITH 1980 110 REM ROYAL COLLEGE OF ART, LONDON 120 REM VERSION 1.0 /16 NOV 81 130 INPUT "NUMBER OF CYCLES e.g. 1 to 5 ",T 140 INPUT "BACKGROUND SYMBOL e.g. +",D$ 150 INPUT " MOT I F ( < 2 0 chrs.)",A$ 160 INPUT "TEXT AFTER DESIGN", B$ 170 CLS 180 F = 1 190 READ A , G , S , C , D , N 200 H= (D-C) /N 210 X = 0 220 J=1 230 X=X+S 240 Y=SIN (X) 250 Y1 = 1 + INT ( (Y-C) /H + 0 . 5) 260 1=0 270 1=1+1 280 IF I = Y 1 THEN 310 290 PRINT D $ ; 300 GOTO 420 310 Z=Z+F 320 IF Z>0 THEN 350 330 F=— F 340 GOTO 450 350 IF Z<=LEN(A$) THEN 390 360 F=— F 370 Z=Z-1 380 GOTO 310 390 S$=LEFT$ (A$ , Z) 400 PRINT S $ ; 410 I=I+Z 420 IF I < 4 0 THEN 270 430 PRINT 440 GOTO 230 450 J=J+1 460 IF J>T THEN 490 470 Z=Z+1 40 480 GOTO 310 490 FOR K= 1 TO 39 500 PRINT D $ , 510 NEXT K 520 PRINT 530 PRINT B $ 540 DATA 0 , 6 . 4 , 0 . 2 , -1 , 1 , 20 >RUN NUMBER OF CYCLES e.g. 1 to 5 BACKGROUND SYMBOL e.g. +?. MOTIF ( <2 0 chrs . ) ?Hello David! TEXT AFTER DE S I GN ? That ' s all ? 3 folks SINE This program draws a sine wave on the screen. The computer can draw dotted lines and the feature is used to fill in one part of the sine wave (line 130). The computer can also print letters anywhere on the screen not just on a 40 by 25 grid. Lines 190 to 220 print a message in the shape of another sine curve. 10 REM SINE 20 REM JOHN A COLL 30 REM VERSION 2/16 NOV 81 50 MODE 4 60 VDU5 70 GCOL 0 , 1 80 VDU19 , 1 , 1 , 0 , 0 , 0 90 MOVE 16,400 100 110 FOR X= 0 TO 320 120 IF X< 1 5 0 THEN MOVE 4 *X + 1 6 , 400 130 PLOT 21, 4 *X + 1 6 , 300 * S I N ( X/ 4 8) +400 140 NEXT 160 GCOL 0 , 1 170 H A$="SINE WAVES ARE FAR MORE INTERESTING 180 190 FOR X=1 TO 39 200 MOVE X*1280/40, 300 * S I N (X/6) +500 210 PRINT MI D $ (A$ , X, 1 ) 220 NEXT 230 240 VDU 4 250 END 41 DOUBLE HEIGHT Here is an example of an assembly language program embedded within a BASIC program between the two brackets [ and ] which enables you to type in double height letters on the screen. 10 REM DOUBLE HEIGHT IN TELETEXT 20 WIDTH 36: MODE 7 30 VDU 28,0,23,39,0 40 wr ite= ! & 2 0E AND &FFFF 50 DIM PROG 100 60 FOR PASS = 0 TO 1 70 P%= PROG 80 [ 90 OPT PASS * 3 100 CMP # & D : BNE noter 110 PHA : JSR write 120 LDA#&8D : JSR write 140 LDA# & 0 8 : JSR write 150 LDA#&8D : JSR write 160 PLA : RTS 170 .noter CMP #&20 : BCS legal 180 JMP write 190 .legal PHA JSR write 200 LDA # $ 0B : JSR write 210 LDA # & 0 8 : JSR write 220 PLA : PHA : JSR write 230 LDA #&0A : JSR write 240 PLA : RTS 250 ] 260 NEXT PASS 270 !&20E=!&20E AND &FFFF0000 OR PROG 280 END Line 270 changes the ‘write character’ routine indirection vector so that all output is sent to the new routine given above. This routine tests for a ‘return’ code (line 100) and if it finds one it issues Teletext double height control codes on to the next two lines. Otherwise the routine just prints the characters on two lines one above the other so as to produce a double height character. This routine has a quite different effect in non-Teletext modes. Try it. Press BREAK after you have finished with this program. Before we leave this section, here are a few points about entering lines into BASIC. 1. Control characters, for example CTRL B, will only be ‘reflected’ in BASIC and not entered into any program lines, strings etc. 42 2. Spaces entered in lines will be preserved, including those at the end of the line. This allows blank lines to be entered eg 1 0 space RETURN to separate program sections. Some of the programs above have such blank lines. Because of this you should avoid using COPY past the true end of a line. 3. Most keywords can be abbreviated using a full stop, eg L. for LIST, SA . for SAVE. See chapter 48 for a list of abbreviations. 7 AUTO, DELETE, REM, RENUMBER BASIC provides a number of facilities to help the user enter programs into the computer and modify programs already there. As you will know by now, it is usual to use line numbers 10, 20, 30, 40 etc for programs. This leaves gaps where the user can insert extra lines later on - for example, he or she might insert lines 11, 12, 13 and 14. When typing in a line of program the user types in the line number first and then the rest of the line. For example: 10 PRINT "THIS IS A PROGRAM" The command AUTO instructs the computer to ‘assign’ the line numbers automatically for the user. As an option you can tell the computer to start assigning lines from any number. Thus AUTO 30 0 would make the computer produce line number 300, then 310, then 320, etc. There are other options, too, which are explained in chapter 33. The command DELETE allows the user to delete a group of lines from his or her program. When you are writing a long program you often need to be able to delete a large section of it. The keyword DELETE is followed by two numbers which give the first and last lines that you wish to remove. For example DELETE 150,330 would delete all the lines with numbers between 150 and 330. Single lines can be removed by typing in the line number and pressing RETURN. REM is a very useful statement. It enables you to put remarks in your program to remind you (not the computer) what is going on. If you are developing a big program - or loading a simple program that you have not used for some time it is very easy to forget how it works or what it does. Normally people place several RE Ms at the start of a program to give general information and then put a REM at major points further down the program. 44 Once you have entered a program you will very often find that the line numbers are no longer in a numeric sequence. As we have seen the command RENUMBER makes the computer go through the whole program changing all the line numbers so that they start at line 10 and increase by 10 for each successive line. When you have finished a program it is a good idea to RENUMBER it. If you have a program in the computer try RENUMBER RETURN and then LIST the program to see the effect. After that try RENUMBER 900,100 RETURN and you will see, when you list the program, that the computer has renumbered the whole program but the new version has line numbers starting at 900 and this time increasing by steps of 100. It is possible to put more than one statement on a line. For example, the two statements C L S (clear the screen) and PRINT "HELLO" can be put on one line, as long as the individual statements are separated by colons, for example: CLS : PRINT "HELLO" You can put as many statements on a line as you like as long as the line has less than about 230 characters. The argument for using ‘multiple statement lines’ is that it saves some memory space and may make the program work a little faster. But the argument against is that you will notice it becomes much more difficult to follow the program when you list it (see chapter 16). 45 8 Introducing graphics Modes, colours, graphics and windows The BBC Microcomputer can display text and windows in eight different screen modes. Only one mode can be used at a time. When the computer is turned on, and also when the BREAK key is pressed, it is in MODE 7. MODE 7 will display text (40 columns and 32 rows) and/or graphics. MODE 7 differs from all the other modes in many ways and a whole chapter (chapter 28) has been devoted to it. In particular it is not easy to draw lines or triangles in MODE 7 and the colour of the text is changed in a different way. Finally some characters are displayed on the screen differently in this mode - for example the character [ is displayed as <— . The description that follows assumes that you are in MODE 5 . To enter MODE 5 , simply type MODE 5 RETURN Note that pressing BREAK will return you to MODE 7 so avoid using BREAK . The ‘panic button’ is marked ESCAPE. If you press this the computer will stop what it is doing and return control to you. MODE 5 is a four colour mode which means that up to four different colours can be shown on the screen at any time. When you enter MODE 5 two ‘colours’ are displayed - white letters on a black background. As you will be aware from earlier chapters the colour of the text can be changed by using the COLOUR statement, and since this is a four colour mode you can select from: colour 0 Black COLOUR 1 Red COLOUR 2 Yellow COLOUR 3 White The same four colours (black, red, yellow and white) may be selected for the background with the commands: COLOUR 12 8 (128+0) Black COLOUR 12 9 (128+1) Red COLOUR 130 (128+2) Yellow COLOUR 131 (128+3) White The colour can be used to change the colour of the text foreground and 46 background - but not the colour of any graphics: for that you need to use another BASIC keyword - GCOL, which stands for Graphics COLour. Graphics Now for the graphics: when drawing lines and triangles positions on the screen are given with two numbers (the X and Y coordinates). The point A has coordinates 600 across, 0 up. The point B is at position 100,500 and C is at 800,800. The statement DRAW 800,800 will draw a line from the last point ‘visited’ to 600,600. If no point has been visited, the computer will assume that it starts from the point 0,0. To move without drawing a line use the command MOVE . So to draw a line from 1000,0 to 1000,600 type MOVE 10 0 0, 0 RETURN DRAW 1000,600 RETURN DRAW 100, 5 00 will draw another line, and so on. As well as MOVE and DRAW there are PLOT commands for other effects. These are described in a later chapter. The statement GCOL is used to change the graphics colour used by the DRAW statement. GCOL is followed by two numbers, the first is normally zero and the second determines the graphics colours, eg: GCOL 0,0 Black lines GCOL 0,1 Red lines GCOL 0,2 Yellow lines GCOL 0,3 White lines 47 We’ll consider what happens when the first number is not zero later on (chapter 29). As with the text colours, you can change both foreground and background colours. However, before that can be illustrated it will be easier to set up two windows on the screen - one for text and one for graphics so that you are sure which is which. We will then return to the GCOL statement. Windows At the moment the whole screen can be used for text and the whole screen can be used for graphics. In some modes (eg MODE 5) we can restrict each to a specific window - or section of the screen. In modes without graphics (MODE 3, 6 and 7) only text windows can be used. Imagine we want to create two windows as shown below - on the left a graphics window, on the right a text window. Suppose that the text window stretches from the top of the screen right to the bottom but the graphics window stops short of the bottom: Making a graphics window Imagine a graphics window which has its edges a, b, c and d ‘graphics units’ away from the bottom left hand corner of the screen (which is always the starting point for graphics). 1023 i d Graphics window « a ^ b ± 0 1279 48 The statement VDU 2 4 is used (with some numbers after it) to set up a graphics window (VDU stands for Visual Display Unit). For the window shown above the full statement is VDU 2 4 , a ; b ; c ; d ; Note: There is a comma after the 24 and a semi-colon after all the other values. The reason for this punctuation is given in chapter 34. So for our actual graphics window we would put VDU 24,0; 100; 300; 799; In all screen modes which can support easily defined graphics the range of values for a, b, c and d is always the same: 0-1023 vertically, 0-1279 horizontally. Making a text window Unlike graphics, text ‘starts’ at the top left hand corner of the screen, so text windows are defined using that point as zero. Imagine the text window has edges a, b, c and d ‘text units’ away from the top left of the screen, as shown: 0 19 0 31 The statement VDU 2 8 is used to set up the window as follows: VDU 28 , a, b, c, d Note: There is a comma after the 2 8 and between the other values. There is no comma at the end. For the text window we wanted to set up, the statement would be VDU 28,5,24,19,0 To prove that you now have two separate windows try COLOUR 129 CLS 49 to fill the text window with red and GCOL 0,130 CLG to fill the graphics window with yellow. Note: In the various different screen modes the number of text characters which can be accommodated along the screen and down the screen is also different. This affects the range of values for the horizontal distances a and c as follows: MODES 0 and 3 (80 characters to a line) 0 to 79 MODES 1,4,6 and 7 (40 characters to a line) 0 to 39 MODES 2 and 5 (20 characters to a line) 0 to 19 Similarly the values of b and d depend on the MODE: MODES 0,1, 2, 4 and 5 have 32 lines (0 to 31) MODES 3, 6 and 7 have 25 lines (0 to 24) To recap, to set up the windows press BREAK then type the following - with RETURN at the end of each line. You are working in command mode rather than writing a program, so the computer acts on each instruction as you press RETURN. It also means that pressing BREAK while you are using windows would destroy the text and graphics windows and send the computer back to MODE 7. MODE 5 VDU 24, 0; 100; 300; 1000; VDU 28,5,31,19,0 CLS The command CLS clears the text from the screen. Now try typing the following lines: DRAW 0 , 1000 DRAW 100,1000 DRAW 0 , 0 DRAW 1000, 1000 You will find that text is now only appearing in the text window and that graphics are only appearing in the graphics window. If you want to clear the text only, type CLS RETURN If you want to clear the graphics only, type CLG RETURN 50 (Normally CLS clears the whole screen, but where independent text and graphics areas are defined, CLS only clears the text.) You will also notice that although some of the commands have told the computer to draw in areas of the screen outside the graphics window, you will not see this on the screen. Windows may overlap - in fact when you change mode both text and graphics windows fill the whole screen, and you can move windows without destroying what is on the screen, although changing mode does clear the screen. To reset both text and graphics windows to the whole screen, eg in the middle of a program, use VDU 2 6. VDU 5 enables text to be drawn at any position inside a graphics window - see chapter 34. Changing the colours of text and graphics Now back to text and graphics colours. Let us define the text background to be red and the graphics background to be yellow: COLOUR 129 Red text background GCOL 0,130 Yellow graphics background and then clear the text and graphics areas to their background colours: CLS Clear text area CLG Clear graphics area Now to select the foreground colours for the two areas - for example to obtain yellow letters (text foreground) type COLOUR 2 and to get black graphics lines type GCOL 0 , 0 Test this out by typing DRAW 150,500 Although you start up (in MODE 5 ) with the four colours set to black, red, yellow and white, you can select other colours (still of course only four at a time) by using VDU 1 9, as we saw in chapter 1. See chapter 34 for more details of VDU 19. So far we have been working in command mode. Next try typing in this program. You can use MODE 4 to type the program in but nothing will happen until you run the program. So, press BREAK and then the following: 10 MODE 5 20 VDU 24, 0; 0; 500; 1000; 30 VDU 28,10,20,19,5 40 COLOUR 129 51 50 COLOUR 2 60 GCOL 0,130 70 CL S : CLG 80 FOR N = 1 TO 1000 90 PRINT "LINE " ; N 100 GCOL 0 , RND (4 ) 110 DRAW RND (500) , RND (1000) 120 NEXT N >RUN You might like to try saving this program on cassette as described in chapter 5 52 9 More on variables In an earlier chapter the idea of ‘variables’ was introduced. Variables are a fundamental concept in computing, and it is not possible to go far without understanding them. As we have seen, it is possible to say LET X = 12 or just X = 12 and the computer knows that it must label a ‘box’ in its memory with the name X and that the current value of X is 12. With a variable it is possible to alter the value of what is in the box but not the name of the box itself. The statement X = 14 simply changes the value of X from 12 to 14. Similarly we can say X = X + l which looks unusual — like an equation which does not balance. In fact all that this is doing is saying to the computer - whatever the value inside your box X, increase it by 1 from now on. So far we have considered only numeric variables - that is, variables which contain numbers and on which arithmetic can be carried out. But the computer has letters and symbols of various kinds on its keyboard - what about them? Numbers and characters Although we can talk of the ‘number’ 22, we can also consider 22 as a pair of characters - in the same way as A, B, C, ?, $ are characters. In computing it is important to be able to distinguish between numbers and characters. Arithmetic can be carried out on numbers but not on characters. To give you an example to show that this is not such an esoteric idea, consider 22. We can divide 22 by 2 and get 11 if 22 is taken to be a number. But if we talked about a train leaving ‘Platform 22’ the 22 here would be a pair of characters. You cannot, with a great deal of meaning, divide ‘Platform 22’ by 2 and get ‘Platform 11’. Next it’s important to have a look at the other major kind of variable used in computing - one which can hold characters, not numbers. This is called a string variable. 53 String variables String variables are used to store ‘strings of characters’ eg words. They can be recognised easily because they always end with a dollar sign. Here are a few examples of string variables containing various strings of characters. Note that these strings must be enclosed by quotation marks. X$ = "HELLO" DAY $ = "SUNDAY 3RD JANUARY" NAME $ = "ALEX" In the first example X$ is called a string variable and HELLO is called a string. OnceX$ has been set to contain HELLO we can use statements like PRINT X$ in just the same way as we said earlier. Z = 5 PRINT Z String variables can be used to hold any number of characters between zero (empty) and 255 (full). X $ = " " will empty X $ X $ = " A " will set X $ to contain one character Of course you cannot use ordinary arithmetic on string variables. For example NAME $ = "SUSAN" PRINT NAME $ / 10 does not make sense. You can’t divide Susan’s name into ten parts. While you can add, subtract, multiply and divide using numeric variables the only similar operation that can be carried out on string variables is that of ‘addition’. Thus 10 A$ = "TODAY IS " 20 B$ = "SUNDAY" 30 C$ = A$ + B $ 40 PRINT C$ >RUN TODAY IS SUNDAY The importance of understanding string variables cannot be over-emphasised. Later chapters develop this idea. 54 How numbers and letters are stored in the computer’s memory Each memory location in the computer can be used to store any number between, and including, 0 and 255, and yet some way has to be found to store letters and also very large numbers. A number of codes are used in the computer in much the same way that different groups of people have used different codes to count. Thus the number 1984 can be written as MCMLXXXIV in Roman numerals or 1984 in decimal Arabic numerals or 7C0 in hexadecimal Arabic or 11111000000 in binary The need to transmit and store letters has produced another set of codes. The letter ‘J’ is coded in various ways thus • in Morse 10001010 in ASCII binary 4A in ASCII hexadecimal 74 in ASCII decimal The ASCII (American Standard Code for Information Interchange) is by far the most common code used by computers to represent characters. A complete code table is given in Appendix C. When you tell the computer A$ = "HELLO" it stores the ASCII codes for the letters in the word HELLO in successive memory locations. The fact that they are stored as ASCII codes is really irrelevant as far as the user is concerned, it just works. However, there are times when the user needs to know about the ASCII codes and two functions are provided to convert between characters and ASCII codes. The function ASC converts a character into its ASCII code. Thus PRINT ASC ( " J" ) would print 74. The reverse function, of converting an ASCII code into a character, is performed by CHR$ . Thus PRINT CHR$ (74) would print the letter J. In fact, one quite often needs to use PRINT CHR$, so there is a further shortened version of that statement. It is VDU; VDU 7 4 would also print the letter J. 55 Those doing more complicated programming will need to know the exact way that the computer stores strings and numerics in memory. Full information is given at the end of chapter 39. Real and integer variables The numeric variables you have met so far are technically known as real variables. They can be used to store any number between 170 000 000 000 000 000 000 000 000 000 000 000 000 (1.7 X 10 38 ) and 0.000 000 000 000 000 000 000 000 000 000 000 000 001 47 (1.47 x 10 39 ) and can include a decimal point. Of course a similar range of negative numbers can be stored too. The problem with real numbers is that they are only stored to nine figure accuracy, although this is generally accurate enough for most purposes. Note that values can be assigned to variables in exponent format, which is especially useful with very large or very small numbers. For example, instead of typing A= 15000000000000000000000000000000, A= 1 . 5E3 1 could be typed instead. Another type of numeric variable is an integer variable. Integer variable names are distinguished by having a percent sign as the last character of the variable name. They can only store whole numbers between -2,147,483,648 and +2,147,483,647. On the other hand integer variables are held with complete accuracy - so accounting problems can be dealt with the nearest penny in £2M. Arithmetic calculations with integer variables are significantly faster than with real variables. (See chapter 32 for other suggestions for speeding up programs.) The two integer operators MOD and D IV are described in chapter 23. The variables A% to Z % are special in that they are permanently allocated space in memory. Typing RUN or NEW does not destroy them. As a result the variables A% to Z% can be set in one program and then used in another program later on without losing their values. Of course the values will be lost if the machine is switched off but otherwise they will remain, even if BREAK is pressed. The variables A% to Z% are called the resident integer variables. 56 Summary Three main types of variables are supported in this version of BASIC; they are integer, real and string. Integer Real String Example 346 9.847 “HELLO” Typical variable A% A A$ Name SIZE% SIZE SIZE$ Maximum size 2,147,483,647 1.7 x 10 38 255 characters Accuracy 1 digit 9 sig figs - Stored in 32 bits 40 bits ASCII values All variable names can contain as many characters as required and all characters are used to identify the variable. Variable names may contain capital letters, lower case letters and numbers and the underline character. Variable names must start with a letter and must not start with a BASIC keyword. 10 PRINT formatting and cursor control This chapter describes the PRINT statement which is used to put text on the screen or to a printer. It assumes that you understand that a variable (such as X) can be used to hold a number and that a string variable (such as A$) can be used to hold a line of text. The following program will help to illustrate some of the ideas. Press BREAK and then type in the following program. 10 X = 8 20 A$=" HELLO" 30 PRINT X, X, X When this is RUN it produces this: >RUN 8 8 8 This shows that commas separating items in the print list (the print list is the list of things to be printed - X , X , X in this case) will force items to be printed in columns or “fields” ten characters wide. Numbers are printed at the right hand side of each column whereas words are printed on the left hand side. You can see the difference if we add some lines to the program. 10 X = 8 20 A$=" HELLO" 30 PRINT X , X/ 2 , X/ 4 40 PRINTA$ , A$ , A$ >RUN 8 4 2 HELLO HELLO HELLO < field — > width Field widths in different screen modes As we said above, the width of each ‘field’ is automatically set to ten characters when the computer is switched on. Since the computer can operate in different screen modes, displaying 20, 40 or 80 characters to the line, clearly the number of fields which can be displayed on 58 the screen will differ depending on the MODE. So try typing in a new line and running the program above. 5 MODE 5 or 5 MODE 0 80 character modes 40 character modes 20 character modes (MODES 0 and 3) (MODES 1,4,6 and 7) (MODES 2 and 5) Note: the widths of the fields can be altered by the use of a special command, 0 % (see below). Commas between items in the print list always put things in columns or ‘fields’. On the other hand semi-colons between items in the print list cause items to be printed next to each other, without spaces: 10 X = 8 20 A$=" HELLO" 30 PRINTA$ ; X; A$ ; X; X >RUN HELL08HELL088 Of course if the first item is a number it will be printed to the right of a ‘field’ unless it is preceded by a semi-colon. 10 X = 8 20 A$=" HELLO" 30 PRINT X; A$ ; A$ >RUN 8HELLOHELLO or 10 X = 8 20 A$=" HELLO" 30 PRINT ;X;A$;A$ >RUN 8HELLOHELLO 59 As well as printing variables and string variables as shown above the computer can print any characters placed in between double quotes exactly as they have been typed in, provided they are in a PRINT statement. The next program asks for your name and remembers it in the string variable N $ . 10 PRINT "WHAT IS YOUR NAME"; 20 INPUT N $ 30 PRINT "HELLO" ; N$ . HOW ARE YOU?" >RUN WHAT IS YOUR NAME ?JOHN HELLO JOHN. HOW ARE YOU? Notice the semi-colon at the end of line 10 that makes the computer stay on the same line while it waits for you to provide it with a value for N $ . Without the semi-colon this happens: >RUN WHAT IS YOUR NAME ? JOHN HELLO JOHN. HOW ARE YOU? Note also the space after the word HELLO and before the word HOW in line 30. Without these spaces the words run together to produce HELLOJOHN . HOW ARE YOU? It is also legitimate to do calculations in a print list - for example 10 X = 4 . 5 20 PRINT X, X+2 , X/3, X*X > >RUN 4.5 6.5 1.5 20.25 but look what happens if instead of X = 4 . 5 we put X = 7 10 X = 7 20 PRINT X, X+2 , X/3, X*X >RUN 7 92.33333333 49 because X/3 is 2.33333333 it makes the number move left in the field until it immediately follows the previous field which contains a 9 and appears to give a result 92.33333333, which is misleading. For this reason, amongst others, the next section is important if you want to print out a lot of numbers. 60 Altering the width of the field and the way in which numbers are printed It is often useful to be able to specify the width of the field when printing columns of figures or words and also to be able to specify the number of decimal places to which numbers will be printed. On the BBC Microcomputer this can be done by setting a special ‘variable’ (called @%) in a particular way. For the moment this must be treated as a bit of ‘magic’ but, for example, if we write @%=&20209 then this statement tells the computer to print in a field nine characters wide, and that number will be printed with a fixed number of decimal places - in this case, to two decimal places. The following program shows this being used: 5 @%=&20209 10 X = 7 20 PRINT X, X+2 , X/3 , X*X >RUN 7.00 9.00 2.33 49.00 For the more technically minded @ % is made up of a number of parts. & 2 02 09 Means hexadecimal numbers follow Format number 2 ie fixed number of decimal places Two decimal places characters Field width of nine @ %=& 2 0 3 0 9 would give Format 2, three decimal places and field width of nine characters. 5 @%=&20309 10 X = 7 20 PRINT X, X+2 , X/3 , X*X >RUN 7.000 9.000 2. 333 49.000 If you want four decimal places and a field width of 12 you would put the following: 5 0%=&2O4OC 10 X = 7 20 PRINT X, X+2 , X/3 , X*X 61 >RUN 7.0000 9.0000 2.3333 49.0000 A few points: 1. The maximum number of significant figures is ten. 2. Format 1 gives figures as exponential values Format 2 gives figures to a fixed number of decimal places. Format 0 is the ‘normal’ configuration. 3. To set the print format back to its initial value (Format 0 and field width ten), set @ % = & 90A . The & tells the computer that the numbers which follow are ‘hexadecimal’ numbers - that is, numbers based not on 10s but on 16s. Here is a list of hexadecimal numbers (which include the letters A to F). Decimal number Hex number 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 0 1 2 3 4 5 6 7 8 9 A B C D E F 10 11 12 13 14 If you want the computer to print a number or variable in hex then you must put the symbol ~ before it. For example PRINT -12 will give C . 62 TAB(X) As well as controlling the print layout by using the comma and semi-colon you can use the TAB statement to start printing at a particular place on the screen. You will remember that there can be 20, 40 or 80 characters to the line depending on the MODE . MODE 7 has 40 characters. Try this: 10 PRINT "012345678901234567890" 20 F=1 6 30 REPEAT 40 PRINT TAB ( 10 ) ; F ; TAB (15) ; 2*F 50 F=F+1 60 UNTIL F=1 8 >RUN 012345678901234567890 16 32 17 34 TAB (10) prints the value of F ten spaces from the left and then TAB (15) prints the value of 2*F 15 spaces from the left, on the same line. Note the semi- colon after TAB (10) - this causes the computer to begin printing at that point. Be sure to place an open parenthesis immediately after the word TAB. If you leave a space between them, thus: TAB (10) the computer will not understand and will report No such variable If you are beyond the place that you tell the computer to tab to, for example in position 15 with request to TAB (10), then the computer moves to the next line and then tabs ten spaces. Type in this replacement line: 40PRINT TAB ( 15 ) ; F ; TAB ( 10 ) ; 2*F >RUN 012345678901234567890 TAB(X,Y) A useful extension of the TAB statement allows print to be placed at any specific character location anywhere on the screen. You will remember that in MODE 7 the text coordinates are 63 Y 241 I This program counts to 1000, printing as it goes: 5 CLS 10 Q = 1 20 REPEAT 30 PRINT TAB (18,5) ; Q 40 Q=Q+ 1 50 UNTIL Q = 1 0 0 0 The two numbers in parentheses after TAB represent the X and Y text coordinates where printing should start (see also the third program in chapter 23). Advanced print positioning Using PRINT TAB (X, Y) allows text etc to be printed in any character ‘cell’ in the appropriate MODE. In MODE 5 there are 20 cells across the screen and 32 cells (lines) down the screen. Sometimes it is useful to be able to position characters on a much finer grid. The statement VDU5 enables text to be printed at the exact position of the graphics cursor. The statement MOVE can be used to position text. Note that this will not work in MODE 7. You will remember that the graphics screen is addressed as shown below 1023 I 1 Y 0 0 X 1279 in all modes except MODE 7. 64 Each character cell is 32 graphic units high and, in a 40 character mode such as MODE 4, 32 units wide. Suppose we want to subscript a letter to produce for example the chemical formula H 2 . This can be done as follows 10 MODE 4 20 VDU 5 30 MOVE 500,500 40 PRINT "H"; 50 MOVE 532,484 60 PRINT "2" 70 VDU 4 Note that the letter H is positioned with its top left corner at 500,500. The 2 is then printed one character to the right (532) and a half a character down (484). Again the top left of 2 is at 532,484. A neater way of achieving the same effect is to replace line 50 with PLOT 0,0, -16 One further feature of the BBC Microcomputer which is not normally available on ‘personal’ computers is the ability to superimpose characters. One obvious use of this facility is to generate special effects such as accents and true underlining. The short program below prints the word role with the accent correctly placed. 10 MODE 4 20 VDU 5 30 X = 5 0 0 40 Y = 5 0 0 50 MOVE X , Y 60 PRINT "role" 70 MOVE X+ 3 2 , Y + 1 6 80 PRINT " A " 90 VDU 4 Once in VDU 5 mode the screen will not scroll up when you reach the bottom of the page, instead the writing will start from the top of the screen again. In addition characters will be superimposed on those already on the screen. When in VDU 5 mode you can only print things in the graphics window and not in the text window, and colour is selected with the GCOL statement. VDU 5 will not work in text-only modes such as MODES 3 , 6 and 7 . 65 Cursor control The text cursor is the flashing line on the screen which shows where text will appear if it is typed in on the keyboard. The text cursor also indicates where text will be printed on the screen by a PRINT statement. The cursor can be moved around the screen by a number of special ‘control codes’, some of which are as follows. Code Effect 8 Move cursor left 9 Move cursor right 10 Move cursor up 11 Move cursor down These code numbers can be used with either the VDU command or the PRINT command - eg to move left four spaces, use either VDU 8 , 8 , 8 , 8 or PRINT CHR$ ( 8 ) ; CHR$ ( 8 ) ;CHR$(8) ;CHR$(8) Clearly the VDU command is simpler to type in in most cases. In addition to the codes shown above the user can use the PRINT TAB (X, Y) statement to move the cursor directly to any character position on the screen. As we’ve seen in MODE 7 the screen can contain up to 25 lines (numbered 0 to 24) of up to 40 characters per line. 0 Y 24 The position marked on the diagram above is 18 positions across and six lines down. The cursor could be moved directly there with the statement PRINT TAB (18,6); Note that the opening parenthesis must immediately follow the word TAB thus TAB ( and not TAB ( . 66 Exactly the same effect can be obtained with the statement VDU 31,18,6 The cursor can be moved to the ‘home’ position at the top left of the screen with the statement VDU 30 If the user wishes to clear the screen as well as move the cursor to the home position then he or she can use the statement VDU 12 The last of the VDU commands directly to do with cursor control is VDU 12 7 which moves the cursor left and deletes the character there. If you wish to delete the next four characters and then return the cursor to its initial place you could use VDU 9,9,9,9,127,127,127,127 Cursor on/off In some applications the flashing cursor can be a distraction. The cursor can be turned off with the statement VDU 23,1,0;0;0;0; The cursor can be turned back on with the statement VDU 23,1,1;0;0;0; or by changing screen mode using a MODE statement. 11 Input 67 The previous chapter showed how to get information out of the computer and on to the screen. This chapter deals with getting things from the keyboard into the computer. When a program is running there will often be a need for it to request some information from the person at the keyboard. 10 PRINT "HOW OLD ARE YOU" 20 INPUT AGE 30 IF AGE < 1 8 THEN PRINT "YOU ARE TOO YOUNG AT 40 IF AGE = 18 THEN PRINT "CONGRATULATIONS ON BEING H . I 50 IF AGE > 1 8 THEN PRINT "YOU ARE PAST IT IF YOU ARE H . r 70 PRINT ; AGE >RUN HOW OLD ARE YOU ? 2 2 YOU ARE PAST IT IF YOU ARE 22 Line 20 of the above program prints a question mark on the screen and then takes in everything that is typed on the keyboard until RETURN is pressed. Line 20 says INPUT AGE so the computer is expecting a number since AGE is a numeric variable rather than a string variable (see chapter 9). If words are supplied instead of numbers then the computer assumes that the number is zero. >RUN HOW OLD ARE YOU ?I DON'T KNOW YOU ARE TOO YOUNG AT 0 Because line 20 said INPUT AGE a number was expected. If you want to INPUT a string (word or group of words) then you must place a string variable (eg NAME $ ) on the input line. 10 PRINT "WHAT IS YOUR NAME" 20 INPUT NAME $ 30 PRINT "HELLO " ; NAME $ ; " HOW ARE YOU?" >RUN WHAT IS YOUR NAME ? JOHN HELLO JOHN HOW ARE YOU? 68 You must follow the word INPUT with a numeric variable if you are expecting a number and with a string variable if you are expecting a string. As you will have seen from the examples above you usually need to print a question on the screen to tell the person at the keyboard what you are waiting for. In the last example the question was ‘What is your name’. Instead of placing this in a separate PRINT statement you can include the question on the INPUT statement. 20 INPUT "WHAT IS YOUR NAME ", NAME $ 30 PRINT "HELLO " ; NAME $ ; " HOW ARE YOU?" >RUN WHAT IS YOUR NAME ? SUSAN HELLO SUSAN HOW ARE YOU? Notice the punctuation between the question ‘What is your name’ and the string variable NAME $ . It is a comma. Notice also that the computer printed a question mark after the question when the program was run. It always prints a question mark on an INPUT statement if a comma is used to separate the question from the string variable. If you leave the comma out of the program the computer will leave the question mark out when the program is RUN. 20 INPUT "WHAT IS YOUR NAME " NAME $ 30 PRINT "HELLO " ; NAME $ ; " HOW ARE YOU?" >RUN WHAT IS YOUR NAME STEPHEN ALLEN HELLO STEPHEN ALLEN HOW ARE YOU? The INPUT statement, which we have explored above, requires that the user presses the RETURN key after he or she has entered the reply. Until the RETURN key is pressed the user can delete errors with the DELETE key or delete the whole entry so far with CTRL U . Several inputs can be requested at one time. If you type 10 INPUT A , B 20 PRINT A , B two numbers will be expected by the computer. They can either be typed in separated by commas, or both can be followed by RETURN. The INPUT statement will ignore leading spaces and anything after a comma unless the reply is inside quotation marks. 10 INPUT A $ 20 PRINT A $ >RUN ? ABC , DEF ABC 69 The INPUT LINE statement can be used in the same way as INPUT, but it will accept everything that is typed, including leading spaces and commas. Replace line 10 by 10 INPUT LINE A$ >RUN ? ABC , DEF ABC, DEF Of course if you make the program 10 INPUT A$ , B $ 20 PRINT A$ , B $ you will get >RUN ? ABC , DEF ABC DEF because now two different inputs are needed in line 10. 70 12 GET, INKEY Sometimes it is useful to be able to detect a key as soon as it is pressed without having to wait for the RETURN key to be pressed. For example most games react immediately when a key is pressed. There are a group of four functions which respond to single keystrokes. GET GET$ INKEY INKEY $ The GET and GET$ functions wait until a key is pressed; the INKEY and I NKE Y $ pair give up after a while if no key is pressed. 100 A$ = GET$ will wait (for ever) until a key is pressed but 100 A$ = INKEY $ (200) will wait for only two seconds (200 hundredths of a second). If no key is pressed within two seconds then the computer will move on to the next line of the program and A$ will be empty. If a key was pressed after say one second then the computer will immediately move on to the next line of the program and will put the character typed into A $ . 100 PRINT "DO YOU WANT TO GO ON" 110 PRINT "YOU HAVE 2 SECONDS TO REPLY" 120 A$ = INKEY$ (200) 130 IF A$ = " " THEN PRINT "TOO LATE YOU MISSED IT" 140 IF A$="Y" THEN PRINT "COURAGEOUS FOOL!" 150 IF A$="N" THEN PRINT "COWARD" One of the most common uses of GET $ is to wait at the bottom of a page for the user to press any key when he or she is ready to go on. 100 A$ = GET$ GET and INKEY are very similar to GET$ and INKEY $ but instead of returning a character which can be put into a string variable they return a number which is the ASCII code of the character. The ASCII code of ‘Y’ is 89 and the ASCII code of ‘N’ is 78, so the last program could be re-written as 71 100 PRINT "DO YOU WANT TO GO ON" 110 PRINT "YOU HAVE 2 SECONDS TO REPLY" 120 A= INKEY (200 ) 130 IF A=— 1 THEN PRINT "TOO LATE YOU MISSED IT" 140 IF A= 8 9 THEN PRINT "COURAGEOUS FOOL!" 150 IF A = 7 8 THEN PRINT "COWARD" You will see that ‘no reply’ returns the value -1 when using INKEY and returns an empty string when using INKEY $ . Advanced features Another important use of INKEY and GET is with the group of four direction keys at the top of the keyboard. Normally these are used for editing, but a special statement can make these keys produce ASCII codes like all the other keys on the keyboard. They can then be used by a program for some special purpose - for example to move a point around the screen. The statement *FX 4,1 makes the editing keys produce ASCII codes and the statement * F X 4,0 returns the keys to their editing function. The keys produce the following codes: COPY 135 or (&87) 4- 136 or (&88) -» 137 or (&89) J. 138 or (&8A) t 139 or (&8B) For example: 10 * FX 4,1 20 MODE 4 30 X = 500 40 Y = 500 50 REPEAT 60 PLOT 6 9 , X , Y 70 K = GET 80 IF K= 1 3 6 THEN X = X-4 90 IF K= 1 3 7 THEN X = X + 4 100 IF K= 1 3 8 THEN 1 II >H 110 IF K= 1 3 9 THEN Y = Y + 4 120 UNTIL Y = 0 130 *FX 4 , 0 This program waits at line 70 for a key to be pressed. The program shown above would often be part of a much larger program in which case you would not want everything to stop until a key is pressed. Here it would be better to use K= INKEY (0) at line 70 which will let the computer have a quick look to see if a key has been pressed but not wait at all. 72 10 *FX 4 , 1 20 MODE 4 30 X = 5 0 0 40 Y = 5 0 0 50 REPEAT 60 PLOT 6 9 , X , Y 70 K=INKEY ( 0 ) 80 IF K= 1 3 6 THEN 1 X II X 90 IF K= 1 3 7 THEN X = X + 4 100 IF K= 1 3 8 THEN Y = Y — 4 110 IF K= 1 3 9 THEN Y = Y + 4 120 UNTIL Y = 0 130 *FX 4 , 0 13 TIME, RND 73 TIME The BBC Microcomputer contains an ‘elapsed time’ clock. That means that the clock ticks away at a hundred ticks per second but it does not know the real time. However, you can set it and read it. Once set it will stay running until you turn the power off or you do a ‘hard reset’ (see chapter 25). It can be set to any value, for example 0: TIME = 0 This program will print a running stopwatch in the middle of the screen: 5 CLS 10 T = TIME 20 PRINT TAB (10, 12) ; (TIME-T) /100; 30 GOTO 20 There is a program to print a 24 hour clock in chapter 23. RND When writing games (and simulations), we very often want the computer to make a random choice - or to pick a random number. The most useful function for this is RND ( X ) which picks a random number between 1 and X. The program below prints out a new random number between 1 and 6 every time a key is pressed - like throwing a dice. 10 PRINT RND (6) 20 G=GET 30 GOTO 10 and this program draws random triangles in random colours 10 MODE 5 20 PLOT 85, RND (1200) , RND (1000) 30 GCOL 0 , RND ( 3 ) 40 GOTO 20 Sometimes it is useful to be able to reset the random number generator to a known value. That may sound a bit strange but when testing a program it is sometimes convenient to have a predictable set of ‘random numbers’! To do this the number in parenthesis after the RND must be a negative number. Thus X=RND (-8) will ensure that the number sequence resulting from RND is repeatable. 74 14 REPEAT...UNTIL, TRUE, FALSE Computers are fundamentally pretty stupid things but their power comes from their ability to repeat things many times - sometimes many millions of times in one second. In this version of BASIC two types of repeating loops can be used. They are called REPEAT . . . UNTIL and FOR . . . NEXT loops. This chapter explains REPEAT. . .UNTIL loops and the next deals with FOR. . .NEXT loops. Do you remember the story about a man starting with one grain of rice and doubling it each time he won a bet? How many times would he have to double his grains of rice to own more than a million grains? In the following program C is a counter showing how many times the number of grains has doubled and X represents the number of grains of rice. 10 X = 1 20 C = 0 30 REPEAT 40 X = X* 2 50 C=C+1 60 UNTIL X>1000000 70 PRINT C,X >RUN 20 1048576 Lines 30 to 60 are called a REPEAT . . . UNTIL loop and everything within the loop is repeated until X is greater than one million. The ‘terminating condition’ in this program is that X is greater than 1000000. The next program terminates after 15 seconds. Line 40 reads the starting time and the program repeats until the present time minus the starting time is greater than 1500 hundredths of a second — the internal clock ticks a hundred times a second. 10 PRINT "SEE HOW MANY SUMS YOU" 20 PRINT "CAN DO IN 15 SECONDS" 30 PRINT 40 S TARTT IME = T IME 50 REPEAT 60 F = RND (12) 75 70 G = RND ( 12 ) 80 PRINT "WHAT IS ";F;" TIMES "G; 90 INPUT H 100 IF H=F*G THEN PRINT "CORRECT" ELSE PRINT "WRONG" 110 PRINT 120 UNTIL T IME- START T IME >15 0 0 130 PRINT "TIME UP" >RUN SEE HOW MANY SUMS YOU CAN DO IN 15 SECONDS WHAT IS 6 TIMES 9772 WRONG WHAT IS 1 TIMES 474 CORRECT WHAT IS 9 TIMES 8772 CORRECT TIME UP REPEAT . . .UNTIL loops are very useful and should be used frequently. The next program selects random letters (line 20) and times how long it takes you to find and press the appropriate key. It uses two REPEAT . . . UNTIL loops. One of them is used to wait for a particular key to be pressed on the keyboard. 10 REPEAT 20 Z = RND ( 2 6 ) + 6 4 30 PRINT 40 PRINT "PRESS THE KEY MARKED ";CHR$(Z) 50 T=TIME 60 REPEAT UNTIL GET=Z 70 PRINT "THAT TOOK YOU " ( T I ME -T ) / 1 0 0 " SECONDS" 80 UNTIL Z=0 >RUN PRESS THE KEY MARKED Y THAT TOOK YOU 1.1 SECONDS PRESS THE KEY MARKED G THAT TOOK YOU 1.03 SECONDS Lines 10 and 80 are the main loop and line 60 is a single line REPEAT. ..UNTIL loop. 76 Look at line 80. This will stop the REPEAT . . . UNTIL loop if Z=0. However Z is calculated in line 20 and will have a value between 65 and 90. It will never equal zero, so the program will never stop on its own - you have to press the ESCAPE key. Line 80 says 80 UNTIL Z=0 Z=0 will never be ‘true’. Z=0 will always be ‘false’, so line 80 can be replaced with 80 UNTIL FALSE which just means ‘go on for ever’. This is a far better way of doing things than using Z=0 because you might decide to change Z next time you looked at the program. It is also better to use REPEAT . . . UNTIL loops in this way than to put at line 80 80 GOTO 20 Using REPEAT. . .UNTIL keeps this section of the program well organised. See chapter 19 for a comment on GOTO. If you delete line 10, then the computer will meet an UNTIL statement at line 80 with no idea of where the loop is meant to start. >RUN PRESS THE KEY MARKED A THAT TOOK YOU 2.09 SECONDS No REPEAT at line 80 In summary REPEAT . . . UNT I L should be used for loops which must terminate on some specific condition. 15 FOR...NEXT 77 This structure makes the computer repeat a number of statements a fixed number of times. Try the following: 10 FOR X = 8 TO 20 20 PRINT X, X+X 30 NEXT X >RUN 8 16 9 18 10 20 11 22 12 24 13 26 14 28 15 30 16 32 17 34 18 36 19 38 20 40 You can see that the computer looped through line 20 with X taking on the value 8, then 9, then 10 etc up to 20. Each time through the loop, X increased by 1. The ‘step size’ can be changed easily. 10 FOR X = 8 TO 20 STEP 2.5 20 PRINT X, X+X 30 NEXT X >RUN 8 16 10.5 21 13 26 15.5 31 18 36 In the two previous examples the value of X (which is called the ‘control variable’) increased each time through the loop. The ‘control variable’ can be made to decrease by using a negative step size. 78 10 FOR S = 100 TO 90 STEP -1 20 PRINT S, S/2, S/5 30 NEXT >RUN 100 50 20 99 49.5 19.8 98 49 19.6 97 48.5 19.4 96 48 19.2 95 47.5 19 94 47 18.8 93 46.5 18.6 92 46 18.4 91 45.5 18.2 90 45 18 Here is a program which uses several FOR . . . NEXT loops. Some are ‘nested’ within each other in the way that one REPEAT . . . UNTIL loop was included within another. 10 FOR ROW = 1 TO 5 20 FOR STAR = 1 TO 10 30 PRINT"*"; 40 NEXT STAR 50 FOR STRIPE = 1 TO 20 60 PRINT 70 NEXT STRIPE 80 PRINT 90 NEXT ROW 100 FOR ROW = 1 TO 6 110 FOR STRIPE = 1 TO 30 120 P R I N T " = " ; 130 NEXT STRIPE 140 PRINT 150 NEXT ROW 79 >RUN ********** ********** ********** ********** ********** The listing shown above is not very easy to follow - try typing LISTO 2 and then re-listing the program. > L I S TO 2 > L I S T 10 FOR ROW = 1 TO 5 20 FOR STAR = 1 TO 10 30 PRINT"*"; 40 NEXT STAR 50 FOR STRIPE = 1 TO 20 60 PRINT 70 NEXT STRIPE 80 PRINT 90 NEXT ROW 100 FOR ROW = 1 TO 6 110 FOR STRIPE = 1 TO 30 120 PRINT "= " ; 130 NEXT STRIPE 140 PRINT 150 NEXT ROW This causes each of the ‘nested’ FOR . . .NEXT loops to be indented which can make it easier to follow. Lines 20 to 40 print out ten stars. Lines 50 to 70 print out 20 equal signs. Lines 10 and 90 ensure that the above are repeated five times. Lines 100 to 150 print out six rows of 30 equal signs. 80 A note on LISTO L I S T O stands for LIST Option and it is followed by a number in the range 0 to 7. Each number has a special effect and details are given in the BASIC keywords chapter under LISTO. However, the two most useful values are 0 and 7. LISTO 0 lists the program exactly as it is stored in memory. LISTO 1 lists the program with one space after each line number. Most programs in this book have been listed like this. LISTO 7 lists the program with one space after the line number, and two extra spaces every time a FOR . . .NEXT loop or a REPEAT . . .UNTIL loop is detected. If you are using the screen editor then make sure that you list the program with LISTO 0 or else you will copy all those extra spaces into the line! A few points to watch when using FOR . . .NEXT loops: 1. The loop always executes at least once. 10 FOR X = 2 0 TO 0 20 PRINT X 30 NEXT >RUN 20 The loop finishes with the ‘control variable’ larger than the terminating value. In the next two examples the terminating value is 10. 10 FOR Z=0 TO 10 STEP 3 20 PRINT Z 30 NEXT 40 PRINT "OUT OF LOOP" 50 PRINT Z > >RUN 0 3 6 9 OUT OF LOOP 12 10 FOR Z=0 TO 10 STEP 5 20 PRINT Z 81 30 NEXT 40 PRINT "OUT OF LOOP" 50 PRINT Z > >RUN 0 5 10 OUT OF LOOP 15 Note that it is not necessary to say NEXT Z in line 30: it is optional, though it could be argued that it is clearer to put the Z in. 2. You should never jump out of a FOR . . . NEXT loop. It is generally accepted that this is poor style. If you do this your programs will become extremely difficult to follow - there are always better alternatives usually involving the use of a procedure, or setting the control variable to a value greater than the terminating value for example 10 FOR X = 0 TO 1000 15 PRINT 20 PRINT "TYPE IN A SMALL NUMBER" 30 PRINT "OR ENTER -1 TO STOP THE PROGRAM" 40 INPUT J 50 IF J=-l THEN X= 2000 60 PRINT "12 TIMES ";J;" IS 12*J 70 NEXT X > >RUN TYPE IN A SMALL NUMBER OR ENTER -1 TO STOP THE PROGRAM ? 3 2 12 TIMES 32 IS 384 TYPE IN A SMALL NUMBER OR ENTER -1 TO STOP THE PROGRAM ? 4 5 6 12 TIMES 456 IS 5472 TYPE IN A SMALL NUMBER OR ENTER -1 TO STOP THE PROGRAM ?-l 12 TIMES -1 IS -12 82 The REPEAT . . . UNTIL loop provides a much better way of dealing with this sort of problem. 3. If you omit the FOR statement an error will be generated. First a correct program: 10 FOR X=1 TO 5 20 PRINT "HELLO" 30 NEXT >RUN HELLO HELLO HELLO HELLO HELLO and then the program with line 10 deleted 20 PRINT "HELLO" 30 NEXT >RUN HELLO No FOR at line 30 4. Every FOR statement should have a matching NEXT statement. This can be easily checked by using LISTO 7 (list option 7). If the FOR . . . NEXT loops are correctly nested then the END in line 50 will line up with the FOR in line 5. 5 FOR H= 1 TO 4 10 FOR X=1 TO 2 20 PRINT "HELLO" ,H,X 30 NEXT X 40 NEXT H 50 END > L I S TO 7 > L I S T 5 FOR H= 1 TO 4 10 FOR X=1 TO 2 20 PRINT "HELLO", H,X 30 NEXT X 40 NEXT H 50 END >RUN HELLO HELLO HELLO 1 1 2 1 2 1 83 HELLO HELLO HELLO HELLO HELLO 2 3 3 4 4 2 1 2 1 2 If the NEXT X in line 30 is deleted the computer does its best to make sense of the program. 5 FOR H= 1 TO 4 10 FOR X=1 TO 2 20 PRINT "HELLO", H,X 40 NEXT H 50 END >RUN HELLO 1 HELLO 2 HELLO 3 HELLO 4 This is not the way to write programs! Mis-nested FOR. . .NEXT loops will cause problems. 5. In summary FOR . . . NEXT loops should be used when you wish to go through a loop a fixed number of times. 1 1 1 1 84 16 IF...THEN...ELSE More on TRUE and FALSE The IF . . .THEN statement has been used in several of the programs earlier in this book - for example, in the program in chapter 14 which checked your multiplication. Line 100 was IF H = F * G THEN PRINT "CORRECT" ELSE PRINT "WRONG" As you will realise, this type of statement enables the computer to make a choice as it is working its way through the program. The actual choice that it makes will depend on the values of H, F and G at the time. As a result, the same program can behave in very different ways in different circumstances. Multiple statement lines It was explained earlier (chapter 7) that you can put more than one statement on a line and this can be particularly useful with the IF . . .THEN statement. Take, for example: 10 X = 4 : Y= 6 : PRINT "HELLO" 20 PRINT ; X + Y : X=X+Y : PRINT ; X+Y >RUN HELLO 10 16 which is just the same as 10 X = 4 20 Y= 6 30 PRINT "HELLO" 40 PRINT ; X+Y 50 X=X+ Y 60 PRINT ; X+Y This helps to understand how the computer treats multiple statement lines using the IF. . .THEN statement. In the first example which follows, K=6 and therefore the computer obeys everything after the word THEN until the word ELSE. Note that a colon only separates statements - the word ELSE must be found if you want the other course of action to follow. 85 10 K= 6 20 IF K= 6 THEN K=9: PRINT "K WAS 6" ELSE PRINT "K WAS NOT 6": PRINT "END OF LINE" >RUN K WAS 6 (Note that line 20 was so long that it overflowed on the printer but it is all part of line 20.) Changing line 10 to K=7 causes the computer to execute everything after the ELSE and as a result it prints K WAS NOT 6 END OF LINE IF . . .THEN is often used with more complicated conditions involving the words AND, OR and NOT. For example: IF X = 5 AND Y= 6 THEN PRINT "GOOD" IF X = 5 OR Y= 6 THEN PRINT "TOO LARGE" The word NOT reverses the effect of a condition, thus IF NOT ( X= 6 ) THEN PRINT "X NOT 6" These are powerful features which are easy to use. For the slightly more advanced It was explained above that you can use multiple statement lines with IF . . .THEN but this leads to messy programs. It is far better to use procedures if you want a whole lot of things to occur. Thus: 100 IF H=F*G THEN PROCGOOD ELSE PROCBAD This helps to keep the program readable which is very important, not just from an aesthetic point of view but from the very practical point that a readable program is much easier to get right! More on TRUE and FALSE In chapter 14 the concept of TRUE and FALSE was introduced. A variable can have a numeric value (eg 6 or 15) or it can be TRUE or FALSE. In fact this is just playing with words (or perhaps we should say numbers) since the computer understands TRUE to have the value -1 and FALSE to have the value 0. 10 IF 6=6 THEN PRINT "YES" ELSE PRINT "NO" >RUN YES 86 This prints YES because 6=6 is TRUE . 5 H = — 1 10 IF H THEN PRINT "YES" ELSE PRINT "NO" >RUN YES The above program prints YES because H is TRUE since it has the value -1. 5 H = 0 10 IF H THEN PRINT "YES" ELSE PRINT "NO" >RUN NO This program sets H = FALSE at line 5 so the program prints NO. -1 implies TRUE and 0 implies FALSE. What about other values of H? In fact all non-zero values (except non-integers between -1 and +1) are regarded as TRUE, as the following shows: 5 H = -5 5 10 IF H THEN PRINT "YES" ELSE PRINT "NO" >RUN YES Here are some other peculiar examples: 10 G= (6=6) 20 PRINT G >RUN -1 because (6=6) is TRUE . 10 IF 5-6 THEN PRINT "TRUE" >RUN TRUE This works because (5-6) is -1 which is TRUE . These tricks are more than academic. They can be very useful - not least when you are trying to fathom out what on earth the computer thinks it is doing! 17 Procedures 87 The BBC Microcomputer has a very complete version of BASIC - often called ‘Extended BASIC’ and in addition it includes the ability to define and use procedures and functions. It is probably the first version of BASIC in the world to allow full procedure and function handling. These extremely powerful features enable the user to structure his or her programs easily and in addition provide a real introduction to other computer languages like PASCAL. A procedure is a group of BASIC statements which can be ‘called by name’ from any part of a program. 10 REM REACT 20 REM JOHN A COLL 30 REM BASED ON AN IDEA BY THEO BARRY, OUNDLE 40 REM VERSION 1/16 NOV 81 50 @ % = & 2 0 2 0 A 60 ON ERROR GOTO 470 70 MODE 7 80 90 PROCINTRO 100 REPEAT 110 PROCF IRE 120 PROCSCORE 130 UNTIL FNS TOP 140 END 150 160 DEF PROCINTRO 170 PRINT "This program tests your reactions" 180 PRINT 190 PRINT "Press the space bar to continue" 200 REPEAT UNTIL GET=32 210 CLS 220 ENDPROC 230 240 DEF PROCF IRE 250 CLS 260 PRINT "Press the space bar" 270 PRINT "as soon as a cross appears" 280 T=T IME 290 R = RND (200) +100 300 REPEAT UNTIL TIME>T+R 88 310 PRINT TAB (17, 10);"+" 320 *FX 15,1 330 REPEAT UNTIL GET=32 340 DELAY=TIME-T-R 350 ENDPROC 360 370 DEF PROCSCORE 380 PRINT TAB ( 0 , 22); 390 PRINT "You took "; DELAY/100;" seconds" 400 ENDPROC 410 420 DEF FNS TOP 430 PRINT "Do you want another go?" 440 REPLY$=GET$ 450 = (REPLY$= "N" ) OR ( REPLY $ = " n " ) 460 470 @ % = & 9 0 A The program above shows how named procedures and functions can be used. The main part of the program is between line 90 and line 140. 90 PROCINTRO 100 REPEAT 110 PROOF IRE 120 PROCSCORE 130 UNTIL FNS TOP 140 END The program tests a person’s reactions by measuring how long it takes him or her to notice a cross on the screen. As you will see from the section above, line 90 calls a procedure which gives an introduction. The procedure is called PROCINTRO and it produces the following on the screen. This program tests your reactions Press the space bar to continue Then the program repeats PROCFIRE and PROCSCORE until the user indicates that he or she does not wish to continue. PROCFIRE produces this : Press the space bar as soon as a cross appears + and PROCSCORE produces this: You took 2.03 seconds 89 It all seems very straightforward and logical - and it is. Using procedures enables you to split a problem up into a number of small manageable sections and to use (or call) those sections with a sensible name. The main section of most programs should be just a number of procedure calls as are lines 90 to 140. The procedures themselves should be in a separate section - after the END statement. Let us examine PROCINTRO more closely. 160 DEF PROCINTRO 170 PRINT "This program tests your reactions" 180 PRINT 190 PRINT "Press the space bar to continue" 200 REPEAT UNTIL GET=32 210 CLS 220 ENDPROC Notice how it is defined: line 160 is the start of the definition and the procedure ends at line 220; between those lines are normal BASIC statements. Lines 170, 180 and 190 just print messages on the screen. Line 200 waits until the space bar is pressed, after which line 210 clears the screen. There are a number of more complex things that can be done with procedures and another program will illustrate the use of parameters - variables passed to the procedure from the main program. 10 REM HYPNO 20 REM TIM DOBSON / ACORN COMPUTERS 30 REM VERSION 2/16 NOV 81 40 MODE 5 50 VDU29, 640; 512; 60 70 FOR X= 5 1 0 TO 4 STEP -7 80 GCOLO , X 90 PROCBOX (X) 100 NEXT 110 120 REPEAT 130 GCOL RND (4 ) , RND ( 4 ) 140 FOR K= 0 TO 500 STEP 8 150 PROCBOX (K) 160 NEXT K 170 GCOL RND (4 ) , RND ( 4 ) 180 FOR K= 5 0 0 TO 0 STEP -9 190 PROCBOX (K) 200 NEXT K 210 UNTIL FALSE 90 220 END 230 240 DEF PROCBOX ( J ) 250 MOVE -J,-J 260 DRAW -J,J 270 DRAW J,J 280 DRAW J,-J 290 DRAW -J,-J 300 ENDPROC The program uses a procedure called PROCBOX which draws a box. The size of the box is determined by the parameter J in the procedure. However you will see that in line 90 the procedure is called with the statement PROCBOX (X) . The initial value of X will be 510 because that is the starting value of the FOR loop at line 70. This value of X (510)will be passed to the parameter J in the procedure. As a result the procedure PROCBOX will draw a box of ‘size’ 5 10. J is called the ‘formal parameter’ for the procedure since it is used in the procedure itself. However the X in line 90 and the K in line 150 are referred to as actual parameters. Whatever value K has in line 150 will be transferred to the formal parameter J. A procedure may have any number of parameters but there must be exactly the same number of actual parameters when the procedure is called as there are formal parameters in the procedure definition. Thus if a procedure was defined like this 1000 DEF PROCSWITCH (A, B, C$) 1040 ENDPROC it could not be called with a statement like 150 PROCSWITCH (X, Y) but this would be acceptable: 150 PROCSWITCH (length, height, NAME $ ) Local variables in procedures 10 J = 2 5 20 FOR X=1 TO 5 30 PROCNUM (X) 40 PRINT "OUT OF PROCEDURE J= ";J 50 NEXT X 60 END 70 DEF PROCNUM ( J) 91 80 PRINT "IN PROCEDURE J= ";J 90 ENDPROC >RUN IN PROCEDURE J= 1 OUT OF PROCEDURE J= 25 IN PROCEDURE J= 2 OUT OF PROCEDURE J= 25 IN PROCEDURE J= 3 OUT OF PROCEDURE J= 25 IN PROCEDURE J= 4 OUT OF PROCEDURE J= 25 IN PROCEDURE J= 5 OUT OF PROCEDURE J= 25 In the program above the variable J is used in two ways. The main program starts at line 10 and ends at line 60. The procedure is defined between lines 70 and 90. Line 10 declares that J has the value 25 and the value of J is not changed in the main program. However J is used as the formal parameter in the procedure. All formal parameters are local to the procedure which means that their value is not known to the rest of the program. Inside the procedure, J takes on the value of the actual parameter X, but outside the procedure it has a different value. The distinction is made between global variables and local variables. Global variables are known to the whole program, including procedures, whereas local variables are only known to those procedures in which they are defined and to procedures within that procedure. In the program above, X is a global varible and it looks as if J is global too, since it is defined in line 10 of the main program. In fact that J is global but the use of the parameter J in the procedure creates another variable J which is local to the procedure. If a different parameter had been used in the procedure definition then J would have remained global. Thus in the program below the formal parameter has been changed to K in line 70, which leaves J as a global variable. 10 J=2 5 20 FOR X=1 TO 5 30 P ROCNUM ( X ) 40 PRINT "OUT OF PROCEDURE J= ";J 50 NEXT X 60 END 70 DEF P ROCNUM ( K ) 80 PRINT "IN PROCEDURE J= ";J 90 ENDPROC >RUN IN PROCEDURE J= 25 92 OUT OF PROCEDURE J= 25 IN PROCEDURE J= 25 OUT OF PROCEDURE J= 25 IN PROCEDURE J= 25 OUT OF PROCEDURE J= 25 IN PROCEDURE J= 25 OUT OF PROCEDURE J= 25 IN PROCEDURE J= 25 OUT OF PROCEDURE J= 25 The program is pointless in its present form for several reasons - mostly because it doesn’t actually do anything with K in the procedure! Now that J is global its value could be altered anywhere - including inside the procedure. Line 75 increases J by 10: 10 J=2 5 20 FOR X=1 TO 5 30 P ROCNUM ( X ) 40 PRINT "OUT OF PROCEDURE J= ";J 50 NEXT X 60 END 70 DEF P ROCNUM ( K ) 75 J=J+10 80 PRINT "IN PROCEDURE J= ";J 90 ENDPROC >RUN IN PROCEDURE J= 35 OUT OF PROCEDURE J= 35 IN PROCEDURE J= 45 OUT OF PROCEDURE J= 45 IN PROCEDURE J= 55 OUT OF PROCEDURE J= 55 IN PROCEDURE J= 65 OUT OF PROCEDURE J= 65 IN PROCEDURE J= 75 OUT OF PROCEDURE J= 75 It has been pointed out that all formal parameters are local to the procedure in which they are defined (and to inner procedures) but other variables can be declared as LOCAL if required. We very often use the variable X as a counter for aFOR. . .NEXT loop and as a result you have to be careful not to use it twice in the same section of a program. Declaring X as local to a procedure ensures that its use locally will not affect the value of X outside the procedure. 10 J=2 5 20 FOR X=1 TO 5 93 30 PROCNUM(X) 40 PRINT "OUT OF 50 NEXT X 60 END 70 DEF PROCNUM(K) 72 LOCAL X 75 FOR X= 1 TO 10 80 J=J + J/X 85 NEXT X 90 ENDPROC >RUN OUT OF PROCEDURE OUT OF PROCEDURE OUT OF PROCEDURE OUT OF PROCEDURE OUT OF PROCEDURE PROCEDURE J= " ; J J= 275 J= 3025 J= 33275 J= 366025 J= 4026275 In the program above, X is used twice - once in the main program (lines 20 and 50) and secondly, and very differently, as a local variable in the procedure. J remains global. It is wise to declare variables as LOCAL in procedures and functions wherever possible except when the variable is a formal parameter. A formal parameter is automatically local and therefore does not need to be declared. 94 18 Functions Functions are in many ways similar to procedures but there is one major difference - they always calculate a result which may be a number or a string. BASIC already contains a number of functions. For example the function SQR returns the square root of a number. The square root of 16 is 4 so the statements Y = SQR (16) and PRINT SQR (16) make sense. The first example calculates the square root of 16 and places the result in Y. Compare this to a procedure - for example the one above, to draw a box. The procedure makes things happen (a box appears on the screen) but it does not produce a numeric or a string value. Functions always produce a numeric or string result. If you have a reasonable understanding of procedures and parameters then you can probably cope with this example of a function: 10 PRINT "GIVE ME THREE NUMBERS 20 INPUT A , B , C 30 PRINT "THE SUM OF THE NUMBERS IS 40 PRINT FNSUM (A, B, C) 50 END 100 DEF FNSUM ( X , Y , Z ) 105 LOCAL K 110 K=X+Y+Z 120 =K >RUN GIVE ME THREE NUMBERS ?2,4,4 THE SUM OF THE NUMBERS IS 10 Again this program is not of much use - we are using a sledge hammer to crack a nut - but we had better learn to walk before we run! The function is defined in lines 100 to 120 and three parameters are passed to the function. A, B and C are the actual parameters and the numbers in A, B and C are passed to formal parameters X, Y and Z. For the sake of illustration a local variable K has been used. Line 110 sets K equal to the sum of X, Y and Z. Line 120 shows the way in which a function is ended. It says that the function FNSUM has the value of K. 95 The example above was spread out to show how a function can be constructed - it could have been compressed to 10 PRINT "GIVE ME THREE NUMBERS 20 INPUT A , B , C 30 PRINT "THE SUM OF THE NUMBERS IS 40 PRINT FNSUM (A, B, C) 50 END 100 DEF FNSUM ( X , Y , Z ) 120 = X+Y+Z or even to the single line function shown below 10 PRINT "GIVE ME THREE NUMBERS 20 INPUT A , B , C 30 PRINT "THE SUM OF THE NUMBERS IS 40 PRINT FNSUM (A, B, C) 50 END 100 DEF FNSUM ( X , Y , Z ) = X+Y+Z Of course we could have managed without a function at all... 10 PRINT "GIVE ME THREE NUMBERS 20 INPUT A , B , C 30 PRINT "THE SUM OF THE NUMBERS IS 40 PRINT A+B+C 50 END ...and clearly that would have been the right thing to do in this case. However as soon as your programs reach 40 or 50 lines you should be using procedures extensively and functions occasionally. As mentioned at the start of this chapter, functions can be used to calculate a numeric or a string result. The function which follows returns the middle letter of a string. The string is passed as a parameter 100 DEF FNMID (A$) 110 LOCAL L 120 L=LEN(A$) 140 =MID$ ( A $ , L/2 , 1 ) Again, the function is terminated by a statement starting with an equal sign. To use the above function type in the following additional lines. 10 INPUT Z$ 20 PRINT FNMID ( Z $ ) 30 END Notice that the function is placed after the END statement where it will not be executed unless it is called by name. 96 19 GOSUB This statement allows the program temporarily to divert to another section. Think about the process of writing a letter. In essence it is really a straightforward procedure - but in practice while the main aim is to write the letter there are often several diversions like the need to get another sheet of paper or answer the phone. These small ‘sub-tasks’ are essential but if we write a description of every single thing that occurred while writing a letter the reader would probably be so confused that he or she wouldn’t realise what the overall aim was. However if the job is described as a series of subroutines or procedures then the main task will emerge more clearly. The subroutine and the GOSUB statement were introduced some years ago to help people who write BASIC programs to break their programs up into recognisable modules. In recent years more flexible and more easily used tools have become available - namely procedures and functions - and these two should be used in preference to GOSUB. None the less, BBC BASIC maintains the GOSUB statement for compatibility with other versions of BASIC. A temperature scale conversion program is shown in two forms below. Both produce exactly the same output on the computer screen but one has been written using GOSUB and GOTO and the other using procedures. First with GOSUB and GOTO: 10 REM TEMPERATURE CONVERSION 20 REM WITHOUT STRUCTURED BASIC 30 REM THIS IS NOT THE WAY TO WRITE PROGRAMS! 40 REM JOHN A COLL 50 REM VERSION 1.0 /22 NOV 81 60 MODE 7 70 @ % = & 2 0 2 0 A 80 PRINT "ENTER THE TEMPERATURE FOLLOWED BY" 90 PRINT "THE FIRST LETTER OF THE TEMPERATURE" 100 PRINT "SCALE, e.g. 100C or 72F or 320K" 110 PRINT 120 PRINT "Enter the temperature 130 INPUT REPLY $ 140 TEMP = VAL ( REPLY $ ) 150 SCALE $ =RI GHT $ (REPLY$, 1) 160 GOODSCALE=FALSE 170 IF SCALE $ = " C " THEN GOSUB 370 180 IF S CALE $ = " F " THEN GOSUB 390 97 190 IF S CALE $ = " K " THEN GOSUB 430 200 IF NOT ( GOODSCALE AND TEMP > = - 2 7 3 . 1 6 ) GOTO 260 210 PRINT ' ' 220 PRINT TEMP; " Celsius" 230 PRINT TEMP + 2 7 3.16; " Kelvin" 240 PRINT TEMP *9/5 + 32;" Fahrenheit" 250 PRINT 260 IF GOODSCALE THEN 310 270 CLS 280 PRINT "You must follow the temperature with" 290 PRINT "the letter ""C"", ""F"" or ""K"" " 300 PRINT "and nothing else" 310 IF TEMP>=-273 . 16 THEN 360 320 CLS 330 PRINT "The temperature you have given is" 340 PRINT "too cold for this universe! Try again" 350 PRINT 360 GOTO 110 370 GOOD S C ALE = TRUE 380 GOTO 460 390 REM CONVERT TO CELSIUS 400 TEMP= (TEMP-32 ) *5/ 9 410 GOOD S C ALE = TRUE 420 GOT 0 4 6 0 430 REM CONVERT TO CELSIUS 440 TEMP=TEMP-2 73 . 16 450 GOOD S C ALE = TRUE 460 RETURN Lines 430 to 460 are referred to as a ‘subroutine’, and these lines of the program can be called from line 190 by the statement GOSUB 43 0. Notice that this statement does not give the reader any idea of the purpose of the subroutine. The statement RETURN at the end of the subroutine returns it to the statement after the original GOSUBstatement. Compare the last program with the one that follows. 10 REM TEMPERATURE CONVERSION 20 REM JOHN A COLL 30 REM VERSION 1.0 /22 NOV 81 40 MODE 7 50 @%=&2020A 60 PRINT "ENTER THE TEMPERATURE FOLLOWED BY" 70 PRINT "THE FIRST LETTER OF THE TEMPERATURE 80 PRINT "SCALE, e.g. 100C or 7 2 F or 3 2 OK " 90 REPEAT 98 100 PRINT 110 PRINT "Enter the temperature 120 INPUT REPLY $ 130 TEMP = VAL ( REPLY $ ) 140 SCALE$ = RIGHT$ ( REPLY $ , 1 ) 150 GOODSCALE=FALSE 160 IF S CALE $ = " C " THEN PROCCENT 170 IF SCALE $ = " F " THEN PROCFAHR 180 IF SCALE $ = " K " THEN PROCKELVIN 190 PROCEND 200 UNTIL FALSE 210 END 220 230 DEF PROCCENT 240 GOOD S C ALE = TRUE 250 ENDPROC 260 270 DEF PROCFAHR 280 REM CONVERT TO CELSIUS 290 TEMP= (TEMP-32 ) *5/ 9 300 GOOD S C ALE = TRUE 310 ENDPROC 320 330 DEF PROCKELVIN 340 REM CONVERT TO CELSIUS 350 TEMP=TEMP-2 73 . 16 360 GOOD S C ALE = TRUE 370 ENDPROC 380 390 DEF PROCEND 400 IF GOODSCALE AND TEMP > = - 2 7 3 . 1 6 THEN PROCRESULTS 410 IF NOT GOODSCALE THEN P ROC I LLEGAL_S CALE 420 IF TEMP < -273.16 THEN PROC I LLEGAL_TEMP 430 ENDPROC 440 450 DEF PROCRESULTS 460 PRINT ' 1 470 PRINT TEMP; " Celius" 480 PRINT TEMP + 2 7 3.16; " Kelvin" 490 PRINT TEMP *9/5 + 32; " Fahrenheit" 500 PRINT 510 ENDPROC 520 530 DEF PROC I LLEGAL_SCALE 99 540 CLS 550 PRINT "You must follow the temperature with" 560 PRINT "the letter ""C"", " II p M II II II II II II 570 PRINT "and nothing else" 580 ENDPROC 590 600 DEF PROC I LLEGAL_TEMP 610 CLS 620 PRINT "The temperature you have given is" 630 PRINT "too cold for this universe! Try again" 640 PRINT 650 ENDPROC Obviously the second version is long (about a third longer) but it is much more understandable and this is of crucial importance for medium and large programs. GOTO You may have noticed the use of the GOTO statement in many of the examples above. GOTO is a very useful statement which tells the computer to skip to a particular line number. Beginners in programming find it easy to use. However, it should be used with care because it can lead to what some people call ‘spaghetti’ programming, a tangle of loops backwards and forwards which makes it very difficult indeed to follow what is going on. The example at the end of chapter 4 shows this in an extreme form. If you are writing short programs then by all means use GOTO. For example, the following program prints out the ASCII code of any key which is pressed - useful if you can’t find an ASCII code chart: 10 PRINT GET 20 GOTO 10 It would be taking things too far to expect people to write 10 REPEAT 20 PRINT GET 30 UNTIL FALSE However, when you write programs of more than, say, 50 lines it is a very good idea to try to use the ‘structure’ provided instead of GOTO statements. It is generally accepted that it is still useful to use GOTO statements as a last resort when handling error conditions. Use whatever techniques make your program (a) work and (b) easy to follow. 100 20 ON GOTO, ON GOSUB There is often a need, in a computer program, to proceed in one of a number of directions. For example your program might present a ‘menu’ of eight options for the user to choose from. When the user has made the choice your program will need to branch off in the appropriate direction. There are a number of ways of doing this. Here is one in part of a program. 100 MODE 7 110 PROCINTRO 120 REPEAT 130 PROCMENU 140 IF M= 1 THEN PROCOscar7 150 IF M=2 THEN PROCOscar8 160 IF M=3 THEN PROCUOSAT 170 IF M= 4 THEN PROCorbit 180 IF M=5 THEN PROCtransmit 190 IF M= 6 THEN PROCshowfigs 200 IF M= 7 THEN PROCMercator 210 IF M= 8 THEN PROCLocator 220 IF M= 9 THEN PROCgetdatetime 230 UNTIL M=— 1 240 END Lines 140 to 220 provide exits to a number of procedures all of which will automatically return to the main program. Which procedure is selected depends on the value of M as selected by the user during the procedure PROCMENU. The above method is easy to understand and is recommended but there are other methods which should be noted. The statement ON. . .GOTO also provides a number of exits. 100 ON M GOTO 1000,1200,1250,1600 would provide an exit to line 1000 of the BASIC program if M=l. If M=2 then control will pass to line 1200 and so on. An alternative format is 100 ON M GOSUB 1000,1200,1350 In this case control is passed to the subroutines indicated and then returned to the next line. 101 Both these techniques are widely used but are less clear than the procedures as indicated at the beginning of this chapter. ON. . .GOTO and ON. . . GOSUB may be used with ELSE to trap variable which is out of range. 60 ON F % GOTO 100,210,350 ELSE PROCfind will perform PROCfind if F% is any value other than 1, 2 or 3. use of an ON 102 21 Even more on variables Arrays Very often we use the computer to store and manipulate sets of data rather than just a single value. For example, we might want to calculate wages for a group of people or sort a group of 20 numbers into order. The 20 numbers might well be associated with 20 names. Arrays make it a lot easier to deal with groups of names and numbers. To get to a more manageable example let’s consider working with five names and their associated year of birth. We could store the five names in five variables like this: Nl$ = "SARDESON" N2$ = "MATTINSON" N3$ = "MOIR" N4$ = "ALLEN" N5$ = "MOUNT" That is quite reasonable and it works. If you say PRINT N2$ the computer will then print out MATTINSON. However, you cannot tell it to print out the fifth entry or the fourth entry. The computer doesn’t have any way of knowing that N5$ is the fifth entry. Using arrays, though, we can pick out the fifth entry in a long list and that is very useful. The first thing we have to do is to tell the computer how large an array we are going to use. This is done with a D IM statement - eg DIM N$ (5) creates an array (a table) and we can then say N $ ( 1 ) = " SARDESON" N$(2) = "MATTINSON" N$ ( 3 ) = "MOIR" N$ (4 ) = "ALLEN" N$ (5) = "MOUNT" If we follow that with X = 1 and then say 103 PRINT N$ (X) the computer will print " SARDESON " . Note that the X was a variable which, in this case, had the value of 1. Here is a complete program - as far as we have got. 10 DIM N$ (5) 20 N$ (1) =" SARDESON" 30 N$ ( 2 ) = " MATT INSON " 40 N$ (3) = " MO I R " 50 N $ (4) = "ALLEN " 60 N$ (5) = " MOUNT " 70 PRINT "WHICH ENTRY DO YOU WANT" 80 INPUT X 90 PRINT N $ (X) 100 GOTO 70 We could also define an array to contain the five years of birth: 200 DIM Y (5) 210 Y ( 1 ) =1964 220 Y ( 2 ) =1960 230 Y ( 3 ) =1950 240 Y ( 4 ) =19 5 9 250 Y ( 5 ) =19 6 2 It would be easy to add lines to this program to make the computer search for various things. Of course with only five entries it would undoubtedly be quickest to do the whole thing manually - but with a hundred, a thousand or a million entries the computer would be faster - and certainly more accurate. A few examples of extra lines will make the use of these arrays clearer. Delete lines 70 to 100. To print out everyone born before 1963 300 FOR X=1 TO 5 310 IF Y ( X ) <19 6 3 THEN PRINT N$(X) 320 NEXT X or to print out everyone whose name contains more than five letters 400 FOR X=1 TO 5 410 J$ = N$ (X) 420 IF LEN ( J$ ) >5 THEN PRINT J$ 430 NEXT 104 or to print out everyone whose name begins with M 500 FOR X=1 TO 5 510 J$ = N$ (X) 520 IF LEFT$ (J$, 1) = " M " THEN PRINT J$ 530 NEXT All these things can only be done if the computer is able to select a position in a list and it can only do this with arrays. Note: For an explanation of how the last examples worked, see chapter 22. You will have noticed that we used the array N $ ( X ) to store strings (the names of the people), and array Y ( X ) to store numbers (the years of birth). Each element of the array N $ ( X ) can store as long a name as you want (up to 255 characters) and you can dimension N $ to have as many entries as you want. For example, DIM N$(1000) would create a string array with space for 1000 different names. N $ ( X ) is called a ‘string array’ since it is used to store strings. The array Y (X) is called a ‘numeric array’ and again it can have as many elements (entries) as you need - eg DIM Y(2000). You can also have ‘integer numeric arrays’ like DIMJ% (100) . As usual on the BBC Microcomputer the story doesn’t finish there! There is another whole group of arrays which we haven’t met yet. The arrays we have met (both string and numeric) are all ‘single-dimension arrays’ and could be illustrated by this diagram. Y(l) Y(2) Y(3) Y(4) Y(5) 1964 1960 1950 1959 1962 Now suppose we wanted to store the day and month of the birthday as well as the year. We need more boxes. 21 12 4 24 19 2 2 2 10 12 1964 1960 1950 1959 1962 A set of data like that is called a ‘5 by 3 array’ and the (empty) boxes can up by the statement 10 DIM (5,3) The array could then be filled with the statements 20 Y (1, 1) =21 30 Y (1 , 2 ) =2 40 Y (1, 3) =1964 50 Y (2, 1) =12 60 Y (2 , 2 ) =2 105 70 Y (2, 3) =1960 ... etc. In practice it would involve a lot less typing, and make the program shorter, if all the figures were held in DATA statements. You may well need to skip this section at first and return to it when you have understood chapter 22 which deals with the keywords READ, DATA and RESTORE. If you use READ and DATA to fill the above five by three array the program could look like this: 10 DIM Y (5, 3) 20 FOR COLUMN = 1 TO 5 30 FOR ROW= 1 TO 3 40 READ Y (COLUMN, ROW) 50 NEXT ROW 60 NEXT COLUMN 500 DATA 21,2,1964 510 DATA 12,2,1960 520 DATA 4,2,1950 530 DATA 24,10,1959 540 DATA 19,12,1962 The program above takes successive numbers from the DATA statements and inserts them into the array. Once this program has been run the array will be set up - filled with the figures - and other sections of the program (not shown above) could search the array as required. The array above is a ‘two-dimensional array’ used to store numbers. The phrase ‘two dimensional’ refers to the fact that there are five entries in one dimension and three entries in another dimension - a total of 15 entries. A three-dimensional array could be defined with the statement DIM W (4 , 5, 6) and a four-dimensional array with DIM T (2 , 2 , 5, 3 ) This last array would have 2x2x5x3 (60) individual entries. Actually, array elements can be numbered from zero instead of one, so an array declared with DIM V (3) has, in fact, got four elements which are V(0), V(l), V(2) and V ( 3 ) . Similarly the array T ( 2 , 2 , 5 , 3 ) has 3x3x6x4 (216) elements and will take up over 1000 bytes of memory. Multi-dimension arrays are voracious memory eaters - only use them when needed and, if at all possible, use every element that you set up. There is no limit, other than lack of memory, on the number of dimensions in an array. 106 At the start of this chapter we set up a string array with the statement DIM N$ (5) . This contains six elements, N$(0) to N $ ( 5 ) . The length of each string element is limited to the usual 255 characters but you can have as many elements as you wish and as many dimensions - just as for numeric arrays. String arrays are even more ravenous for memory than numeric arrays - use them sparingly! Just to make sure that the various possibilities are clear, here is a program to set up a string array with first names as well as last names. The program reads names and dates into two arrays: 10 DIM Y (4 , 2 ) 20 DIM N$ (4, 2) 30 FOR COLUMN = 0 TO 4 40 FOR ROW= 0 TO 2 50 READ Y (COLUMN, ROW) 60 NEXT ROW 70 FOR ROW= 0 TO 2 80 READ N$ (COLUMN, ROW) 90 NEXT ROW 100 NEXT COLUMN 500 DATA 21,2,1964, JAME S , C , SARDE SON 510 DATA 12, 2, 1960, A, MICHAEL, MATTINSON 520 DATA 4 , 12 , 1 960 , CHARLES , C, MOIR 530 DATA 2 4 , 10 , 1 959 , STEPHEN, R, ALLEN 540 DATA 19,12,1962, GAVIN,, MOUNT 107 22 READ, DATA, RESTORE One very common way of storing a whole set of information along with the computer program is to use DATA statements. You will remember that computer programs can be stored on cassette and sets of data can be stored in the program as well. For example, it might be necessary in a program to convert the month given as a number into a name. The program below stores the names of the month as DATA. 5 REPEAT 10 PRINT "GIVE THE MONTH AS A NUMBER" 20 INPUT M 30 UNTIL M> 0 AND M<13 40 FOR X=1 TO M 50 READ A $ 60 NEXT X 70 PRINT "THE MONTH IS " ; A$ 100 DATA JANUARY, FEBRUARY, MARCH, APRIL 110 DATA MAY, JUNE, JULY, AUGUST, SEPTEMBER 120 DATA OCTOBER, NOVEMBER, DECEMBER >RUN GIVE THE MONTH AS A NUMBER ? 6 THE MONTH IS JUNE Lines 10 to 30 repeat until a sensible value for M is entered - it must be between 1 and 12. In the example run a value of 6 was given to M. In this case the FOR. . .NEXT loop between lines 40 and 60 will repeat six times. Each time through it READS the next piece of DATA into A $ until finally A $ will be left containing JUNE. It might make it clearer if an extra line is temporarily inserted at line 55 to print out the value of A$ and X each time through the loop. >55 PRINT A$ , X > > L I S T 5 REPEAT 10 PRINT "GIVE THE MONTH AS A NUMBER" 20 INPUT M 30 UNTIL M> 0 AND M<13 40 FOR X=1 TO M 50 READ A $ 108 55 PRINT A$ , X 60 NEXT X 70 PRINT "THE MONTH IS " ; A$ 100 DATA JANUARY, FEBRUARY, MARCH, APRIL 110 DATA MAY, JUNE, JULY, AUGUST, SEPTEMBER 120 DATA OCTOBER, NOVEMBER, DECEMBER > >RUN GIVE THE MONTH AS A NUMBER ? 6 JANUARY 1 FEBRUARY 2 MARCH 3 APRIL 4 MAY 5 JUNE 6 THE MONTH IS JUNE This is one way of getting to the (say) sixth element of a list but there is another way of using an array. Sometimes there is more than one set of data and it is useful to be able to set the ‘data pointer’ to a selected set of data. The next program has two sets of data each containing a set of prices and car names. One set of data refers to British Leyland cars and the other to Lotus cars. 10 REPEAT 20 PRINT "DO YOU PREFER BL OR LOTUS CARS?" 30 A$ = GET $ 40 PRINT A$ 50 IF A$="B" THEN RESTORE 170 ELSE RESTORE 270 60 INPUT "HOW MUCH ARE YOU WILLING TO SPEND ",P 80 PRINT "IN THAT CASE, YOU CAN AFFORD THESE:" 90 FOR X=1 TO 8 100 READ NAME $ 110 READ PRICE 120 IF PRICE

RUN DO YOU PREFER BL OR LOTUS CARS? B HOW MUCH ARE YOU WILLING TO SPEND ?6000 IN THAT CASE YOU CAN AFFORD THESE: 3198 MINI 1000 CITY 4 699 METRO HLE 5419 MAESTRO 1 . 3L 5660 MONTEGO 1 . 6 DO YOU PREFER BL OR LOTUS CARS? L HOW MUCH ARE YOU WILLING TO SPEND ? 1 9 0 0 IN THAT CASE, YOU CAN AFFORD THESE: Out of DATA at line 100 You will notice that line 50 usees the RESTORE statement to set the data ‘pointer’ to either line 170 where BL data is stored or to line 270 where Lotus data is stored. This ensures that data is read from the correct list. Lines 90 to 130 attempt to read off eight sets of data from the data lists, but fail when Lotus data is selected as only three sets of data are provided. The message Out of DATA at line 100 indicates the failure to find enough entries in the data table. Methods of overcoming the problem are given in chapter 27 which deals with error handling. 110 23 Integer handling Two special arithmetical functions are provided which produce integer (ie whole number) results. These integer functions are DIV and MOD (Division and MODulus). The result of a normal division has two parts - the whole number part and the remainder. Normally the remainder is quoted as a decimal fraction. Thus 11/4 = 2.75 or 2% However the functions DIV and MOD enable the whole number part and the remainder to be calculated separately. Thus 11 DIV 4=2 (ie 4 goes into 11 two times) and 11 MOD 4=3 (ie the remainder is 3). A simple division test shows how they can be used. 5 CLS 10 PRINT "Division test!" 20 PRINT "Answer with a whole number, and a" ' " remainder " 30 REPEAT 40 X = RND (100) 50 Y = RND ( 10 ) 60 PRINT ' "What is ";X;" divided by ";Y 70 INPUT A 80 INPUT "Remainder? "B 90 IF A=Y DIV Y AND B=X MOD Y THEN PRINT "That's correct" ELSE PRINT "That's wrong" 100 PRINT 1 "Press any key to continue" 110 T=GET 120 UNTIL FALSE DIV and MOD are used whenever you are trying to convert units - for example seconds into minutes. Thus 500 seconds is 500 DIV 60 minutes and 500 MOD 6 0 seconds - that is 8 minutes 20 seconds. For example this program prints a 24 hour clock Ill 5 PRINT "Please input the time" 10 INPUT "Hours ",H 20 INPUT "Minutes ",M 30 TIME=H* 360000 + M* 6000 40 CLS 50 REPEAT 60 SEC= ( T IME DIV 100) MOD 60 70 MIN= ( T IME DIV 6000) MOD 60 80 HR= ( TIME DIV 360000) MOD 24 90 PRINT TAB ( 7 , 12) HR; " : " ; MIN ; " : " ; SEC ; SPC ( 2 ) 100 UNTIL FALSE The clock is improved if you type VDU 23,1,0;0;0;0; which switches off the flashing cursor (see chapter 10). The next program would keep time to the end of the century - if you left the computer switched on that long! 10 lastminute=0 20 MODE 7 30 PROCOFF 40 PROGgetdatetime 50 CLS 60 REPEAT 70 PROCshowtime 80 UNTIL FALSE 90 END 100 110 DEF PROCgetdatetime 120 CLS 130 PRINT"Please supply the day, month and year" 140 PRINT "as numbers e.g. 24 10 1984" 150 PRINT 160 170 REPEAT 180 PRINT TAB (5 , 10 ); "Day 190 INPUT TAB) 12, 10) ""day 200 UNTIL day>0 AND day<32 210 220 REPEAT 230 PRINT TAB (5 , 12 ); "Month " ; 240 INPUT TAB (12,12)"" month 250 UNTIL months- 0 AND month<13 260 270 REPEAT 280 PRINT TAB (5 , 14 ) ; " Year " ; 290 INPUT TAB (12, 14) "" year 112 300 UNTIL year> 1 799 AND year<2500 OR year>0 AND year < 9 9 310 IF year<99 THEN year=year+l 90 0 320 330 CLS 340 PRINT"and now the time please" 350 PRINT "using a 24 hour clock" 360 370 REPEAT 380 PRINT TAB (5 , 10 ); "Hours 390 INPUT hour 400 UNTIL hour>-l AND hour<24 410 420 REPEAT 430 PRINT TAB (5 . 12 ); "Minutes 440 INPUT minute 450 UNTIL minute>-l AND minute<60 460 470 T IME = 1 0 0 * 6 0 * (minut e + 6 0 * hour ) 480 ENDPROC 490 500 510 DEF PROCshowtime 520 IF T IME> 8 6 4 0 0 0 0 THEN T IME=T IME- 8640000 530 hour=T IME DIV 360000 MOD 24 540 minut e=T IME DIV (100*60) MOD 60 550 second=TIME DIV 100 MOD 60 560 IF (hour=0 AND minute=0 AND lastminute=5 9 ) THEN PROCincdate 570 lastminute=minute 580 PRINT TAB (0 , 0) ; "Date = ";day; ” 590 RESTORE 600 600 DATA Jan , Feb , Mar , Apr , May , June, July, Aug , Sept, Oct, Nov, Dec 610 FOR X=1 TO month 620 READ months 630 NEXT X 640 PRINT month$ ; " ";year;" 650 PRINT "GMT = 660 IF hour< 1 0 THEN PRINT " 670 PRINT; hour; " : " ; 680 IF minute29) THEN day=l : month=3 820 IF (month=2 ) AND (day=29) THEN IF NOT FNLEAP (year) THEN day = l : month = 3 830 IF ( (month=4 OR month=6 OR month=9 OR month=ll) AND (day=31 ) ) THEN day=l: month=month+l 840 IF day>31 THEN day=l : month=month+l 850 IF month>12 THEN month=l : year=year+l 860 ENDPROC 870 880 8 90 DEF FNLEAP ( Y) 900 REM RETURNS TRU IF Y IS LEAP YEAR 910 IF Y MOD 4 = 0 AND (Y MOD 100o0 OR Y MOD 400 = 0) THEN =TRUE ELSE =FALSE 114 24 String handling It has been explained that the BBC Microcomputer can store words or other groups of characters in string variables. There are a number of functions which can be used with strings. For example if A$= "NOTWITHSTANDING" then the string function LEFT$ can be used to copy, say, the left three letters of A$ into another string - B $ . 10 A$ = " NOTWI THS TAND ING " 20 B $ = LEFT $ ( A $ , 3) 30 PRINT B $ >RUN NOT Similarly MID$ can be used to extract the middle section of a string. Change line 20 thus: 10 A$ = " NOT HW I THS TAND ING " 20 B $ =M I D $ (A$ ,4,9) 30 PRINT B $ >RUN WITHSTAND Line 20 can be read as ‘B$ is a copy of the middle of A$ starting at the fourth letter and continuing for nine letters’. As a result of this flexible use of the word ‘middle’, MID$ can in fact be used to copy any part of a string. Change the program again: 10 A$ = " NOTWI THS TAND ING " 20 B $ =M I D $ ( A$ , 1, 7) 30 PRINT B $ >RUN NOTWITH As well as LEFT $ and MID$ there is the string function RIGHT$ which copies the rightmost characters of a string. 10 A$ = " NOTWI THS TAND ING " 20 B$=RIGHT$ ( A $ , 4 ) 30 PRINT B $ >RUN DING 115 It is easy to join two strings together to make a long string by using the ‘string concatenation operator’ which is a plus sign. Its title sounds grand but its purpose is obvious - but quite different from its arithmetic use. 10 A$ = " NOTWI THS TAND ING " 20 B $ = LEFT $ ( A $ , 3) 30 C$=" LIKELY" 40 D$=B$+C$ 50 PRINT D $ >RUN NOT LIKELY The numeric function LEN can be used to count up the number of characters in a string - in other words how long it is. 10 A$ = " NOTWI THS TAND ING " 20 X=LEN(A$) 30 PRINT X >RUN 15 LEN is very useful if you don’t know how long a string is going to be. For example in this palindrome testing program A$ is copied backwards letter by letter into B $ . 5 REPEAT 7 B $ = " " 10 INPUT "What would you like to reverse? " 'A$ 20 FOR T=LEN(A$) TO 1 STEP-1 30 B $ =B $ + MID$(A$,T,1) 40 NEXT T 50 PRINT '"If you reverse"' A$ ’ " you get" ' B$ 60 UNTIL A $ = " " The numeric function INSTR can be used to see if there is a particular letter (or group of letters) in another string. 10 A$ = " NOTWI THS TAND ING " 2 0 B $ = " T " 30 X=INSTR (A$ , B$ ) 50 PRINT X >RUN 3 You will notice that it finds that there is a T at position 3 in A$ . Sometimes it is useful to be able to start the search further along the string. To do this you can add a third parameter which gives that position to start the search. 116 10 A$ = " NOTWI THS TAND ING " 2 0 B $ = " T " 30 X=INSTR (A$, B$, 4) 50 PRINT X >RUN 6 If no match is found then INSTR returns zero. 10 A$ = " NOTWI THS TAND ING " 2 0 B $ = " Z " 30 X=INSTR (A$, B4, 4) 50 PRINT X >RUN 0 This looks like the elements of the game called hangman! But first three more string related functions. It is possible to make a string containing many copies of another string by using the string function S TRING$ . So to make a string containing 20 copies of ’’ABC” we write: 10 A $ = " ABC " 20 B$=STRING$ (20, A$) 30 PRINT B $ >RUN ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC There is also a function STR$ which converts a number into a string. 10 A= 4 5 : B = 3 0 20 A $ = S TR$ (A) 30 B $ = S TR$ (B) 40 PRINT A+B 50 PRINT A $ +B $ >RUN 75 4530 > ie line 40 treats A and B as numbers and line 50 as string characters. Note that STR$ is affected by the special variable @ % if this has been set (see chapter 10). The opposite function is VAL. This extracts the number from the start of the string, which must start with a plus + or minus - sign or a number. If it 117 doesn’t a zero is returned. Numbers which are embedded in other characters are ignored. So 10 A$="124ABC56" 20 PRINT VAL ( A$ ) will print 12 4. However, back to the outline of a hangman program. 10 MODE 7 20 W=RND(12): REM 12 WORDS TO CHOOSE FROM 30 FOR X= 1 TO W 40 READ A $ 50 NEXT X 60 REM WE HAVE SELECTED A RANDOM WORD 70 REM NOW GIVE THE USER CHANCES TO 80 REM GUESS LETTERS IN THE WORD 90 L=LEN(A$) 100 CORRE C T = 0 110 TRI E S = 0 120 PRINT TAB (0,5) ; "The word has L letters " 130 PRINT TAB (0 , 6) ; "You have ";2*L; "tries" 140 REPEAT 150 PRINT TAB (10 , 7 ); "GUESS A LETTER"; 160 G$ = GET $ 170 PRINTTAB (25, 7) ; G$ 180 P=0 190 REPEAT 200 P=INSTR (A$, G$, P+1) 210 IF P < > 0 THEN PRINT TAB ( P + 1 2 , 1 5 ) ; G $ 220 IF P < > 0 THEN CORRECT = CORRECT +1 225 IF P=L THEN P=0 230 UNTIL P=0 240 TRIES=TRIES+1 250 PRINT TAB (0 , 0) ; "TRIES ";TRIES; TAB (20, 0) ; "CORRECT CORRECT 260 UNTIL (CORRECT=L OR TRIES=2*L) 270 IF CORRECT =L THEN PRINT TAB(10,19); "Congratulations" 280 IF TRIES=2*L THEN PRINT TAB (0, 16) ; "The word was " ; A$ 290 DATA NOTWITHSTANDING 300 DATA INQUISITION 310 DATA MONUMENTAL 320 DATA PRESCRIPTION 118 330 DATA CARNIVOROUS 340 DATA TENTERHOOK 350 DATA DECOMPRESSION 360 DATA FORTHCOMING 370 DATA NEVERTHELESS 380 DATA POLICEWOMAN. 390 DATA SOPHISTICATED 400 DATA GUESSTIMATE There are a number of improvements to be made to this program. Its screen layout is poor and also it lets you guess the same letter twice. It is possible to use some mathematical operators on strings. For example one can check to see if two strings are ‘equal’ or if one string is ‘greater’ than another. Obviously the words ‘equal’ and ‘greater’ have slightly different meanings when applied to strings. A few examples may help to clarify things. As far as the computer is concerned, ‘XYZ’ is greater than ‘ABC’ because X is further down the alphabet than A. Similarly, ‘ABC’ is greater than ‘AB’ because ‘ABC’ is a longer string. You can use the following comparisons with strings: = equal to < > not equal to < less than > greater than < = less than or equal to > = greater than or equal to The following are legal statements: IF A $ = "HELLO" THEN PRINT "HOW ARE YOU" IF B $ > "FIFTEEN" THEN GOTO 1000 Notice that if B$ contained SIX it would be regarded as ‘greater than’ FIFTEEN because it starts with an S whereas FIFTEEN starts with an F . 10 B $ = "SIX" 20 IF B $ > " E I GHT " THEN STOP This program would stop because the word SIX begins with an S which is regarded as ‘greater than’ the letter E . Strings are compared character by character using ASCII codes. If two strings start with an identical sequence of letters, for example PIN and PINT then the longer string is regarded as the larger one. 119 25 Progra mmin g the red user defined keys At the top of the keyboard is a group of special red keys which are called user defined keys. Instead of producing a fixed character the user can ‘define’ these keys to generate any character or string of characters that is required. For example, to set up key fl so that it produces the word PRINT every time it is pressed you can type *KE Y 1 PRINT RETURN To set key f2 to produce the word DATA you enter *KE Y 2 DATA RETURN If you want to enter more than one word into a user defined key then you can enclose the words in quotes *KE Y 3 "IF X=" RETURN though quote marks are not necessary. When you are developing programs it is very useful to have one of the keys set up to change to MODE 7 and then LIST the program automatically. If you were typing in the commands MODE 7 and LIST you would normally follow each with a RETURN, and you have to include something equivalent to pressing the RETURN key when you set the key up. In fact to set up key fO you enter this: *KE Y 0 MODE 7 |M LIST |M The two characters | and M together are understood to mean the same thing as pressing the RETURN key. In fact the | in front of any letter makes the computer generate a control character. You may remember that to enter ‘paging mode’, where the computer stops at the bottom of every page, you can type CTRL N. That instruction can be added to the key fO definition as well, if you wish. *KE Y 0 MODE 7 |M |N LIST |M It is important to remember than any *KEY definition must be the last statement on a line because once the computer finds a * at the start of a statement it passes the rest of the line to the Machine Operating System and not to BASIC. The Machine Operating System does not understand : which BASIC would understand as a multiple statement separator. The same thing 120 applies to *FX statements - only one is allowed per line. However, it is acceptable to use colons to separate statements within the key definition. For example: *KE Y 6 MOVE 0,0 : DRAW X,Y |M If you want to you can set up the user defined keys in a program in exactly the same way that they are set up in command mode. Thus 10 *KE Y 7 " | B LIST|M |C" would let key 7 turn the printer on, list the program and then turn the printer off. If you wish to include an ASCII code greater than 128 (&80) then you can do this by using the sequence | ! to add 128 to the value produced. For example: *KE Y 8 " | ! | V" would put a single ‘character’ in key 8 and the ASCII value of the character would be made up from the two parts. The | ! is worth 128 and the ASCII value of CTRL V is 22, giving a total value of 150. The BREAK key Pressing the BREAK key causes a ‘soft reset’ which does not reset the clock or clear the definitions of the user defined keys. However, pressing BREAK while the CTRL key is pressed will cause a ‘hard reset’ which resets everything. Pressing SHIFT and BREAK together is used on disc and runs a program without any further instructions. As you know, when you press the BREAK key the computer is reset and nothing can change that. Your program will stop and all variables will be lost; even your program will appear to be lost. However, there are a number of things that can be done to alter the course of events. First, a program can be recovered by typing OLD RETURN and then RUN RETURN. Alternatively, the BREAK key can be ‘redefined’ by using the expression *KE Y 10 "OLD | M RUN |M " which treats the BREAK key as another user definable key. Other keys The BBHm and COPY keys can also be redefined considered to be user defined keys 11 to 15 (see also chapter 43). they can be 121 COPY 7 t 11 12 13 14 15 122 26 Operator priority An operator is something like + , / , < , etc, which affects one or more items - for example comparing them, or adding them. Mathematical operators are familiar. Most act on two numbers - for example 3 + 7 addition 2-5 subtraction 4*6 multiplication 1/9 division 7 D I V 4 integer division 7 MOD 4 integer remainder 3 A 4 raise to a power These operators are referred to as binary operators since they require two operands (ie two things to operate on). -5 This shows one of the few ‘unary’ operators that we are used to. The - just acts on the 5 to make it a negative number. This version of BASIC has a large number of operators and it is very important that the user is aware of their order of priority. You will remember that in mathematics multiplication must be completed before addition. The same applies to other operators - there is a strict hierarchy and you must be aware of it if the computer is to do what you expect. The overall order of precedence for operators is as follows. Group 1 unary minus unary plus NOT functions parentheses ( ) indirection operators (see chapter 39) Group 2 A raise to the power Group 3 * multiplication division integer division integer remainder / DIV MOD 123 Group 4 + addition - subtraction Group 5 = equal to < > not equal to < less than > greater than < = less than or equal to > = greater than or equal to Group 6 AND logical and bitwise AND Group 7 OR logical and bitwise OR EOR logical and bitwise Exclusive OR All operators in each group have equal priority and will be dealt with on a left to right basis — in other words in order in each line. Some of the operators should be familiar by now, others may need explanation. Group 1 NOT is most often used to reverse the result of a test, eg IF NOT ( X= 5 ) THEN .... Clearly this example could be written IF X<>5 THEN but the operator NOT is often needed when using functions, eg IF NOT FNVALID THEN... Functions include all the predefined functions such as SQR,SIN,ASC etc and user defined functions like FNVAL I D . Parentheses can be used to ensure that everything within the parentheses is evaluated before any other calculations take place. Indirection operators are described in chapter 39. Group 2 Raise to the power, eg 3 A 2 = 9 3 A 3 = 2 7 Group 3 and Group 4 These contain all the usual arithmetic operators. Nothing unexpected here. 124 Group 5 This contains the relational operators which mean ‘greater than’, ‘less than’, etc. They are used in expressions such as IF X>10 THEN . . . Group 6 Logical AND is used to ensure that two or more conditions hold true before some action is taken, eg IF X>10 AND Y= 6 THEN... For further details see under AND in the chapter on BASIC keywords. Group 7 Logical OR is also used with multiple conditions, eg IF X>10 OR Y= 6 THEN The action is taken if one or more of the conditions is true. EOR is normally only used as a bitwise operator and the user is referred to the BASIC keywords chapter for details. 125 27 Error handling If the computer is unable to deal with a situation such as this: PRINT 3/0 then it will report the fact to you with an ‘error message’ and then stop, waiting for your next command >PRINT 3/0 Division by zero If you are just playing at the keyboard this entry is not a problem - in fact one of the main virtues of BASIC is that it does try to give you an indication of why it is unable to proceed. However if you are writing a program for someone else to use, and you do not want them to be bothered with error messages then you must take the precautions to deal with every possible error that might arise. The major tool in error handling is the statement ON ERROR GOTO 5000 (The 50 00 is an example — it could GOTO any line number you like.) Once the computer has encountered an ON ERROR GOTO statement it will no longer report errors and stop - instead it will go to line 5000 (or wherever you have told it to go to). The statement ON ERROR OFF makes the computer handle errors normally again. The computer has an error number for every error it may encounter and you can use the error number to enable you to know what has gone wrong. The error number is stored in the variable ERR. The error number for an attempt to divide by zero is 18 for example. 10 ON ERROR GOTO 2000 20 PRINT "HELLO" 30 PRINT 3/0 40 PRINT "BYE " 50 END 2000 PRINT ERR >RUN HELLO 18 The computer also remembers the line at which it detected the error and this number is stored in the variable ERL. 126 10 ON ERROR GOTO 2000 20 PRINT "HELLO" 30 PRINT 3/0 40 PRINT "BYE" 50 END 2000 PRINT ERR 2010 PRINT ERL >RUN HELLO 18 30 As you will see from the above the computer detected error number 18 in line number 30. Instead of just printing an error number the computer can be made to deal with the problem. Look at the next program which will generate an error when X gets to zero. 100 X=- 5 110 PRINT X, 3 / X 120 X=X+1 130 IF X< 5 THEN GOTO 110 140 END >RUN -5 -0.6 -4 0.75 -3 -1 -2 -1.5 -1 -3 0 Division by zero at line 110 If we put in error handling routine we can let the computer deal with the problem itself. 10 ON ERROR GOTO 1000 100 X=- 5 110 PRINT X, 3 / X 120 X=X+1 130 IF X< 5 THEN GOTO 110 140 END 1000 IF ERR= 1 8 THEN PRINT: GOTO 120 1010 REPORT >RUN -5 -0.6 -4 -0.75 -3 -1 127 -2 -1.5 -1 -3 0 1 3 2 1.5 3 1 4 0.75 In the example program above error 18 was dealt with successfully but line 1010 causes it to REPORT other errors in the normal way without trying to deal with them. It is usually easy, but tedious, to anticipate all the likely errors but careful planning is needed if all the error handling is to be effective. In particular you should be aware that when an error occurs you cannot return into a FOR . . . NEXT or REPEAT . . . UNTIL loop or into a procedure or function or subroutine. So long as you are aware of these limitations you can program around them. 128 28 Teletext control codes and MODE 7 MODE 7 is a Teletext compatible display mode which is very economical in its use of memory. It can provide a full colour text display with limited, but full colour, graphics. This mode is strongly recommended for applications which do not require very fine graphic detail. MODE 7 uses the standard Teletext control codes to change colours rather than the BBC BASIC COLOUR and DRAW statements. It cannot be overemphasied that MODE 7 requires different codes and statements from those available in MODES 0 to 6. The MODE 7 display consists of 20 lines of 40 characters. Each line will normally consist of white letters and numbers (text) on a black background. If the user wishes to change the colour of the text or the background then a control code must be sent to the screen with a PRINT or VDU statement. By way of explanation let’s examine one typical line of characters on a MODE 7 screen. The screen display consists of 25 lines (numbered 0 to 24) each containing up to 40 characters (0 to 39). Let’s examine a typical line and see what is displayed on the screen. 0 i 2 3 4 5 6 7 8 9 10 11 12 13 14 E E c □ o] Jj ~~1 i G H Hi 0 K 0 - - y i * r White letters Red letters Green letters Blue letters 129 To get this on the screen you will need to type >MODE 7 >PRINT "ABC" ; CHR$ (129) ; "DEF" ; CHR$ (130) ; "GHI" ; CHR$ (132) ; " JKL " You will see that there is a space on the screen between the letters ABC and the letters DEF. That space is in fact occupied by an invisible ‘control code’. The control code ‘appears’ on the screen as a spce but it affects everything to its right. The control code at position 3 is code number 12 9 and that has the effect of turning all letters and numbers that follow into red. The code at position 7 is number 13 0 which produces green alphanumerics (letters and numbers). Here is a list of a few more control codes 12 9 Alphanumeric red 13 0 Alphanumeric green 131 Alphanumeric yellow 13 2 Alphanumeric blue 13 3 Alphanumeric magenta 13 4 Alphanumeric cyan 13 5 Alphanumeric white To change the colour of the text To get these codes on the screen you have to type PRINT CHR$ (X) just before the text you want to alter, where X stands for the code you want. Often you will wish to put the codes in between two words and you do that by placing both the words and the CHR$(X) in one long PRINT statement as shown below 10 MODE 7 20 PRINT "WHITE" ; CHR$ (131) ; "YELLOW" ; CHR$ (135) ; "AND BACK TO WHITE" To make characters flash Another code, 13 6, makes everything that follows on that line flash. Try 10 MODE 7 20 PRINT "HELLO" ; CHR$ ( 136) ; "FLASHER ! " It must be emphasised that every line starts off in white, non-flashing, normal height, black background and so on. If you want to print a whole page in red letters then every line must start with control code 12 9. You should by now understand how to put the control codes into a PRINT statement and now we can see what other effects are available. 130 To change the background colour requires three control codes. Suppose that you want blue letters on a yellow background then you must use the following sequence of control codes 131 Yellow alphanumeric 15 7 New background 13 2 Blue alphanumeric 10 MODE 7 20 PRINT CHR$ (131 ); CHR$ ( 157 ); CHR$ ( 132 ); "BLUE LETTERS ON YELLOW" As you will gather to change the background colour you must first select letters of the desired colour, then declare a new background and then reselect the colour of the letter. Note that you cannot have a flashing background, so the following sequence 13 6 Flash 131 Yellow alphanumeric 15 7 New background 13 2 Blue alphanumeric will produce flashing blue letters on a steady yellow background. 10 MODE 7 20 PRINT CHR$ (136) ; CHR$ (131) ;CHR$ (157) ;CHR$ (132) ; "BLUE LETTERS ON YELLOW" Flash is turned off with control code 13 7. To produce double height characters It is possible to write characters with double their normal height using control code 141. Obviously this takes up two of the normal display lines. What is not so obvious is that you must therefore print exactly the same text on two successive lines. Try the following: 10 MODE 7 20 PRINT CHR$ (141) ; "THIS IS DOUBLE HEIGHT" As you will see it only produces the top half of the letters. Add line 30 and it works properly. 10 MODE 7 20 PRINT CHR$ (141) ; "THIS IS DOUBLE HEIGHT" 30 PRINT CHR$ (141) ; "THIS IS DOUBLE HEIGHT" 131 Of course you can have (for example) double height, flashing red letters on a white background 141 Double height 15 7 New background 12 9 Red alphanumeric 13 6 Flashing 10 MODE 7 20 PRINT CHR$ (141 ) ; CHR$ ( 157 ) ; CHR$ ( 12 9) ; CHR$ ( 136) ; "THE LOT" 30 PRINT CHR$ (141 ) ; CHR$ ( 157 ) ; CHR$ ( 12 9) ; CHR$ ( 136) ; "THE LOT" Double height is turned off with control code 14 0. As you can see, it can be very tedious typing in CHR$ (129) every time you want a Teletext control code. To make things easier it is possible to use the red user defined function keys in combination with the SHIFT key to generate these special codes. While pressing SHIFT the function keys normally produce the codes shown in the following table. fO 128 No effect fl 129 Alphanumeric red f2 130 Alphanumeric green f3 131 Alphanumeric yellow f4 132 Alphanumeric blue f5 133 Alphanumeric magenta f6 134 Alphanumeric cyan f7 135 Alphanumeric white f8 136 Alphanumeric flash on f9 137 Alphanumeric flash off As you will see fO is set to produce code 12 8 and the other keys produce higher numbers. 12 8 is said to be the ‘base address’ for the keys. The base address can be altered with *FX 2 2 6 if you wish. See chapter 43 for more details. Once you have a Teletext control code on the screen you can use the editing keys (eg COPY) to copy it into another string. This can be very useful. 132 Graphics In addition to displaying coloured letters it is possible in MODE 7 to do a certain amount of work with graphics. The graphics available in Teletext mode are more complicated to use than in other modes but with a little patience very good effects can be achieved. An additional set of control codes are used to change lower case letters into small graphic shapes. The shapes are all based on a two by three grid, the same total size as a large letter. If you want to use those graphic shapes instead of lower case letters then they must be preceded with one of the following control codes: 14 5 Red graphics 14 6 Green graphics 14 7 Yellow graphics 14 8 Blue graphics 14 9 Magenta graphics 15 0 Cyan graphics 151 White graphics Note that upper case letters will still show as letters in the same colour that you have selected for the graphics. Thus 10 PRINT CHR$ (145) ; "ABCdefGHI jkl" will show the following on the screen in red. @i0ssa0H0[iH (The full list of graphics shapes is given in Appendix B.) 133 Graphics codes It is possible to calculate the code for any particular graphics shape in the following way. Each of the six cells is represented by a specific code number: 1 2 4 8 16 64 In addition you should add in 32 + 128 (ie 160). For example the ASCII code for is 2 + 8 + 16 + 32 + 128 = 186. Making a large shape In the next chapter you can see how to use user defined characters to draw a space ship. By way of comparison, a similar but cruder space ship can be made in Teletext mode. Here is the design and the code number for each graphic character: 134 To make these display as graphics characters each line must be preceded by (for example) code 14 6 (green graphics). So the following codes must be printed on the screen: 146, 250, 245 146, 255, 255 146, 191, 239 These codes can be sent using PRINT CHR$( ) as long as you are careful to get each code in the correct place, eg 10 MODE 7 20 X = 2 0 30 Y= 1 0 40 PRINT TAB ( X , Y ) ; CHR$ (146) ;CHR$ (154) ;CHR$ (250) ;CHR$ (245) 50 PRINT TAB (X,Y + 1) ;CHR$ (146) ;CHR$ (154) ;CHR$ (255) ;CHR$ (255) 60 PRINT TAB (X,Y + 2) ;CHR$ (146) ;CHR$ (154) ;CHR$ (191) ;CHR$ (239) Instead of using PRINT TAB (X,Y) it is probably easier in this case to use ASCII codes to move the cursor-down one line (code 1 0 ) and back four spaces (code 8 four times). It is probably also easier to use the VDU statement rather than PRINT CHR$ ( ).If these two things are done then the program becomes 5 MODE 7 6 PRINT TAB (20,10); 10 VDU 146,154,250,245 20 VDU 10,8,8,8,8 30 VDU 146,154,255,255 40 VDU 10,8,8,8,8 50 VDU 146,154,191,239 100 PRINT A complete list of the Teletext codes is given in Appemdix A and Appendix B. Teletext graphics codes for the more adventurous As you will have realised, the statements MOVE and DRAW are not available in the Teletext mode(MODE 7 ). If you wish to draw lines in this mode you will need to use a suitable procedure - so here is one. It uses a look-up table, called S % , to remember the numbers corresponding to each of the six pixels (picture elements) in a Teletext graphics character. The look-up table must be set up at the beginning of the program: 135 10 DIM S% 7 20 ! S%=&08040201 30 S% ! 4 = & 4 0 1 0 (Refer to chapter 39 for an explanation of the above techniques). Next a row of Teletext control codes must be written down the left hand side of the screen to convert every line into a graphics display: 40 PROCGR and the associated procedure is: 200 DEF PROCGR 210 LOCAL Y % 220 VDU 12 230 FOR Y % = 0 TO 18 240 VDU 10,13,697 250 NEXT 260 ENDPROC The main program - in this case to plot a ‘sine curve’ - follows: 50 FOR X = 0 TO 75 STEP 0.25 60 PROCPLOT (X, 28+28*SIN (X/10) ) 70 NEXT 80 END and lastly here is the procedure to plot the point: 300 DEF PROCPLOT (X% , Y% ) 310 LOCAL C % , A% 330 VDU 31, X% DIV2+1, 19-Y% DIV3 340 C%=S%?((X% AND 1 ) + ( 2 -Y%MOD 3 ) * 2 ) 350 A% = 13 5 360 VDU (USR &FFF4 AND &FF00) DIV256 OR C% OR 128 370 ENDPROC There is no need to know how it works but here is an explanation in case you are interested. The X and Y coordinates are put into X % and Y % and then line 330 moves the text cursor to the position X % , Y % . Line 340 uses the look-up table (S%) to calculate the ‘value’ (in terms of ASCII code) of the selected pixel at X % , Y % . Setting A % = 1 3 5 and jumping to the subroutine at &FFF4 returns the ASCII code of the character which includes the spot X%, Y%. See chapter 43, which explains this OSBYTE call for a similar example. The character read from the screen is then ORed with the new pixel (C%) and written with the VDU statement. 136 The & 9 7 in line 240 produces white graphic dots. Other values will give other colours. For example & 9 1 would draw a red graph. Two improvements could be made; first we could test for illegal values of X % and Y % with 320 IF X%<0 OR X% > 7 5 OR Y%<0 OR Y%>56 THEN ENDPROC and secondly remember the position of the cursor before we entered the procedure and restore the cursor to that position at the end of the procedure. Notice that you do not need to reference actual memory coordinates and this is vital if your programs are to work via the Tube. You may not think it is important now but you will find that it is advisable to write programs using the machine code calls provided and not to get into the habit of addressing the memory directly. 29 Advanced graphics 137 As we saw earlier, the following keywords can be used in a variety of statements which produce high resolution graphics effects on the screen: MODE Selects a particular graphics MODE GCOL Selects the colour and drawing ‘style’ of any graphics (except inMODES 3, 6, or 7) DRAW Draws lines (except in MODES 3,6,or7) MOVE Moves the graphics cursor (except in MODES 3 , 6 , or 7 ) PLOT Draws lines, dotted lines, points and colours in triangles (except in MODES 3, 6, or 7) These will only become familiar with use. What follows is a description of how to use some of the keywords to produce a selection of results on the screen. How to change the screen display modes The screen mode can be changed at any time by typing MODE X, where X is the value 0 to 7 from the following list: MODE 0 Uses two colours with very high resolution and requires 16K of memory to ‘map’ the screen MODE 1 Uses four colours with high resolution and requires 16K of memory MODE 2 Uses four colours with medium resolution graphics - 16K MODE 3 Text only - 16K MODE 4 Two colours and high resolution graphics — 8K MODE 5 Four colours and medium resolution graphics - 8K MODE 6 Text only - 8K MODE 7 Teletext (which is the subject of a separate chapter of this book)- 8K In MODES 0, 1, 2, 4 and 5 the screen is divided up into imaginary rectangles, like a piece of graph paper. In MODE 0 there are 640x256 squares; in MODES 1 and 4 there are 320x256 and in MODES 2 and 5 there are 160x256 (in other words, the higher the resolution of the graphics, the smaller the rectangle). The higher the resolution , the more memory is used up in the process of ‘mapping’ the screen. 138 MODES 128-135 do not use any of main memory, and are the ‘shadow screen’ equivalents of MODES 0 - 7 . See chapter 42 for more details. How to draw lines In Appendix E you will find a graphics planning sheet which shows the way the screen can be thought of as a piece of graph paper, with each point having a horizontal (X) and a vertical (Y) value. The ‘origin’ is point 0,0 and is at the bottom left of the screen. Top right is 1279,1023. How to draw a square in the centre of the screen 1. First set up a screen mode which can support graphics. Use the MOVE statement to move the graphics ‘cursor’ from its home position (0,0) to a point where we can start drawing (say 400 units along and 400 units up). 100 MODE 5 110 MOVE 400, 400 2. Draw a line horizontally to point 800,400. The DRAW command draws a line from the last point ‘plotted’ to a point defined in the DRAW statement. 120 DRAW 800 , 400 3. Finish the box with three more DRAW statements 130 DRAW 800 , 800 140 DRAW 400, 800 150 DRAW 400, 400 Run the program. Changing the colour of the square The normal colour in MODE 5 is white. Add the following line: 105 GCOL 0 , 1 This changes the lines to ‘logical’ colour 1, which in MODE 5 is red. MODE 5 only has four colours. When the machine is switched on they are black, red, yellow and white. However, logical colours can be changed by using one of the VDU commands (see later). So, if you know what you are doing, you can select any four colours in MODE 5 . How to fill in with colour PLOT 85,X,Y (see the BASIC keywords chapter) draws and then fills in a triangle drawn from the last two plotted points to the point defined by X and Y. The colour is the current graphics foreground colour. Add the following line: 160 PLOT 85,800,800 139 Run the program. This will fill in one half of the square. Now add: 170 PLOT 85,800,400 Run the program and the whole square should become red. How to change colours At any particular moment the computer can print and draw on the screen using four colours. This page uses two ‘colours’: a white background colour and a black foreground colour - that is, black writing on a white background. Similarly, the computer has a text background colour and a text foreground colour but, in addition, it is aware of a graphics foreground colour (used to draw lines), and a graphics background colour. When you change MODE the computer resets all these colours as follows: - Text foreground colour: white - Text background colour: black - Graphics foreground colour: white - Graphics background colour: black The number of colours that can appear on the screen at one time depends on the MODE selected. In MODES 0,3,4 and 6 you can only have two colours at any time and they are normally black and white. In MODES 1 and 5 you can have up to four colours at any time and they are normally black, red, yellow and white. In MODE 2 you can have up to 16 different coloured effects. Let us consider MODE 5 for a moment and explore the effects that are available. MODE 5 is a four colour mode and the default colours are black, red, yellow and white. As you may have gathered, text and graphics are dealt with separately so to change the colour that will be used for text output type COLOUR 0 To give black text COLOUR 1 To give red text COLOUR 2 To give yellow text COLOUR 3 To give white text However, to change the colour used for graphics, for example to produce lines with the DRAW statement, you use these statements GCOL 0,0 Black graphics GCOL 0,1 Red graphics GCOL 0,2 Yellow graphics 140 GCOL 0,3 White graphics The two groups of statements above change the ‘text foreground’ and ‘graphics foreground’ colours. You will have noticed that, so far, 1 represents red, 2 is yellow and so on. To change the background colours we add 128 to these numbers. Thus COLOUR 12 9 will give a red text background, and GCOL 0,12 9 would set the graphics background colour to red. For text, for example, to change from white lettering on black, to black lettering on red in MODE 5, type MODE 5 COLOUR 129 COLOUR 0 CLS All text will now be in black and red. Graphics will still appear in white. To change text colours in the middle of a program simply insert the appropriate colour statements before the print statements to which they refer. For graphics use the GCOL statement which stands for ‘Graphics COLours’. GCOL has two numbers after it (see the BASIC keywords chapter). The second number refers to the logical colour which is to be used for graphics in the future. The first number is usually set at 0. So for example, to get red graphics lines on a yellow background, type COLOUR 131 GCOL 0 , 1 CLS But suppose that you wanted a blue background in MODE 5. So far the only available colours have been black, red, yellow and white and there is a limit of four colours in MODE 5. You can make one of four colours blue if you want to. You do this with the statement VDU 19,0,4,0,0,0 and then the four available colours would be blue, red, yellow and white. In MODE 5 , only four colours are available at a time and they are referred to as ‘logical’ colours 0 to 3. In MODE 5 ‘logical’ colour 0 is normally black, ‘logical’ colour 1 is normally red and so on but you can change the ‘actual colour’ of the ‘logical colours’ easily by using the VDU 19 statement followed by five numbers, separated by commas. In a two colour mode such as MODE 4 we can do similar things eg MODE 4 141 VDU 19,1,2,0,0,0 changes logical foreground colour 1 (which is initially white) to actual colour 2 which is green, and VDU1 9 , 0 , 5 , 0 , 0 , 0 changes logical background colour 0 to actual colour 5, which is magenta. (Note: The zero at the end are for future expansion of the system.) The computer will now produce these colours until either it is switched off, the BREAK button is pressed, or the MODE is changed. These instructions to change the colours can be embedded in a program thus making it possible to alter the colours while a program is running. Here is a list of the numbers for each ‘actual colour’ that the computer can produce. Actual colour number Displayed colour 0 Black 1 Red 2 Green 3 Yellow 4 Blue 5 Magenta 6 Cyan 7 White 8 Flashing black/white 9 Flashing red/cyan 10 Flashing green/magenta 11 Flashing yellow/blue 12 Flashing blue/yellow 13 Flashing magenta/green 14 Flashing cyan/red 15 Flashing white/black So ‘actual colour’ numbers are any numbers between 0 and 15. To make logical 3 a flashing red/cyan effect you write VDU 19,3,9,0,0,0 and here are some other examples VDU 19,1,2,0,0,0 Logical colour is green VDU 19,3,5,0,0,0 Logical colour 3 is magenta VDU 19,0,12,0,0,0 Logical colour 0 is flashing blue/yellow 142 Having set the logical colours up in this way you could then select logical colour 0 (flashing blue/yellow) as the text foreground colour whith COLOUR 0,oras the graphics foreground colour with GCOL 0,0. The table shows the set up for each time you change MODE . Foreground colour Background colour Logical Actual colour Logical Actual colour number number Modes 0,3, 4, 6 0 Black (0) 128 Black (0) 1 White (7) 129 White (7) Modes 1,5 0 Black (0) 128 Black (0) 1 Red (1) 129 Red (1) 2 Yellow (3) 130 Yellow (3) 3 White (7) 131 White (7) Mode 2 0 Black (0) 128 Black (0) 1 Red (1) 129 Redd) 2 Green (2) 130 Green (2) 3 Yellow (3) 131 Yellow (3) 4 Blue (4) 132 Blue (4) 5 Magenta (5) 133 Magenta (5) 6 Cyan (6) 134 Cyan (6) 7 White (7) 135 White (7) 8 Flashing black/white (8) 136 Flashing black/white (8) 9 Flashing red/cyan (9) 137 Flashing red/cyan (9) 10 Flashing green/magenta (10) 138 Flashing green/magenta (10) 11 Flashing yellow/blue (11) 139 Flashing yellow/blue (11) 12 Flashing blue/yellow (12) 140 Flashing blue/yellow (12) 13 Flashing magenta/green (13) 141 Flashing magenta/green (13) 14 Flashing cyan/red (14) 142 Flashing cyan/red (14) 15 Flashing white/black (15) 143 Flashing white/black (15) You should note that the GCOL statement is followed by two numbers. The first number can be used to control the way that the colour (which is selected by the second number) is affected by what is already on the screen. The statement GCOL 0,3 143 tells the computer that the graphics colour to be used is to be logical colour 3 and that this is to appear no matter what was on the screen under the new line or triangle. Values other than 0, for the first number, have other effects. For example, a value of 4, as in GCOL 4 , 0 has the effect of drawing a line which is the ‘inverse’ logical colour to the colour that it is currently crossing over. In two colour MODES the inverse of logical colour 0 is logical colour 1. In four colour modes the following applies. Logical colour Inverse 0 3 1 2 2 1 3 0 With ‘default’ actual colour of black, red, yellow and white the inverse colours would be white, yellow, red and black. In MODE 2 , the 16 colour MODE, all steady colours translate to flashing colours and vice versa as the next table shows for the default colours. Logical colour Default displayed colour Inverse 0 Black Flashing white/black 1 Red Flashing cyan/red 2 Green Flashing magenta/green 3 Yellow Flashing blue/yellow 4 Blue Flashing yellow/blue 5 Magenta Flashing green/magenta 6 Cyan Flashing red/cyan 7 White Flashing black/white 8 Flashing black/white White 9 Flashing red/cyan Cyan 10 Flashing green/magenta Magenta 11 Flashing yellow/blue Blue 12 Flashing blue/yellow Yellow 13 Flashing magenta/green Green 14 Flashing cyan/red Red 15 Flashing white/black Black 144 Other values of the first number following GCOL enable the logical colour to be plotted to be ANDed, ORed or Exclusive-ORed with the logical colour presently on the screen. The user is referred to the BASIC keywords chapter for a description of AND , OR and E OR but the following examples may help. In MODE 5 with the background set to red by the following statements MODE 5 GCOL 0,129 CLG an attempt to draw a yellow line ORed with the background colour by using the statement GCOL 1 , 2 would in fact produce a white line since the background logical colour is 1 and the new line is to be drawn in logical colour (1 OR 2) ie logical colour 3. The same principles apply to GCOL 2, which ANDs the new colour with the previously displayed logical colour and to GCOL 3, which Exclusive-ORs the colours together. Obviously, an understanding of AND, OR and EOR is required before all the GCOL statements can be used. The effects which can be produced are very useful when it comes to producing sophisticated animations. How to plot a point on the screen The PLOT command can also be used to plot points. Type MODE 4 (which is a two colour MODE ) and type PLOT 69,500,500 This will plot a point in white at coordinates X=500, Y=500. Type PLOT 69, 600, 500 and another point will appear at X=600, Y=500. How to remove a point selectively Typing CLG would clear the whole graphics area. However, instead of CLG , type PLOT 70,500,500 and you will see that one of the points has gone. This is because PLOT 7 0 prints the ‘inverse’ colour at the given point. This is particularly useful when ‘animating’, for example, a point (see below). In MODE 4 the ‘inverse’ of logical colour 1 is logical colour 0. 145 Suppose we want, for example, a yellow dot on a blue background. We need to change the foreground colour from white to yellow and the background colour from black to blue. To do this, use the VDU 1 9 command as described earlier: VDU 19,1,3,0,0,0 This alters logical colour 1 (this means the normal foreground colour) to actual colour 3 (ie yellow). Now type VDU 19,0,4,0,0,0 This alters logical colour 0 (which is the normal background colour) to actual colour 4 (ie blue). PLOT 69, 500, 500 will now produce the desired effect. Animation How to make a ball and move it on the screen This time we’ll write a program in steps to make a yellow ‘ball’ consisting of four dots which move on the screen on a red background. 1. Setting up MODE and colours 10 MODE 4 20 VDU 19,1,3,0,0,0 30 VDU 19,0,1,0,0,0 2. Next a ‘procedure’ for creating a ball. This procedure will be ‘called up’ whenever we want to create a ball on the screen at a point (X,Y). It is good practice to put a procedure like this at the end of a program, so we’ll give it a high line number. X and Y are the parameters for this procedure. 1000 DEF PROCBALL (X,Y) 1010 PLOT 7 0 , X , Y 1020 PLOT 7 0 , X , Y + 4 1030 PLOT 7 0 , X + 4 , Y 1040 PLOT 70, X+4, Y+4 1050 ENDPROC We use PLOT 7 0 rather than P L O T 6 9 to help the animation which follows. 3. Making the ball travel horizontally at height Y = 500. 40 REM HORIZONTAL MOVEMENT 50 FOR N = 1 TO 1000 60 PROCBALL (N,500) 70 PROCBALL (N,500) 146 80 NEXT N 90 END You will see that this prints the ball at the point (N,500) and then ‘unprints’ it only to print it again one step further on, and so on. To speed the ball up, alter line 50: 50 FOR N = 1 TO 1000 STEP 10 How to create your own ‘graphics’ characters Each character which you type in at the keyboard has an associated ASCII code. When the computer is told to print this character it looks up the code and prints the appropriate character as an eight by eight matrix of dots. The letter ‘A’, for example, has the code value 65 and ‘a’ has the value 97. (See Appendix A for the other codes.) However, certain code values have been left to be defined by the user. They include values 224 to 255. (See chapter 34 for more details.) They can be defined by use of the VDU 2 3 command. How to make a character (eg a man) Create the character by planning it on an eight by eight square grid. X t>i ->* CM C£J HtDCOHOO'flN a b c d e f g h Note the numbers along the top of the grid which start at the right and double at each column to the left. To store the character shown above as code number 240, type in VDU 23,240,28,28,8,127,8,20,34,65 The numbers which follow VDU 2 3, 240 tell the computer the pattern of dots in each horizontal row. These values are the ‘byte’ patterns corresponding to the eight cells of each row. They can be calculated in a number of ways and entered as a decimal or hexadecimal number. The simplest way for the novice is to add up the values shown in the diagram above. Thus row a consists of 16+8+4=28, row b is also 28, the third row is 8, the fourth row is 64+32+16+8+4+2+1=127, and so on. So to create the little man, type the following program: 5 MODE 5 147 10 VDU 23,240,28,28,8,127,8,20,34,65 20 PRINT CHR$ (240) ; 30 GOTO 20 Note the last two lines, which print him over and over again. How to make him move We have created a character which can be reproduced in any MODE (except MODE 7). By printing him and then erasing him at successive positions he can be made to move across the screen in a similar way to the ball. However, since he exists as a character he is treated as text, not graphics. This means that he is made to appear by using PRINT - as above - and the position can be defined by using the TAB statement. Try this: 5 MODE 4 10 VDU 23,240,28,28,8,127,8,20,34,65 20 PRINT TAB (20,10); CHR$(240) The character appears at text position 20,10. This is a 40 character wide MODE so 20,10 is roughly in the middle of the screen. Now try this: 20 FOR X = 1 TO 19 30 PRINT TAB ( X , 1 0 ) ; CHR$(240) 40 NEXT X This will print the man 19 times across the screen. Now type these additional lines: 40 FOR T = 1 TO 100: NEXT T 50 PRINT TAB (X, 10) ; "" 60 NEXT X and he appears to run across the screen. By alternating lines 30 and 50 so that the value in parentheses is (X,X) he can be made to move diagonally across the screen. Note that line 40 acts as a time delay. 148 How to make a larger character This time, a space ship. So we have 5 MODE 4 10 VDU 23,240,8,8,28,28,62,62,62,62 20 VDU 23,241,62,62,62,62,62,62,62,62 30 VDU 23,242,62,62,62,127,127,127,93,93 40 X = 20: Y = 1 0 50 PRINT TAB (X, Y) ; CHR$240 ; 60 PRINT TAB (X, Y+l ) ; CHR$241 ; 70 PRINT TAB (X, Y+2 ) ; CHR$242 ; This produces the space ship in the middle of the screen. To make it take off, change the program by adding these lines: 7 VDU23 , 1 , 0 ; 0 ; 0 ; 0 ; 35 X = 2 0 40 FOR Y = 24 TO 0 STEP -1 80 FOR T = 1 TO 100: NEXT T 90 PRINT TAB ( X , Y ) ; " 100 PRINT TAB (X, Y + l); " 110 PRINT TAB (X, Y+2); " "; 120 NEXT Y Now add an extra character to produce flames at the bottom of the space ship for the initial take-off. 149 32 VDU 23,243,28,60,30,60,126,108,162,162 75 IF Y> 1 2 THEN PRINT T AB ( X , Y + 3 ) ; CHR$ 2 4 3 ; 115 PRINT TAB ( X , Y + 3 ) ; " How to make the movement smoother The rocket does not appear to move up the screen smoothly but in a series of jumps. This is because when you use TAB (X, Y) there are only 32 possible lines you can print on. The VDU 5 statement (see also chapter 10, where it is used for positioning accents, etc) causes text to be written at the graphics cursor. This means that you can move to any point on the screen on the normal 1280x1024 graphics ‘grid’ and print text or user defined characters. To do this MOVE X , Y is used. VDU 4 undoes the effect of the VDU 5 statement. It causes text to be written where the text cursor is. Then the first character can be printed using PRINT CHR$ (240); or VDU 240. These two alternatives are equivalent but VDU 240 means less typing! We now need to backspace one character to our original position and move down one character cell so we can print the next character. We can’t use TAB ( X , Y ) because it won’t work after a VDU 5 statement, so we use two of the four ‘cursor movement commands’ (see chapter 10). VDU 8 Backspace cursor one character VDU 9 Forwardspace cursor one character VDU 1 0 Move cursor down one line VDU 11 Move cursor up one line Later on we’ll also use VDU 12 7 which has exactly the same effect as pressing the DELETE key. All the VDU commands are listed at the back of the book - there’s no need to remember them all. We need to use VDU 8 and then VDU 10, or the other way around. You can string VDU commands together, so we can use VDU 240,8,10 which will print the first character and then move the cursor into position to print the next one. To print the whole rocket and flames this is repeated three times: here it is done in PROCROCKET. 5 MODE 4 7 VDU 23,1,0;0;0;0;: REM turn off cursor 10 VDU 23,240,8,8,28,28,62,62,62,62 20 VDU 23,241,62,62,62,62,62,62,62,62 30 VDU 23,242,62,62,62,127,127,127,93,93 150 32 VDU 23,243,28,60,30,60,126,108,162,162 35 X% = 6 0 0 37 VDU 5 38 GCOL 4,1 40 FOR Y % = 1 2 0 TO 1023 STEP 10 50 PROCROCKET: REM draw the rocket 90 PROCROCKET: REM now delete it 120 NEXT Y % 130 END 1010 DEF PROCROCKET 1020 MOVE X % , Y % 1025 VDU 240 1030 VDU 10,8,241 1040 VDU 10,8,242 1050 IF Y % < 5 0 0 THEN VDU 10,8,243 1060 ENDPROC Notice that ‘integer’ variables (followed by a % sign) are used: these make the program run considerably faster than ‘real’ variables. The rocket is printed at line 50 by PROCROCKET. Line 90 prints the rocket again in its inverse colour (as specified in line 38 GCOL 4,1), and so deletes it. As an alternative to this, the rocket could be deleted by replacing line 90 with PROCdelete, deleting line 38, and adding the new procedure 2000 DEF PROCdelete 2010 MOVE X% , Y % 2020 VDU 9,127 2030 VDU 10,9, 127 2040 VDU 10,9, 127 2050 VDU 10,9, 127 2060 ENDPROC This just deletes the rocket with VDU 12 7. We can get smoother movement if we just delete the bottom character every time. This removes most of the flicker and what remains becomes a good effect. The price is that the detail at the bottom of the rocket is lost, so this method only works if your character gets wider at the bottom! Delete line 90, and type this new procedure: 38 GCOL , 1 40 FOR Y % = 1 2 0 TO 1023 STEP 4 1010 DEF PROCROCKET 1020 MOVE X% , Y % 1025 VDU 240,10,8 1030 VDU 241,10,8 151 1040 VDU 242 1050 VDU 10,8,243 1055 VDU 127 1060 ENDPROC If you change one line 1050 IF Y % < 5 0 0 THEN VDU 10,8,243 ELSE VDU 10 the flames will cut out after take-off again. Making a complete lunar landing game Using the procedures we have developed for creating and moving the rocket on the screen, we can incorporate these into a complete game which can test your skill at landing a space ship on the moon. This complete program uses a range of techniques described elsewhere in the book and was written by Jim Murray. Included are a few notes to explain what is going on. 5 ON ERROR REPORT : GOTO 245 10 MODE 5 20 VDU 23,240,8,8,28,28,62,62,62,62 30 VDU 23,241,62,62,62,62,62,62,62,62, 40 VDU 23,242,62,62,62,127,127,127,93,93 50 VDU 23,243,28,60,30,60,126,108,162,162 60 VDU 23,1,0;0;0;0; 70 VDU 19,2,2,0,0,0 80 VDU 28,0,20,14,0 90 @ % = & 9 0 6 110 *FX 11,1 Notes: Line 70 enables us to use green. Line 80 sets up a text window. Line 90 sets @ % - so numbers are printed as we want them. Line 110 sets the auto-repeat delay period to its minimum value. Line 5 disables the effect of line 110 if you press ESCAPE, otherwise it’s difficult to type anything again! 120 PROClabels 130 PROCmoon 140 PROC ini t i al i se 150 VDU 5 160 X% = 9 6 0 165 GCOL 0,3 170 REPEAT 180 burn$=INKEY$ (0 ) 185 *FX 15,1 152 190 IF burn$=" "THEN burnrate%=0 ELSE burnrate%=VAL (burn$) *30 200 PROCcalculate 210 PROCdashboard 220 IF Y % >o ldY% + 4 OR Y% 0.004 THEN PROCcrash ELSE PROCfanfare 245 *FX 12,0 247 *FX 15,1 250 END Notes: Although line 250 says END this is not the end of the program. What follows are the various procedures which have been called by the program as it exists so far. Before we give the procedures, some notes on the earlier lines. Line 120 PROClabels - sets up the titles Line 130 PROCmoon - draws moon’s surface. Line 140 P ROC i nitialise - sets all the variables to initial values. Lines 170 to 230 are the main part of the program - a REPEAT . . . UNTIL loop. Line 180 - checks to see if any key has been pressed. Line 185 - clears the key buffer, otherwise the burn continues for a long time after the key is released. Line 190 - we use INKEY $ to check if anything has been pressed, but this returns a string. Line 190 converts it to a number. Line 200 PROCcalculate - does the maths. Line 210 PROCdashboard - prints up the results. Line 220 - prints the rocket if Y % (the variable used in MOVE X % , Y % ) has gone up or down by 4. In this MODE the rocket is printed in the same place unless the change is greater than 4. Line 225 PROCburn - draws the burning fuel. Line 240 - crash or good landing? - at less than 15 mph it’s good. Line 247 - clears keyboard buffer again so you don’t get a string of numbers printed when the program stops. 700 DEF PROClabels 710 PRINT TAB (0 , 7) "secs" 720 PRINT TAB (0 , 9) "miles" 725 PRINT TAB (0 , 10 ) "feet " 730 PRINT TAB (0 , 12 )" speed" 740 PRINT TAB (0 , 14 ) "fuel" 750 PRINT TAB (0 , 16 ) "burn? " 760 ENDPROC 153 800 DEF PROCmoon 805 GCOL 0,2 810 LOCAL X 820 FOR X=100 TO 1280 STEP 200 830 MOVE X, 0 840 PLOT 8 5 , X , 3 0 850 PLOT 85,X+100,0 860 NEXT X 870 ENDPROC 900 DEF PROCinitialise 910 T IME = 0 : now = 0 920 speed=l : REM in miles/second 930 height=4 6 : REM in miles 935 Y % = 9 2 0 937 oldY%=Y% 940 gravity=0 .001 950 fuel=16500 960 totalmass=33000 965 burnrate%=0 970 ENDPROC 1100 DEF PROCcalculate 1105 IF f uel<=0 THEN f ue 1= 0 : bur nr at e % = 0 1110 burnt ime= ( T IME-now )/ 1 0 0 1120 no w = T IME 1130 slower= (burnrate% / totalmass) *2*EXP (burnr at e % ‘burnt ime / tot almas s ) 1140 height=height-speed*burnttime- burntime*burntime/2* (gravity-s lower) 1150 speed= speed+burnt ime * ( gravity- s lowe r ) 1160 burnt =burnr at e % ‘burnt ime 1170 tot almas s =tot almas s -bur nt 1180 f ue 1= f ue 1 -bur nt 1190 IF he ight < 0 THEN height = 0 1200 Y%=height*20+32 1210 ENDPROC 1300 DEF PROCdashboard 1310 VDU4 1320 PRINT TAB (5, 7) INT (TIME/100) 1330 PRINT TAB ( 5 , 9 ) INT (height ) 1340 PRINT TAB (5, 10) INT ( he ight * 5 2 8 0 ) MOD 5280 1350 PRINT TAB (5, 12) INT (speed*3600) 1360 PRINT TAB (5, 14) INT (fuel) 154 1370 PRINT TAB (5 , 1 6) burnrate% 1375 VDU5 1380 ENDPROC 5000 DEF PROCcrash 5020 SOUND 4,-15,100,70 5030 F ORX = 1 TO 100 5040 MOVE 850 + RND (200) , RND (200) 5045 GCOL, RND (4 ) 505 0 DRAW RND (1280) , RND (1024 ) 5055 NEXT 5060 ENDPROC 6000 DEF PROCf anf are 6010 FOR X=1 TO 11 6015 READ P , D 6017 IF P = 9 9 9 THEN L = 0 ELSE L = -15 6020 SOUND 1 , L , P , D 6025 SOUND 1,0, 0,3 6030 NEXT 6035 DATA 97, 15, 97, 5, 101, 5, 999, 5, 101, 5, 97, 5, 101, 10, 97, 2, 89, 5, 81 ,5,77,10 6040 ENDPROC 8000 DEFPROCburn 8005 GCOL 0 , 1 8010 MOVEX% , oldY% 8015 IF bur nr at e % = 0 THEN VDU10,9,127 ELSE VDU10,243 8025 GCOL 0 , 3 8030 ENDPROC 10000 DEFPROCrocket 10100 MOVEX% , oldY% 10110 VDU 10,9,127,11,9,127, 10120 MOVE X % , Y % 10140 VDU 242,8,11,241,8,11, 10150 oldY%=Y% 10160 ENDPROC Running the program You start off at a height of 46 miles moving at 1 mile/sec or 3600 mph. You have fuel of 165001bs and your weight to start with, including fuel, is 33000 lbs. You fire the rockets by pressing one of the keys 1-9 and holding it down until you want to stop burning. The rate of burning is proportional to the number. You must land at less than 15 mph. 11, 9, 127, 11, 9, 127 240 155 30 Sound The BBC Microcomputer contains integrated circuits specifically designed to generate musical sounds and noises on four ‘channels’. Two statements control the generation of musical sounds; they are SOUND and ENVELOPE . For simple effects the statement SOUND can be used by itself but if the user wishes to have greater control over the quality of the sounds generated then ENVELOPE can be used. At its simplest the sound statement is followed by four numbers, eg SOUND C , A , P , D C is the channel number 0 to 3 A is the amplitude or loudness 0 to -15 P is the pitch 0 to 255 D is the duration 1 to 255 The channel number C, determines which of the four ‘voices’ is to be used. Channel 0 produces ‘noise’ (this channel will be explained in detail later) whereas channels 1, 2 and 3 produce purer notes. The amplitude, A, can be varied between 0 (off) and -15(loud). The pitch, P, selects notes in quarter semi-tone intervals. Middle C is produced when P is set at 52 and other notes are generated with the values of P shown in the table. As you can see the computer can produce notes spanning five full octaves. The values of P are also shown in the table for a stave in key of C but one octave up. The duration, D, determines the length of the note and is given in twentieths of a second. Those used to reading music will find that music marked ‘Moderate J =60’ will sound about right with the following settings for D. ^5 J i® J 29 O 49 156 Note 1 Octave number 2 3 4 5 6 B 0 48 96 144 192 240 C 4 *52 100 148 196 244 *middle C C# 8 56 104 152 200 248 D 12 60 108 156 204 252 D# 16 64 112 160 208 E 20 68 116 164 212 F 24 72 120 168 216 F# 28 76 124 172 220 G 32 80 128 176 224 G# 36 84 132 180 228 A 40 88 136 184 232 A# 44 92 140 188 236 That completes the simple description of the SOUND command. There are two main areas where the SOUND command can be extended. First, instead of working with a fixed sound quality, one can select an ‘envelope’ to vary both the amplitude and the pitch of the note while it is playing; secondly it is possible to ensure that notes are synchronised so that chords start together. In addition to these major extensions there are a number of other things that can be controlled, and these will be described later. If you wish to use an envelope to vary either the amplitude or the pitch of a note (or both) then you must first define the envelope and secondly, instead of using a fixed amplitude in the SOUND statement, you must quote the envelope number for A. Four envelopes are normally permitted and they are numbered 1 to 4. Thus SOUND 1,2,53,20 would produce on channel 1 a note of middle C with a duration of one second and the amplitude and pitch would be controlled by the envelope number 2. The statement ENVELOPE is followed by 14 numbers and the following labels will be used for the 14 parameters. 157 ENVELOPE N, T, PI1, P12, PI3, PN1 , PN2 , PN3, AA , AD , AS , AR, ALA, ALD A brief description of each parameter follows. Parameter Range Function N 1 to 4 Envelope number T bits 0-6 0 to 127 Length of each step in hundredths of a second bit 7 0 or 1 0=auto-repeat pitch envelope l=don’t auto-repeat pitch envelope PI1 -128 to 127 Change of pitch per step in section 1 PI2 -128 to 127 Change of pitch per step in section 2 PI3 -128 to 127 Change of pitch per step in section 3 PN1 0 to 255 Number of steps in section 1 PN2 0 to 255 Number of steps in section 2 PN3 0 to 255 Number of step in section 3 AA -127 to 127 Change of amplitude per step during attack phase AD -127 to 127 Change of amplitude per step during decay phase AS -127 to 0 Change of amplitude per step during sustain phase AR -127 to 0 Change of ampliude per step during release phase ALA 0 to 126 Target level at end of attack phase ALD 0 to 126 Target level at end of decay phase Note that the pitch can take on a value between 0 and 255. If the pitch is greater than 255 (eg 257) then 256 will be repeatedly subtracted from it until it is in range. The amplitude has a range of 0 to 127 in the ENVELOPE statement whereas it had a range of 0 to -15 in the SOUND statement. The amplitude cannot be set outside the range 0 to 127. Note also that the total duration of the attack, decay and sustain periods (but not the release period) is determined by the SOUND statement and not the ENVELOPE statement. The envelope is divided up into a number of steps - usually a hundredth of a second each and both the pitch and amplitude can be changed at the end of each step. 158 The pitch envelope The pitch of the note can be changed in three sections. For each section you can specify the change in pitch at each tick of the clock (step) in the section. Suppose we wish to generate a wailing sound like a police siren. The pitch has to rise and fall like this: During section 1 the pitch changes +2 units per step and section 1 contains 10 steps. In section 2 the pitch changes -2 units per step and there are 20 steps. Section 3 contains 10 steps of +2 units. So thus far the ENVELOPE command looks like ENVELOPE 2,1,2,-2,2,10,20,10 The next six numbers control the amplitude of the sound and might well be 1,0,0,-1,100,100 (these will be explained in a moment). So the total program to show the pitch envelope working would be 10 ENVELOPE 2,1,2,-2,2,10,20,10,1,0,0,-1,100,100 20 SOUND 1,2,100,100 Here is another pitch envelope - it plays three notes in succession. 10 ENVELOPE 3,25,16,12,8,1,1,1,10,-10,0,-10,100,50 It reads: - Envelope number 3. - Each step is 25/100 ie 1/4 second long. - The first section of the pitch envelope uses a pitch change of 16 units. - The second section uses a pitch change of 12 units. - The third section has a pitch change of 8 units. - All three sections have only 1 step in each section. - Now to explain the amplitude envelope. 159 The amplitude envelope Suppose that we wish to imitate a car driving towards us getting louder all the time and then driving past before stopping nearby and then driving away. The amplitude of the sound against time might well look like this: The first phase of the amplitude envelope, where the sound is getting louder is called the ‘attack phase’. The amplitude envelope is specified by giving six parameters. The first (AA) gives the change of amplitude at the end of each step during the attack phase and it must be a positive number. Usually the envelope starts with an amplitude of zero. However it is possible to start with a non-zero amplitude if you have just interrupted a note on the same channel. The attack phase continues until the amplitude reaches the level given by the parameter ALA. 160 For reference the six parameters are defined again here. Parameter Range Function A -127 to 127 Change of amplitude per step during attack phase AD -127 to 127 Change of amplitude per step during decay phase AS -127 to 0 Change of amplitude per step during sustain phase AR -127 to 0 Change of amplitude per step during release phase ALA 0 to 126 Target level at end of attack phase ALD 0 to 126 Target level at end of decay phase In our example the attack phase takes four seconds and each step lasts 1/4 seconds so there will be 16 steps. We want these 16 steps to get us from an amplitude of zero to an amplitude of at least 100 - if we make each step increase the amplitude by seven we will get there in 16 steps. So parameter AA = 7. During the decay phase the amplitude must drop from 100 to 60 in two seconds. During two seconds there are eight steps. So the amplitude drops 40 units (100- 60) in eight steps - so each step must reduce the amplitude by five units. Thus AD=-5. So far we have determined the following parameters of the amplitude envelope. A=7 AD=-5 ALA=100 ALD=50 In our case the amplitude does not change during the sustain period so we can set AS = 0. The sound will go on until the sustain phase is ended. The total time allowed for the attack, decay and sustain phases is given by the duration part of the SOUND command. The release phase then starts. Note that the length of the attack and decay phases is set by the values chosen for AA, AD, ALA and ALD but that the sustain phase can be terminated either by the amplitude reaching zero or the time set by the duration of the SOUND statement running out. The duration has to be set with care to ensure that it doesn’t cut the note off at the wrong moment. At the end of the sustain period the note enters the release phase where the note changes in amplitude at the rate set by AR until it reaches zero. 161 As you may have guessed there are many ways for things to go wrong so that a phase does not complete as expected. For example with ALA set to 100 and ALD set to 50 and a decay rate (AD) of zero the amplitude will not decay at all during the decay phase. However the sound will be moved to the release phase when the duration is reached. The ENVELOPE statement is very complicated and there is a wide range of possible effects. You will have to use it a lot before you can accurately predict what effect you will produce. Some sample envelopes to try out: ENVELOPE 1,1,0,0,0,0,0,0,2,0,-10,-5,120,0 ENVELOPE 2,3,0,0,0,0,0,0,121,-10,-5,-2,120,120 ENVELOPE 3,7,2,1,1,1,1,1,121,-10,-5,-2,120,120 ENVELOPE 4,1,0,0,0,0,0,0,61,0,-10,-120,120,0 ENVELOPE 1,8,1,-1,1,1,1,1,121,-10,-5,-2,120,1 Note synchronisation and other effects The first parameter of the SOUND statement has been considered, up to now, to control only the channel number. It can in fact control a number of other features. For this purpose the channel number should be considered as a four digit hexadecimal number C=&HSFN Parameter Range Function N 0 to 3 Channel number itself F 0 or 1 Flush control S 0 to 3 Synchronisation control H 0 or 1 Continuation control N selects the channel number. F If F is 0 the sound will be placed in a channel queue if a note is playing on that channel. If F = 1 then the channel queue will be flushed (emptied) so that the sound can be generated immediately. S It is possible to synchronise two or more channels so that they do not start until all have received a note marked for synchronous production. The value of S determines how many other channels are to form the chord. Thus for a three note chord all three channels should be fed a note with S set to 2. H This parameter allows the previous effect on the channel to continue if it is set to 1. In this case the amplitude, pitch and duration parameters of the new sound command have no effect. Because the ‘dummy’ note thus created is added to the queue in the normal way it can be used to ensure that the release 162 phase of a sound is completed. Normally the release phase is truncated by the next note on the queue. If H=0 then the note is treated as a ‘real’ note in the usual way. Typical values of C are: C=&201 A note on channel 1 to be synchronised with two others. C=&12 A note on channel 2 is to be played immediately regardless of what was in the channel 2 queue. A more succinct description of SOUND and ENVELOPE is given in the BASIC keywords chapter. 31 File handling 163 You are probably already aware that as well as storing computer programs on cassette or disc, you can store ‘data’. By ‘data’ we mean sets of numbers or words. We might, for example, store a set of names and telephone numbers. This set of data is called a file. A set of BASIC statements are provided to enable you to read information from files, to write information to a new file, to update an existing file, to delete a file, to find out how big a file is, to move to a certain record in a file, to check if you have reached the end of a file and to obtain a catalogue of all the files that exist. There are also several other statements for performing other actions on the files. One of the major features of the BBC Microcomputer system is that exactly the same statements are used no matter which ‘filing system’ is in use. A number of different filing systems are available including cassette, disc and Econet systems. Programs written to work on a cassette filing system will usually work unmodified on a disc system. See chapter 36 for details of other filing systems. First here is a summary of all the file handling statements available in BBC BASIC. * CAT OPENIN OPENOUT OPENUP INPUT# PRINT# BGET# BPUT# PTR# EXT# EOF# CLOSE# Gives a catalogue of all data files and programs on the cassette or floppy disc. It takes a very long time on a cassette. Opens a file so that it can be read. Opens a new (empty) file for writing. Opens a file for reading and/or writing. (Not cassette.) Reads data from a file into the computer. Writes data from the computer into a file. Reads a single character (byte) from a file. Writes a single character (byte) to a file. Can be used either to find out which record we are about to read (or write) or to move to a specific record. (Not cassette.) Indicates how long a file is. (Not cassette.) Indicates whether or not the end of a file has been reached. Indicates to the computer that you have finished with a file. 164 The statement *CAT can be used at any time. However before you can use any of the other file statements you have to open the file. After you have opened a file you can read and write to it as much as you wish. When you have finished with the file you must close it. An analogy may help to make one or two points clearer. The files are all kept in one room and your only method of communicating with them is via five telephone links to five clerks. In addition there is a supervisor who knows which telephone line to use to communicate with the right clerk. It is the clerk’s job to keep all the files organised and you really have no idea how he or she looks after the files - nor does it matter, so long as the method is efficient. To return to our computer, suppose that we wish to create a file called "DRINKS " in which we list every drink we have ever tried. First of all we have to ask the supervisor to allocate a phone line and clerk to us. The statement X = OPENOUT "DRINKS" will place the number of the channel (telephone line) allocated to the file into the variable X. Next we can ask the user for the names of the drinks using INPUT "WHAT IS THE DRINK CALLED?", D$ and then send the name (held in D $ ) to the clerk to be entered in the file. Notice how we have to use the variable X to ensure that it is entered in the correct file. PRI NT #X , D $ We could then repeat this process until the user replied with the word STOP. The program would look like this: 10 X=OPENOUT "DRINKS" 20 REPEAT 30 INPUT "What is the drink called ", D$ 40 P RI NT #X , D $ 50 UNTIL D$="STOP" 60 CLOSE# X When run the program will save the data on cassette (if one is connected). >RUN What is the drink cal led? WH I SKY What is the drink called?VODKA What is the drink called?GIN What is the drink called?WINE What is the drink called?CIDER What is the drink cal led? TOMATO JUICE What is the drink called?STOP 165 That has created a file called "DRINKS" which has been stored on cassette. The program to read the file back in is also straightforward. 10 Y=OPENIN "DRINKS" 20 REPEAT 30 INPUT Y , R$ 40 PRINT R $ 50 UNTIL R$="STOP" 60 CLOSE# Y When this is run, the list will appear on the screen as it is read from the cassette. >RUN WHISKY VODKA GIN WINE CIDER TOMATO JUICE STOP For most applications that is all you will need to know about file handling and you will only use statements like these * CAT X=OPENIN "FILENAME" X=OPENOUT "FILENAME" PRINT#X , A, B $ INPUT#X , A, B $ CLOSE #X "FILENAME" is the name of the file which normally consists of up to ten letters but see chapter 35 for more details. A and B $ represent any (and as many) numeric and string variables as you wish to record. X is a numeric variable used to remember the channel number allocated to the file number. For more specialised applications a number of other functions and statements are provided. BGET# and BPUT# enable single characters to be input and output. They would be used for recording special data, for example, laboratory experiments. EXT# and PTR# are used with disc systems where random access files are required. They cannot be used with cassette systems. 166 EOF# enables a program to detect the end of the file when reading in data. It is normally used in the following way 10 Y=OPENIN "DRINKS" 20 REPEAT 30 I NPUT # Y , A $ 40 PRINT A $ 50 UNTIL EOF# Y 60 C LOSE # Y Telephone book program One of the programs on the WELCOME cassette can be used to keep a personal telephone directory. Clearly it should be possible to save a copy of all your entries on to cassette and to load them back into the computer later. Several modifications must be made to the program to enable this to happen. These modifications are shown below. Once you have modified the program you can then save the corrected version with a new name, for example SAVE "TELE2" First, load the program. Don’t RUN it, but type CTRL N then LIST to list the program in ‘page mode’. To move down the program, press SHIFT. When you come to a page requiring one of the changes set out below, press ESCAPE and edit the line in the normal way. Lines 210 to 280 omit final ' Add new lines: 282 PRINT" 9 - Load data from cassette" 284 PRINT" 0 - Save data to cassette" Change line 290 to 290 SEL = -1 In line 300 change TAB (3,22) to TAB ( 3 , 23) Line 330 change to 330 IF A< 0 OR A> 9 THEN 310 Change line 350 to 350 IF S E L > -1 THEN PRINT TAB(2,SEL + 3 - 10* (SEL = 0) ) ; CHR$ (&8 9) ; Change line 360 to 360 PRINT TAB (2 , A+3-10* (A=0 ) ) ; CHR$ (&88 ) ; A; CHR$ (&8 9) Change line 380 to 167 380 IF SEL=— 1 THEN 300 Change line 500 to 500 ON SEL + 505, 510, 520,5 New lines: 505 PROCSAVE : GOTO 200 590 PROCLOAD : GOTO 200 10000 DEF PROCLOAD 10005 PRINT TAB (0,16); "Play tape and press any key" 10007 Q=GET 10008 PRINT "Please wait" 10010 E%=OPENIN "DATA" 10020 INPUT#E% , X 10030 FOR I%=1 TO X 10032 INPUT#E% , NAME $ ( 1% ) , PHONE $ (1% ) 10034 PROCPACK (1%) 10036 NEXT 10040 CLOSE#E% 10050 ENDPROC 11000 DEF PROCSAVE 11005 PRINT TAB (0,16) ; "Please press 11010 E%=OPENOUT "DATA" 11015 PRINT '"Please wait" 11020 PRINT#E% , X 11030 FOR I%=1 TO X 11032 PRINT#E% , NAME $ ( 1% ) , PHONE $ (1% ) 11036 NEXT 11040 CLOSE#E% 11050 ENDPROC 1 GOTO 30,540,550,560,570,580,590 168 32 Speeding up programs and saving memory space For some applications it is important that a program runs as quickly as possible and a few tips are given here which will, together, substantially increase the execution speed of programs. In other applications space may be at a premium and other suggestions are given for saving space. Sometimes there is a trade-off between the size of a program and speed and the user will have to decide which is more important. The most dramatic saving that can be made is in the speed of execution of programs. The use of integer variables (eg WE IGHT % ), and especially of the resident integer variables A% to Z%, will result in execution times as little as 50% of those achieved with ‘real’ variables. Again, integer division (D IV) is much faster than normal division when working with integers. Using integer arrays rather than real arrays will save 20% of the memory required. Execution speed can also be increased in the following ways. 1. Allocate variable names with an even spread throughout the alphabet - so don’t start all your variables with ‘F’, for example. 2. Omit the control variable after the word NEXT - eg say NEXT rather than NEXT X. This saves a reasonable amount of time. 3. REPEAT. . .UNTIL loops are much faster than IF. . .THEN. . .GOTO loops. 4. Procedures are faster than GOSUBs, and it is faster to pass parameters to a procedure than to use global variables - ie do use PROCBOX (X, Y, Z) rather than PROCBOX. 5. If you have a line which contains a lot of ‘integer’ arithmetic and a little ‘real’ arithmetic then, if possible, place the integer work at the start of the line where it will be executed first. 6. Have as few line numbers as possible - ie use long lines and spread the line numbers out rather than re-numbering with an interval of 1. An interval of 10 is good. As far as space saving is concerned the following can be tried - but both reduce the readability of programs and should not be used unless it is really necessary. 169 7. Omit spaces wherever possible - but you must keep a space or a % or $ sign or some other separator before most keywords to avoid ambiguity. If a variable FRED is in use then you must write Y=FRED OR MARY and not Y=FREDORMARY In the latter case the computer will look for the variable FREDORMARY rather than the two variables FRED and MARY. The space after OR is not required. 8. Omit REM statements. 9. Remember that the whole of user memory can be kept for use by your programs by using the ‘shadow screen’ facility - see chapter 42 for more details. 170 33 BASIC keywords This chapter contains a detailed description of every word that BASIC understands. These words are called ‘keywords’. Some parts of the description are intended for the novice user and others for the person who is familiar with BASIC. Note that your BBC Microcomputer is supplied with the latest version of BBC BASIC, known as BASIC II. If you are already familiar with the earlier version, you may wish to refer to chapter 49 before reading this chapter. Chapter 49 lists those BASIC keywords which exist only in BASIC II. Each keyword is described under a number of headings as follows: Keyword Sometimes followed by a few words explaining the derivation of the word. Purpose A plain-English description of what the keyword does. This is intended for the person who is learning BASIC. The only technical terms used are ‘string’ and ‘numeric’ - if you don’t understand those two words then read chapter 9 first. The mathematical functions SIN, COS, TAN etc are not exlained for the absolute beginner - there just isn’t room to explain everything! Examples This section gives a few one-line examples of the keyword (not complete programs). Some of the examples have a number at the start of the line. This number is an ‘example line number’. The examples are only intended to be illustrative. In some cases a line of BASIC program may overflow onto the next line as elsewhere in this book. Description In this section the keyword is described using normal computer jargon. Syntax The syntax of each keyword’s usage is given more by way of helpful explanation than for its strict accuracy. Purists will, rightly, complain at travesty of Backus- Naur form. Others may find the entries useful. 171 The following symbols are used as part of the syntax explanation: { } Denote possible repetition of the enclosed symbols zero or more times. [ ] Enclose optional items. I Indicates alternatives from which one should be chosen. Means a numeric constant such as ‘4.5’ or T27’ Means a numeric variable such as ‘X’ or ‘length’ Means either a or a , or a combination of these in an expression such as " 4 *X + 6 " Means a string enclosed in quotation marks, eg "ROBERT BULL". Means a string variable such as A $ or NAME $. Means either a or a , or an expression such as A$ + "LINDA" . Means something which is either TRUE or FALSE. Since TRUE and FALSE have values it is possible to use a at any point where a is required. The distinction between these two is, in fact, rather unnecessary. Means any BASIC statement, for example, PRINT orGOSUB orPROC. Means any sequence of letters or numbers that obeys the rules for variables (see chapters 3, 9 and 21). 172 Associated keywords This section is intended to draw your attention to other keywords which either have similar functions or which are normally used in conjunction with this keyword. You will probably find it helpful to read the pages which describe the associated keywords. Demonstration program If appropriate a short program is included to illustrate the use of the keyword. Parentheses are generally optional where sense is unaffected. ABS absolute value 173 Purpose This function turns negative numbers into equivalent positive numbers but leaves positive numbers alone. For example the absolute value of -9.75 is 9.75 while the absolute value of 4.56 is 4.56. The ABS function is often used when calculating the difference between two values if you do not know which is the larger of the two. Thus (K-L) will be positive if K is greater than L, and will be negative if L is greater than K. For example, if K = 9 and L = 12 then (K - L) would be equal to -3. However the value of ABS (K-L) will always be positive. In the example given ABS (K-L) would equal 3. Examples 205 error = ABS (DIFFERENCE ) 100 DIFF=ABS (X2-X1) PRINT ABS ( t emp% -50) Description A function giving the absolute value of its argument. Syntax =AB S () Associated keywords SGN 174 ACS arc-cosine Purpose To calculate an angle whose cosine is known. The calculated angle is in radians but may be converted to degrees by using the function DEG. See DEG for more information. Examples 10 X=ACS(Y) 1205 angle=DEG (ACS (0 . 5678) ) 330 OUT = AC S ( . 234) PRINT ACS (0.5) Description A function giving the arc-cosine of its argument. The result is in radian measure. Syntax =AC S () Associated keywords ASN, ATN, SIN, COS, TAN, RAD , DEG analogue to digital converter value 175 Purpose An analogue signal is one which can have almost any value - including fractional parts. It is contrasted with a digital signal which is expressed in exact numbers. The height of the water in a harbour is an analogue quantity whereas the number of boats it contains is a digital quantity. Watches always used to have analogue dials - ‘The time is about four fifteen’. Electronic things usually work with whole numbers; for example 16h : 15m : 23s There are four ‘analogue to digital’ converters in the BBC Microcomputer. Each analogue to digital converter in the computer accepts a voltage and gives out a whole number indicating how large the voltage is. This voltage might be controlled by, for example, the position of a ‘games paddle’ or ‘joystick’ control which is connected to the computer. Alternatively the computer might be connected to a speed sensor on a piece of machinery or it might measure the temperature of a room. The input voltage range is 0 volt to 1.8 volt. When the input is 0 V the converter produces the number zero. With 1.8 V input the converter produces the number 65520. Why 65520? The circuit in the computer which does the conversion was designed to give out numbers in the range 0 to 4095. However it may well be that future converters can give out numbers over a larger range - enabling the computer to measure things more accurately. In order to ensure that the BBC Microcomputer can be used in this situation we have specified a large range. Instead of producing numbers in the 0 to 4095 range it produces a number in the range 0 to 65520. Therefore instead of numeric results going up in the sequence 0, 1, 2, 3 etc they will go 0, 16, 32, 48, 64 etc. If you prefer the range 0 to 4095 then just divide the value by 16. There are four analogue input channels provided in the BBC Microcomputer and the number in parentheses after the keyword ADVAL refers to the channel whose value you wish to find. The channels are numbered 1, 2, 3, 4. ADVAL ( 0 ) performs a special function in that it can be used to test to see which of the ‘fire’ buttons is pressed on the games paddles. The value returned also indicates which ADC channel was the last one to be updated. The following can be used to extract these two pieces of information from the value returned by ADVAL ( 0 ) . 176 X= AD VAL ( 0 ) AND 3 will give a number with the following meaning X=0 no button pressed X=1 left side fire button pressed X=2 right side fire button pressed X=3 both fire buttons pressed X= AD VAL ( 0 ) DIV256 will give the number of the last analogue to digital channel to complete conversion. If the value returned is zero then no channel has yet completed conversion. AD VAL with a negative number in the parentheses, eg X=ADVAL ( - 3 ) , can be used to see how full any of the internal buffers are. When characters are typed in on the keyboard they are put into a buffer from which they are extracted with statements like INPUT and GET. Other buffers are used internally for other purposes. The exact meaning of the number returned depends on the buffer being tested. X=AD VAL (-1 ) X=AD VAL (-2 ) X=AD VAL (-3 ) X=AD VAL (-4 ) X=AD VAL (-5 ) X=AD VAL (-6 ) X=AD VAL (-7 ) X=AD VAL (-8 ) X=AD VAL (-9 ) Returns the number of characters in the keyboard buffer. Returns the number of characters in the RS423 input buffer. Returns the number of free spaces in the RS423 output buffer. Returns the number of free spaces in the printer output buffer. Returns the number of free spaces in the sound channel 0 buffer. Returns the number of free spaces in the sound channel 1 buffer. Returns the number of free spaces in the sound channel 2 buffer. Returns the number of free spaces in the sound channel 3 buffer. Returns the number of free spaces in the speech buffer. This feature can be used, for example, to ensure that a program never gets stuck waiting for a sound channel to empty, eg: IF ADVAL ( — 7) <> 0 THEN SOUND 2 , ...etc 177 Examples 980 X=ADVAL(3) 125 TEMP=ADVAL (X) intensity=ADVAL (1) Syntax =A D V A L ( ) Description A function which returns the last known value of the analogue to digital channel given in its argument. There are four channels, each of 10 bit resolution, but the returned value is scaled to 16 bits. The analogue to digital converter cycles repeatedly through the selected channels and keeps a table of the result so that the function ADVAL returns very quickly. New samples are taken about every ten milliseconds. Therefore with four channels selected results will be updated every 40ms. See chapter 43 for information on changing the number of channels selected. 178 AND Purpose AND can be used either as a logical operator or as a ‘bit by bit’, or ‘Boolean’ operator. As a logical operator AND is used to ensure that two conditions are met before something is done. For example IF X= 9 AND Y= 0 THEN PRINT "HELLO" Logical AND is most often used as part of an IF . . .AND. . .THEN. . . construction. Boolean AND compares the first bit of one number with the first bit of another number. If both bits are set to a one (rather than a zero) then the first bit in the answer is also set to a one. This process is then repeated for the second bit in each of the two numbers being compared and so on for all 32 bits in the numbers. For example the result of 1 4 AND 7 is 6, since in binary 14 is 0000 0000 0000 0000 0000 0000 0000 1110 7 is 0000 0000 0000 0000 0000 0000 0000 0111 6 is 0000 0000 0000 0000 0000 0000 0000 0110 Examples 300 IF length> 9 AND wt>9 THEN PRINT "YES" 100 IF X=2 AND cost>5 AND J=12 THEN PRINT "NO!!" The above example will only print NO ! ! if all three conditions are met. Description The operation of Boolean AND between two items. Note that the logical and Boolean operations are in fact equivalent. This follows since the value of TRUE is -1 which is represented on this machine by the binary number 1111 1111 1111 1111 1111 1111 1111 1111 Similarly the binary value of FALSE is 0000 0000 0000 0000 0000 0000 0000 0000 Thus PRINT 6 = 6 would print -1 since 6 = 6 is TRUE. 179 Syntax = AND or = AND Associated keywords EOR, OR, FALSE, TRUE, NOT 180 ASC American Standard Code (ASCII) Purpose There are two commonly used methods of talking about characters (things like A, B, 5, ?, and so on). Obviously they are single characters! So we can say D $ = " H " - meaning put the letter H into the box in the computer labelled D$. The computer understands this but it doesn’t actually put an H into the box, Instead it stores a number which represents the letter H (in fact the number is 72). Every character has a unique corresponding number called its ASCII code. (ASCII stands for American Standard Code for Information Interchange. The abbreviation ASCII rhymes with ‘Laski’.) Sometimes it is convenient to find out what number corresponds to a particular character - that is its ASCII code. You can look it up at the back of this book or you can say to the computer PRINT ASC ( "H" ) The function ASC gives the ASCII value of the first letter in the string. Thus PRINT ASC ("Good") gives 71, the ASCII value of ‘G’. The reverse process of generating a one-character string from a given ASCII value is performed by the function CHR$ . Examples 25 X=ASC ( "Today" ) would put the ASCII value of ‘T’ which is 84 into the variable X. 650 value5=ASC (A$ ) Description A function returning the ASCII character value of the first character of the argument string. If the string is null (empty) then -1 will be returned. Syntax = ASC() Associated keywords CHR$ , S TR$ , VAL arc-sine 181 Purpose To calculate an angle whose sine is known. The calculated angle is in radians but may be converted to degrees by using the function DEG. A radian is equal to about 57 degrees. Mathematicians often prefer to work in radians. Examples 340 J=ASN (0 . 3456) 30 angle=DEG (ASN ( . 7654) ) PRINT ASN ( . 5) Description A function giving the arc-sine of its argument. The result is in radian measure. Syntax =A S N () Associated keywords ACS, ATN, SIN, COS, TAN, RAD, DEG 182 ATN arc-tangent Purpose To calculate an angle whose tangent is known. The calculated angle is in radians but may be converted to degrees by using the function DEG. Examples 1250 X=ATN(Y) 240 value=DEG (ATN (22 . 31) ) Description A function giving the arc-tangent of its argument. The result is in radian measure. Syntax =ATN () Associated keywords ACS, ASN, SIN, COS, TAN, RAD, DEG automatic 183 Purpose When typing a BASIC program into the computer it is common to make the first line of the program line number 10, the second line 20 etc. To save having to type in the line number each time, the command AUTO can be used to make the computer ‘offer’ each line number automatically. Used on its own the command AUTO will offer first line 10 and then lines 20, 30, 40 etc. The command AUTO 4 5 5 would instead start the process at line number 455, followed by lines 465, 475, 485 etc. Another option allows the user to select the step size. Thus the command AUTO 4 6 5,2 would cause the computer to offer lines 465, 467, 469, 471 etc. The larges step size is 255. To escape from AUTO mode the user must press the key marked ESCAPE. AUTO mode will be abandoned if the computer tries to generate a line number greater than 32767. Examples AUTO AUTO 220 AUTO 105,5 Syntax AUTO [[, ]] Description AUTO is a command allowing the user to enter lines without first typing in the number of the line. Because AUTO is a command it cannot form part of a multiple statement line. AUTO mode can be left by pressing ESCAPE or generating a line number exceeding 32767. AUTO may have up to two arguments. The first optional argument gives the starting line number and the second optional argument gives the increment between line numbers. 184 BGET# get a byte from file Purpose Numbers and words can be recorded on cassette tape and on disc. The function BGET# enables a single character or number to be read back into the computer from the cassette, disc or network. Before using this statement a file must have been opened using the OP ENIN function or else an error will occur (see chapter 31 for more information about ‘files’). When a file is opened, using OPEN IN, the computer will allocate the file a channel number. This number must be used in all subsequent operations on the file, for example when reading the file or when writing a new file. Again see the chapter on file handling for more information. Examples 6000 character=BGET# (channel) 340 next_letter%=BGET#C Description A function which gets a byte from the file whose channel number is the argument. The file must have been opened before this statement is executed. Syntax =B GE T # Associated keywords OPENIN, OPENUP, OPENOUT, CLOSE#, EXT#, PTR#, PRINT#, INPUT#, BP UT # , EOF# BPUT# put a byte to file 185 Purpose To store a byte on cassette or disc. See chapter 31 for a more detailed description of file handling. The number which is sent to the file can have any value between 0 and 255. If you attempt to store a number that is greater than 255, then 256 will be repeatedly subtracted from the number until it is less than 256. The final number will then be sent to file. (This statement is used to store single bytes - not large numbers. Larger numbers can be stored using PRINT#.) As with B GE T # the file must be ‘open’ before this statement can be used. Examples 30 BPUT# channel, number 700 BPUT #N , 32 450 BPUT# S TAFF_F I LE , A/256 Description A statement which puts a byte to the file whose channel number is the first argument. The second argument’s least significant byte is sent. The file must be open before this statement is executed. Syntax BPUT# , Associated keywords OPENIN, OPENUP, OPENOUT, CLOSE#, EXT#, PTR#, PRINT#, INPUT#, BGET # , EOF# 186 CALL transfer control to a machine code subroutine Purpose This statement makes the computer execute a piece of machine code which the user has previously placed in the computer’s memory. Before using this powerful statement you should have a good understanding of machine code and assembly language as incorrect use can destroy a program completely! Unfortunately there is not enough room in this book to teach assembly language programming but brief guidance on the principles of 6502 assembly language is given in chapter 44. Examples 50 rotate=&0270 60 CALL rotate, J,K,L 200 CALL 1234, A$,M, J$ Description A statement to call a piece of machine code. The number of parameters passed is variable and may be zero. The parameters are variable parameters and may be changed on execution of the subroutine. The addresses of parameters are passed in a parameter block starting at location 0600 hex. On entry to the subroutine the processor’s A, X, Y registers are initialised to the least significant bytes of the integer variables A%, X% and Y%. The carry flag is set to the least significant bit of the C% variable. On entry a parameter block is set up by the computer and it contains the following list: Number of parameters Parameter address Parameter type Parameter address Parameter type Parameter types 0 - 8 bit byte (eg ?X) 4 - 32 bit integer variable (eg !X or X%) 5 - 40 bit floating point number (eg V) 128 - A string at a defined address (eg $X - terminated by a &0D) 129 - A string variable such as A$ } 1 byte 2 bytes 1 byte Repeated as often as necessary 187 In the case of a string variable the parameter address is the address of a string information block which gives the start address, number of bytes allocated and current length of the string in that order. Syntax CALL{ , | < string- var > } Associated keywords USR 188 CHAIN Purpose CHAIN enables a program to be split up into a number of small sections. The CHAIN statement is used to enable one program to LOAD and RUN another program automatically. For example, one program might enable the user to enter the number of hours worked by employees and that program might CHAIN a second program which would print out the payslip. In turn that might CHAIN a third program which would do a cost per hour analysis on the data held on the file. CHAIN is also useful in a game with a lot of instructions. The instructions could all be stored as one file which would then CHAIN the main game — thus releasing a lot of the computer’s memory. CHAIN"" (without the program name) will chain the next program on a cassette, whatever its file name. This will not work with disc filing systems where you must give the file name. For that reason it must not be used in programs which may be used on disc systems. Examples 900 CHAIN " GAME_1 " 1234 CHAIN " NEWPROG " CHAIN A $ Description A statement which will load and run the program whose name is specified in the argument. All variables except @ % and A% to Z % are cleared. Syntax CHAIN Associated keywords LOAD, SAVE 189 CHR$ character string Purpose To generate a character (single letter or number etc) from the number given in string form. The character generated will be the ASCII character at the position given in the ASCII table. See the description of ASC and the full ASCII table in Appendix C. The statement VDU has a similar effect to PRINT CHR$ and may be more useful in some applications. Examples 220 RED$=CHR$ (129) 1070 PRINT CHR$ (8) ; makes the cursor move left one position. PRINT CHR$ (7) causes a short note to be emitted by the loudspeaker. Description A string function whose value is a single character string containing the ASCII character specified by the significant byte of the numeric argument. Thus CHR$ ( - 1 ) would give ASCII character number 255. Note that the statement VDU is probably more useful when sending characters to the screen, since it involves less typing. CHR$ is needed when you wish to put a special character into a string. Syntax =C H R $ () Associated keywords ASC, STR$, VAL, VDU 190 CLEAR Purpose This tells the computer to forget all variables previously in use, including string variables and arrays but excluding the ‘resident integer variables’ 0 % and A% to Z% which are not affected in any way. See chapter 9 for an explanation of integer and string variables. Examples 350 CLEAR CLEAR Description A statement which deletes all variables except the resident integer numeric variables 0 % and A% to Z% Syntax CLEAR Associated keywords None clear the graphics screen 191 Purpose To clear the graphics area of the screen. The graphics area of the screen is left in the colour selected as the ‘current graphics background colour’. See the keyword GCOL for more information. The graphics cursor is then moved to its home position (0,0) which is at the bottom left of the graphics area. Examples 870 CLG CLG Description Clears the current graphics area of the screen and sets this area to the current graphics background colour in addition. The statement then moves the graphics cursor to the graphics origin (0,0). Syntax CLG Associated keywords CLS, GCOL CLOSE# Purpose To inform the computer that you have completely finished with a particular file. The computer then transfers any data still in memory to cassette, disc or Econet as needed. See chapter 31 on file handling for more information. Example 90 CLOSE#N Description A statement used to close a specific disc or cassette file. CLOSE# 0 will close all files. Syntax CLOSE# Associated keywords OPENIN, OPENUP, OPENOUT, EXT#, PTR# , PRINT#, INPUT#, BGE T# , BP UT # , EOF# clear the text screen 193 Purpose To clear the text area of the screen. Any graphics in this area will also be cleared. The text area will be left in the ‘current text background colour’. The text cursor will then be moved to its ‘home’ position at the top left of the text area. See the keyword COLOUR for more information about text background colours. Examples 560 CLS CLS Description Clears the current text area and sets this area of the screen to the current text background colour. The statement then causes the text cursor to move to the text origin (0,0) at the top of the current text area. CLS resets COUNT. Syntax CLS Associated keywords CLG, COLOUR 194 COLOUR Purpose This statement selects the colour in which the computer is to print the text and also its background. The command has a number of variations which are most easily explained by example. Type in the following: MODE 5 COLOUR 1 COLOUR 2 COLOUR 3 and press RETURN at the end of each line as usual. As you will have seen, these commands change the colour of the text. This is often called the ‘text foreground colour’. Now try COLOUR 129 COLOUR 130 COLOUR 128 These numbers change the ‘text background colour’. In any two colour MODE (MODE 0,3,4 or 6 ) the following normally apply: Foreground Background Colour 0 128 Black 1 129 White In any four colour MODE (MODE 1 or 5 ) the following normally apply: Foreground Background Colour 0 128 Black 1 129 Red 2 130 Yellow 3 131 White In MODE 2 the following normally apply: 195 Foreground Background Colour 0 128 Black (normal background) 1 129 Red 2 130 Green 3 131 Yellow 4 132 Blue 5 133 Magenta (blue/red) 6 134 Cyan (blue/green) 7 135 White (normal foreground) 8 136 Flashing black/white 9 137 Flashing red/cyan 10 138 Flashing green/magenta 11 139 Flashing yellow/blue 12 140 Flashing blue/yellow 13 141 Flashing magenta/green 14 142 Flashing cyan/red 15 143 Flashing white/black If you are not familiar with BASIC then you may already have had too much of this! Nevertheless, it is possible, for example in a four colour MODE to select any four colours from the available 16 effects by using another command. Remember that the colours given above (black, red, yellow, white) will be available as soon as four colour MODE is selected - but you can then select other colours later. Try the following: MODE 5 COLOUR 1 VDU 19,1,4,0,0,0 VDU 19,1,5,0,0,0 COLOUR 2 VDU 19,2,4,0,0,0 VDU 19,1,3,0,0,0 As you will see the statement VDU 19, can be used to change the ‘actual colour’ of colour 1 or 2. The number which follows the VDU 1 9, is the number that is referred to by the COLOUR statement. It is referred to as a ‘logical colour’. 196 The number which follows the ‘logical colour’ is referred to as the ‘actual colour’ and is as follows: 0 Black 1 Red 2 Green 3 Yellow 4 Blue 5 Magenta (blue/red) 6 Cyan (blue/green) 7 White 8 Flashing black/white 9 Flashing red/cyan 10 Flashing green/magenta 11 Flashing yellow/blue 12 Flashing blue/yellow 13 Flashing magenta/green 14 Flashing cyan/red 15 Flashing white/black Thus the statement VDU 19,3,6,0,0,0 will set logical colour 3 to be cyan. So if in MODE 4, a two colour MODE, you wanted black letters on a yellow background you would issue the command: VDU 19,1,0,0,0,0 VDU 19,0,3,0,0,0 Alternatively, you could string the whole lot together as VDU 19,1,0,0,0,0,19,0,3,0,0,0 This combination of the COLOUR statement and the VDU 19 statement enables a very wide range of effects to be obtained. There are also calls which enable the flash rates of the colours to be altered as well. See chapter 43 on *FX calls. Syntax COLOUR cnumerio Associated keywords VDU, GCOL cosine 197 Purpose To calculate the cosine of an angle. Note that the number in parenthesis (the angle) is expressed in radians and not in degrees. To convert from degrees to radians use the function RAD . Examples PRINT COS (2.45) 780 X = COS (Y) 655 Number=COS (RAD (45)) Description A function giving the cosine of its argument. The argument must be given in radians. Syntax =C O S () Associated keywords SIN, TAN, ACS, ASN, ATN, DEG, RAD 198 COUNT Purpose COUNT counts all the characters printed using PRINT, whether to screen, printer or RS423 output channel. On the other hand POS returns the current position of the actual text cursor on the screen. Examples 290 A=COUNT 75 f red = COUNT PRINT COUNT Description A function returning the number of characters printed since the last new line. COUNT is set to zero if the output stream is changed. Syntax =C O U N T Associated keywords POS Demonstration program 5 REM to print a row of 16 * signs 7 REM this is not the easiest way! 10 X = 1 6 20 REPEAT PRINT 30 UNTIL COUNT=X DATA 199 Purpose DATA is used in conjunction with the keyword READ, and sometimes with RESTORE, to enable you to make available automatically any data (numbers and words) that will be needed by a program. For example, if you were writing a geography quiz, you might want to use the names of five countries and their five capital cities each time you used the program. The names of the cities and countries can be entered as DATA in the program and will always be there when the program is run. Computers using the language BASIC are really clumsy at handling information like this, as the demonstration program on the next page shows. In the example program the DATA consists of many words. DATA statements can just as well contain numbers - or a mixture of words and numbers. In our example the words were all read into a string array. It is essential that the DATA contains numbers where numeric variables are to be filled. Text information, eg h e 1 1 o , will just give 0. There is no need to put each word in quotes unless leading spaces are important intheDATA words, for example " four spaces". If you wish to have leading spaces then these words should be enclosed in quotes. Since a comma is used to separate items of DATA, if you want a comma in your DATA, you must enclose the DATA in quotes. Examples 100 DATA "Allen, Stephen", Stamp dealer, 01-246 8007, 24 130 DATA "TOP OF ROOF", 450, January Description A program object which must precede all lists of data intended for use by the READ statement. Syntax DATA | { , str-const> | } 200 Associated keywords READ, RESTORE Demonstration program 10 REM geography quiz 20 DIM city $ ( 5 ) 30 DIM country$ (5) 40 FOR x=l TO 5 50 READ city $ ( x ) 60 READ country$(x) 70 NEXT x 80 right=0 110 FOR x=l TO 10 120 r = RND (5) 130 PRINT "What city is the capital" 140 PRINT "of country$(r) 150 INPUT answer $ 160 IF answer $=city$ (r) THEN PROCyes ELSE PROCno 170 NEXT x 180 PRINT "You got right; 190 PRINT " correct out of 10" 200 END 500 DATA Paris, France, Reykjavik 505 DATA Iceland 510 DATA Moscow, Soviet Union 520 DATA Athens, Greece 530 DATA Spitzbergen, Spitzbergen 600 DEF PROCyes 610 PRINT "Well done!" 620 right=right+l 630 ENDPROC 700 DEF PROCno 710 PRINT "No, the capital of 720 PRINT count ry$ ( r ) ; " is ";city$(r) 730 ENDPROC Line 10 is just a REMark which the computer ignores. Lines 20 and 30 tell the computer that we are going to use two string arrays - one to store the names of the five cities and the other to store the names of the five countries. See chapter 21 for an explanation of arrays. Line 40 sets upaFOR. . .NEXT loop that will go around five times. 201 Line 50 reads the next word (which will be a city) into the array city$ and then moves the ‘data pointer’ on to point to the next word (which will be a country). Line 60 reads the next piece of DATA into the count ry$ array. Line 70 is the end marker of the FOR . . . NEXT loop. Lines 110 to 170 loop ten times through a ‘question and answer’ quiz. Lines 500 to 530 contain the DATA used above. Lines 600 to 630 are a procedure to deal with correct replies. Lines 700 to 730 deal with incorrect replies. 202 DEF define Purpose The word DEF is used to inform the computer that a procedure or function is about to be defined. Once the computer has been informed that this procedure or function exists, then the procedure or function can be called by name anywhere in the program. Definitions of procedures and functions must not occur in the body of a program. They should be placed in a separate section which is not executed - for example after the final END in the program. This also aids readability. The language BASIC has many predefined functions which the computer already knows about. For example, the function SQR enables it to calculate the square root of a number. Often though, it is useful to be able to define your own functions. For example, you might want to have a function which calculates the VAT inclusive price of a product from the basic sale price by multiplying by 1.15. A function always produces a result so you can write X=FNST. A procedure, on the other hand, is used to perform a number of actions, but it does not by itself produce a numerical result. For example, a procedure might be set up to clear the screen and draw a number of lines on the screen. You may well feel confused, but do not be put off! The use of procedures and functions may be difficult to understand at first but it is well worth the effort. Their use greatly enhances the readability and reliability of programs. The section below gives a more detailed explanation of the use of procedures and functions. It should be read in conjunction with the examples which follow. Both procedures and functions may contain local variables which are declared using the word LOCAL. In the third example given below, K is declared as a local variable. This means that although K is used in this procedure its value is notdefined when the procedure finishes. In fact the variable K might well be used elsewhere in the program. The variable K, elsewhere within the program, would not be altered by the use of the local variable K within the procedure. Any variable which is not declared as LOCAL will be available outside the procedure, in other words to the rest of the program. Also, both procedures and functions may have parameters passed to them. Look at the first example program below: line 1010 says 1010 DEF FNST (g) =1 . 15*g 203 ‘g’ is called a ‘formal parameter’ for the faction FNST. It tells the computer that one number is going to be passed to the function when the function is used and inside the function we have decided to use the letter g to represent the variable. The procedure is ‘called’ or used like this - for example 230 PRINT "VAT inclusive price 236 PRINT FNST(P) and in this case ‘P’ is the ‘actual parameter’ for the function FNST. Whatever value ‘P’ has will be used inside the fuction wherever reference is made to teh formal parameter ‘g’. This is very convenient since you can use any variable names you like for the parameters inside the procedure. Then you can call the procedure with a quite different set of parameter names from the outside. Very often a procedure will be called from many different places in the program - and the actual parameters may have different names each time the procedure or function is called. If a procedure or function is defined with (say) three formal parameters then, when it is called, three actual parameters must be supplied. See the fifth example below where three parameters are passed to the function. The end of the procedure is indicated with the statement ENDPROC. The end of a multi-line function is indicated by the statement that starts with an = sign. The function is given the value of the expression to the right of the = sign. Examples First example - full program 210 REPEAT 220 INPUT "Basic price ",P 230 PRINT "VAT inclusive price 235 PRINT FNST(P) 240 UNTIL P=0 250 END 1000 REM line numeric function 1010 DEF FNST (g) =1 . 15*g Second example - program section Multi-line string function with one string parameter. 1000 DEF FNREVERSE (A$) 1010 REM reverse the order of the letters in A$ 1015 REM 1020 LOCAL d% , B $ 1030 FOR d%=l TO LEN ( A$ ) 204 1040 B$=MID$ (A$ , d% , 1 ) +B$ 1050 NEXT d% 1060 =B$ Third example - program section Multi-line procedure with one parameter. 200 DEF PROCbye ( X ) 210 REM print bye X times 220 LOCAL K 230 FOR K= 1 TO X 240 PRINT "bye" 250 NEXT K 260 ENDPROC Fourth example - program section This sets the background colour to a new value given in the parameter. 10 DEF PROCINITSCREEN (X) 20 REM clear screen and draw border 25 COLOR 128+X 30 CLS 40 DRAW 1279, 0 50 DRAW 1279,799 60 DRAW 0,799 70 DRAW 0 , 0 80 ENDPROC Fifth example - full program 110 INPUT X, Y, Z 120 M=FNMEAN (X, Y, Z) 130 PRINT "The mean of ",X,Y,Z 14 0 PRINT "is " ; M 150 END 8990 REM Single line numeric function 8995 REM with three parameters 9000 DEF FNMEAN (A, B, C) = (A+B + C) /3 Description A program object which must precede declaration of a user function or procedure. String and numeric functions and procedures may be defined. Multi-line functions and procedures are allowed. All procedures and functions must be placed in the program where they will not be executed, eg after the END statement. 205 Syntax DEF FN | PROC[( | {, | })] Associated keywords ENDPROC, FN, PROC 206 DEG degrees Purpose This function converts angles which are expressed in radians into degrees. A radian is equal to about 57 degrees. Examples 100 X=DEG (PI/2 ) 300 angle=DEG (1.36) PRINT DEG ( P I / 2 ) Syntax =D E G Description A function which converts radians to degrees. Associated keywords RAD, SIN, COS, TAN, ACS, ASN, ATN DELETE 207 Purpose The DELETE command is used to delete a group of lines from a program. It cannot be used as part of a program. You can specify which lines should be deleted with a command of the form DELETE 120, 340 This would remove everything between line 120 and line 340 inclusive. To delete everything up to a certain line number (for example up to line 290) use DELETE 0,290. To delete from line 500 to the last line, use as the last line to be deleted any number greater than the last line number in the program. Since the largest line number allowed is 32767 DELETE 500, 32767 will do the trick, but will take a long time. To delete a single line just type the line number and press RETURN. There is no need to use the DELETE command. Examples DELETE 0,540 DELETE 180,753 DELETE 540, 32000 Syntax DELETE , Description A command enabling a range of lines to be deleted from a program. Since DELETE is a command it cannot be used in a program or as part of a multiple statement line. Associated keywords LIST, OLD, NEW 208 DIM dimension of an array Purpose As well as simple numeric and string variables (such as “X’ and ‘name$’) it is possible to work with ‘arrays’ of variables. These are extremely useful when working with groups of numbers or words. For example if you wanted to work with a set of information about the rooms in a hotel with four floors, each with 30 rooms, then an array of four by 230 entries can be created like this: DIM hotel (4,30) Having set up an array, one can enter information into each of its ‘elements’. For example the cost of the room per night might be £26.50 hotel (1, 22) =26 . 50 hotel (4 , 1) =165 . 00 In practice the statement DIM hotel (4, 30) produces an array of five by 31 entries since the lowest array element is hotel (0, 0) . All the above arrays are called ‘two dimension numeric arrays’. Another array could contain the names of guests: DIM name$(4,30) name $ ( 1 , 2 2 ) = " Fred Smith" name $ ( 4 , 1 ) = " The Queen" That sort of array is called a ‘two dimension string array’. Arrays may have one or more dimensions. A single dimension array would be appropriate for all the houses in a road, eg DIM MainSt (150) That sort of array is called a ‘single dimension numeric array’. All arrays are normally dimensioned very early in the program. It is ‘illegal’ to attempt to change the size of an array by re-dimensioning it later in the program. An array may have as many dimensions and as many elements in each dimension as the computer has space for - but you tend to run out of computer memory pretty fast with large arrays! It is essential that there is no space between the array name and the first parenthesis. Thus DIM A ( 1 0 ) is correct but DIM A (10) will not define an array. 209 Examples 100 DIM part number s ( 1 0 0 0 ) 3000 DIM employeename$ (35) 240 DIM ALL_hours_in_the_week(24, 7) 100 DIM A (X) Description A statement which dimensions arrays. Arrays must be predeclared before use. After dimensioning all elements of arrays are initialised to zero for numeric arrays or null strings in the case of string arrays. The lowest element in an array is element zero. Thus DIMX ( 4 ) would create an array of five elements (0 to 4 inclusive). There is a second and quite different use for the DIM statement. It can be used to reserve bytes in memory for special applications. To reserve 25 bytes, type DIM X 24 Notice two things about this statement: firstly the space between the variable X and the (number of bytes minus 1) and secondly the absence of parentheses around the 2 4 . The address of the start of the group of 25 bytes is given in the variable X in this example. Syntax DIM | ({ , } ) or DIM Associated keywords None 210 DIV division of whole numbers Purpose See the keyword MOD for a full explanation. DIV is an operator which gives the whole number part of the result of a division. Thus PRINT 11 DIV 4 gives 2 (leaving a remainder of 3). Description A binary operator performing integer division between its operands. The operands are truncated to integers before division takes place. Syntax =D I V Associated keywords MOD DRAW 211 Purpose This statement draws lines on the screen in MODES 0 , 1 , 2 , 4 and 5 . The DRAW statement is followed by two numbers which are the X and Y coordinates of the end of the line. The line starting point can either be the end of the last line that was drawn or else a new point if the MOVE statement has been used before the statement DRAW. The screen is addressed as 1280 points wide X-axis, 0-1279 1024 points high Y-axis, 0-1023 regardless of the graphics MODE selected. The origin (position 0,0) is normally at the bottom left of the screen. The line is drawn in the current graphics foreground colour. This can be changed using the GCOL statement. Examples 780 DRAW X , Y DRAW 135,200 Description DRAW X , Y means draw a line to X,Y in the current foreground colour. DRAW X , Y is equivalent to PLOT 5,X,Y. DRAW is one of a large group of line drawing statements. See PLOT for others. Syntax DRAW, Associated keywords MODE, PLOT, MOVE, CLG, VDU, GCOL 212 Demonstration program 140 MODE 5 160 REM Red background 170 GCOL 0, 129 175 CLG 180 REM Yellow foreground 190 GCOL CM O 200 REM draw a box 210 MOVE 100,100 220 DRAW 400, 100 230 DRAW 400, 400 240 DRAW 100,400 250 DRAW 100,100 ELSE 213 Purpose To provide an alternative course of action. ELSE can be used following an IF . . . THEN statement, an ON . . . GOTO statement, and an ON . . . GOSUB statement. See the pages describing the associated keywords and chapter 16 for more details. Examples 560 IF length > 0 THEN PRINT "OK" ELSE PRINT "No good " 100 IF AoB THEN C = D ELSE PRINT "Values match" Description Part of the IF. . .THEN. . .ELSE structure. Syntax I F [T HEN ][E L S E ] or ONGOTO{ , } [ELSE] or ONGOSUB{ , ) [ELSE] Associated keywords IF, THEN, ON 214 END Purpose This informs the computer that it has reached the end of the program. END is optional but may be used as many times as required in a program. Example 9000 END Description Optional end of program which may occur anywhere and as often as is required. The command END has a special use in that it causes BASIC to search the program in memory for a valid end program marker. BASIC then updates its internal pointers. This may be useful after unusual loading procedures. If the user changes the value of PAGE then internal pointers such as TOP will not be reset until an END statement or command is met. Syntax END Associated keywords STOP end procedure 215 Purpose This indicates the end of a procedure definition. See the keyword DEF for more information. Examples 1000 DEF PROCdash (param) 1010 REM print dashes lots of times 1020 REM in fact "param" dashes in total 1025 REM 1030 LOCAL counter 1040 FOR counter=l TO param 1050 PRINT"-" ; 1060 NEXT counter 1070 ENDPROC 2010 DEF PROCtriangle (A, B, C, D, E, F) 2020 REM fill a triangle with colour 2050 MOVE A , B 2060 MOVE C , D 2070 PLOT 8 5 , E , F 2100 ENDPROC Description Part of the DEF PROC . . .ENDPROC structure. Syntax ENDPROC Associated keywords DEF, FN, PROC, LOCAL 216 ENVELOPE Purpose The ENVELOPE statement is used with the SOUND statement to control the volume and pitch of a sound while it is playing. All natural sounds change in volume (loudness or amplitude); for example, the sounds from a piano start off loudly and then fade away. An aircraft flying overhead starts off softly, gets louder and then fades away. The variation of amplitude (loudness) for the aircraft, as it flies overhead, looks something like this: This variation of amplitude with time is described as an ‘amplitude envelope’. Some sounds change in pitch. For example, a wailing police siren: This variation of pitch with time is called a ‘pitch envelope’. The BBC Microcomputer can use both pitch and amplitude envelopes and these are set up with the ENVELOPE statement. Example 10 ENVELOPE 1,1,4,-4,4,10,20,10,127,0,0,-5,126,126 20 SOUND 1,1,100,200 217 Description The ENVELOPE statement is followed by 14 parameters. ENVELOPE N,T,PI1,P12,PI3,PN1,PN2,PN3, A A , AD , AS , AR, ALA, ALD Parameter Range Function N 1 to 4 Envelope number T bits 0-6 0 to 127 Length of each step in hundredths of a second bit 7 0 or 1 0 = auto-repeat pitch envelope 1 = don’t auto-repeat pitch envelope PI1 -128 to 127 Change of pitch per step in section 1 PI2 -128 to 127 Change of pitch per step in section 2 PI3 -128 to 127 Change of pitch per step in section 3 PN1 0 to 255 Number of steps in section 1 PN2 0 to 255 Number of steps in section 2 PN3 0 to 255 Number of steps in section 3 AA -127 to 127 Change of amplitude per step during attack phase AD -127 to 127 Change of amplitude per step during decay phase AS -127 to 0 Change of amplitude per step during sustain phase AR -127 to 0 Change of amplitude per step during release phase ALA 0 to 126 Target level at end of attack phase ALD 0 to 126 Target level at end of decay phase The N parameter specifies the envelope number that is to be defined. It normally has a value in the range 1 to 4. If the BASIC statement BP UT # is not being used then envelope numbers up to and including 16 may be used. The T parameter determines the length in hundredths of a second of each step of the pitch and amplitude envelopes. The pitch envelope normally auto-repeats but this can be suppressed by setting the top bit of T - ie using values of T greater than 127. The six parameters PI1, P12, P13, PN1, PN2 and PN3 determine the pitch envelope. The pitch envelope has three sections and each section is specified with two parameters: the increment, which may be positive or negative, and the number of times the increment is to be applied during that section, that is the number of steps. A typical pitch envelope might look like 218 Time in centi-seconds In the above example T = one hundredth of a second. PI1 = +10 PN1 = 12 PI2 = -5 PN2 = 27 PI3 = +50 PN3 = 3 The pitch envelope is added to the pitch parameter (P) given in the SOUND statement. In the above example it must have been 40 since the pitch starts at 40. If bit 7 of the T parameter is zero then at the end of the pitch envelope, at a time given by the equation time = (PN1 + PN2 + PN3)*T hundredths of a second the pitch envelope will be set to zero and will repeat automatically. Note that the pitch can only take on values in the range 0 to 255 and values outside this range are treated as MOD 25 6 of the value calculated. The six parameters AA, AD, AS, AR, ALA and ALD determine the amplitude envelope. Although the current internal sound generator has only 16 amplitude levels the software is upward compatible with a generator having 128 levels. The shape of the amplitude envelope is defined in terms of rates (increments) between levels, and is an extended form of the standard ADSR system of envelope control. The envelope starts at zero and then climbs at a rate set by AA (the attack rate) until it reaches the level set by ALA. It then climbs or falls at the rate set by AD (the decay rate) until it reaches the level set by ALD. However, if AD is zero the amplitude will stay at the level set by ALA for the duration (D) of the sound. The envelope then enters the sustain phase which lasts for the remaining duration (D) of the sound. The duration is set by the SOUND statement. During the sustain phase the amplitude will remain the same or fall at a rate set by AS. 219 At the end of the sustain phase the note will be terminated if there is another note waiting to be played on the selected channel. If no note is waiting then the amplitude will decay at a rate set by AR until the amplitude reaches zero. If AR is zero then the note will continue indefinitely, with the pitch envelope auto- repeating if bit 7 of parameter T is zero. A typical amplitude envelope might look like ALA = 120 ADA = 80 AA = 30 (120 in four hundredths of a second) AD = -4 (-40 in 10 hundredths of a second) AS = 0 AR = -5 (-80 in 16 hundredths of a second) Note that the amplitude cannot be moved outside the range 0 to 126. Syntax ENVELOPE , , , , , , , , , , , , , Associated keywords ADVAL, SOUND 220 EOF# Purpose This function is used to determine whether the end of the file has been reached or not. The function returns the value 0 or -1. It returns the value -1 if the end of the file has been reached. The number following EOF # is the channel number of the file. Refer to chapter 31 for more information. Examples 100 X=EOF# (channel) 200 REPEAT UNTIL EOF#(y) Description The function used to determine whether the end of the file has been reached or not. Syntax =E OF# () Associated keywords OPENIN, OPENUP, OPENOUT, EXT#, P TR# , PRINT#, INPUT#, BGE T# , BP UT # , CLOSE# EOR Exclusive-OR 221 Purpose This is a special logical operator often used to complement certain bits in a byte selectively. Refer to the keyword AND for an introduction to the concepts involved. The process of Exclusive-OR tests whether the corresponding bits in two numbers are the same or different. If the corresponding bits in the two numbers are different then the resultant bit will be a one, if they are the same it will be a zero. Another way of looking at this process is that it complements (changes 0 to 1 and 1 to 0) those bits in one number which are at logic 1 in the other number. Thus if X is 0000 1100 0011 0000 1110 1011 Yis 1011 1111 0000 1010 0010 1000 Then X EOR Yis 10110011 0011 1010 1100 0011 Examples 100 d%= A% EOR &FFFF00 200 R= X EOR Y Description An operator performing the operation of logical or bitwise Exclusive-OR between the two operands. Syntax =E OR or =EOR Associated keywords NOT, AND, OR 222 ERL error line number Purpose This enables the program to find out the line number where the last error occurred. See chapter 27 for more information. Examples 8500 X=ERL 8100 IF ERL = 1 0 0 THEN PRINT "I didn't understand" 300 IF ERL= 1 0000 THEN CLOSE#0 Description A function returning the line number of the line where the last error occurred. Syntax =E RL Associated keywords ON ERROR..., ON ERROR OFF , REP ORT , ERR ERR error 223 Purpose If the computer finds an error that it cannot resolve, it may give up and report the error on the screen. In addition it remembers an ‘error number’. For example, if you try to calculate with numbers which are too large for the computer it will report Too big and remember error number 20. Pressing the ESCAPE key behaves as an error (error number 17) and you can detect this and act on it if you wish. It is possible to make the computer deal with most of these errors itself by writing special sections of the program to deal with the inevitable! These sections of the program need to know what the error was and where it occurred. The function ERR enables your program to find the error number of the last error which occurred. This is generally used to enable the program to respond helpfully to an error caused by the user. Examples 1000 wrong=ERR 100 IF ERR= 1 7 THEN PRINT "YOU CAN'T ESCAPE!" 1230 IF ERR= 1 8 THEN PRINT "You can't divide by zero ! " Description Returns the error number of the last error which occurred. Syntax =E RR Associated keywords ON ERROR..., ON ERROR OFF , ERL , REP ORT 224 EVAL evaluate Purpose This function is mainly used to enable the user to type an expression, such as a mathematical equation, into the computer while a program is running. For example, suppose that a program has to plot a graph; you need a way of getting your equation into the computer while a program is running. In most versions of BASIC this is very difficult to do. With the BBC BASIC the equation is put into a string and then EVAL is used to tell the computer to work out the string. The function is not common in other versions of BASIC so a few more specific examples are given of instructions which can be evaluated by the statement EVAL A$. A$="M*X+C" A$ = "SIN (x/120) +COS (x/30) " Note that EVAL can be used to evaluate functions (SIN, COS, SQR etc) but cannot be used to execute a statement like MODE 4 . Examples 100 X=EVAL(A$) 234 value=EVAL (z$) Description A statement which applies the interpreter’s expression evaluation program to the characters held in the argument string. An easy way to pass a function into a program from a user input. Syntax =E V A L () or =EVAL() Associated keywords VAL, STR$ 225 Demonstration program 10 INPUT A $ 20 FOR X=1 TO 5 30 Y=EVAL(A$) 40 PRINT Y 50 NEXT X >RUN ? 5 *X 5 10 15 20 25 This second program makes the computer act as a calculator. 5 REPEAT 10 INPUT B $ 20 PRINT EVAL B$ 30 UNTIL FALSE >RUN ? 3 + 4 7 ? S I N (RAD ( 45 ) ) 0.707106781 226 EXP exponent Purpose This mathematical function calculates e (2.7183...) raised to any specified power. Examples 120 Y=EXP (X) 3000 pressure = EXP (height ) Description A function returning e to the power of its argument. Syntax =E XP () Associated keywords LN, LOG EXT# extent 227 Purpose This function finds out how large a particular file is. It only works with disc and Econet filing systems — not with cassette files. The number returned is the number of bytes allocated to the file. The file to be investigated must have been opened during the OPENIN, OPENOUT or OPENUP statements. See chapter 31 for more information on file handling. Examples 100 X = EXT# (employee ) PRINT EXT# (N) Description A function which returns the length in bytes of the file opened on the channel given in its argument. Syntax =E XT# () Associated keywords CLOSE#, PTR#, INPUT#, PRINT#, BGE T# , BPUT#, OPENIN, OPENUP, OPENOUT, EOF# 228 FALSE Purpose Sometimes the computer has to decide whether or not something is true. For example: 10 X = 1 2 20 IF X = 2 0 THEN PRINT "X EQUALS 20" Clearly, in this example, the statement x = 2 0 is false. As program will never print X EQUALS 2 0. 100 X= 1 2 110 REPEAT 120 PRINT "HELLO" 130 UNTIL X = 2 0 would repeatedly print HELLO because X will never be anything other than 12. X= 2 0 is FAL S E . The same effect can be achieved by writing 110 REPEAT 120 PRINT "HELLO" 130 UNTIL FALSE which means repeat for ever. In fact, the computer has a numerical value of FALSE, which is zero. Thus PRINT FALSE will print 0 . Similarly PRINT 5=4 will also print 0 , since 5 = 4 is false. It is often useful to say in a program for example CLOCKSET = FALSE and then later you can say IF CLOCKSET THEN PRINT "THE CLOCK IS CORRECT" 229 Examples 100 oldenough = FALSE 245 UNTIL FALSE Description A function returning the value zero. Syntax =F A L S E Associated keywords TRUE 230 FN function Purpose FN preceding a variable name indicates that it is being used as the name of a function. Both string and numeric functions may be defined. See the keyword DEF and chapters 17 and 18 for a more detailed description of functions and procedures. Since a function always returns a value. It often appears on the right of an equal sign or in a print statement. Procedures, on the other hand, do not return results. Example 1000 DEF FNmean2 (x, y) = (x + y) /2 Description A reserved word used at the start of all user defined functions. Syntax DEF FN[( | { , | } )] Associated keywords ENDPROC, DEF, LOCAL FOR 231 Purpose The word FOR is one of the words used in a FOR. . .NEXT loop. This makes the computer execute a series of statements a specified number of times; for example 120 FOR X=1 TO 5 130 PRINT X 140 NEXT X would print out the numbers 1 , 2 , 3 , 4 , 5 . The variable X in the above example initially takes on the value 1 and the program then goes through until it reaches the word NEXT. The program then returns to the line or statement FOR X=1 TO 5 and X is increased in value by 1. The program continues the loop, increasing the value of X in steps of 1, until X reaches 5. After that, the program no longer loops; instead it moves onto the next statement after the FOR . . .NEXT loop. As an option the ‘step size’ can be changed. In the example above X increased by 1 each time around the loop. In the next example XYZ increases by 0.3 each time around the loop. 230 FOR X Y Z = 5 TO 6 STEP 0.3 240 PRINT XYZ 250 NEXT XYZ The above program would print out the numbers 5 5 . 3 5 . 6 5 . 9 The value of X Y Z on exit from the above program would be 6.2. The step size may be negative if you wish to make the value of the control variable decrease each time around the loop. 870 FOR r 2 d2 % = 9 9 TO 60 STEP -12 880 PRINT r 2 d2 % ; "Hi there" 890 NEXT r2d2 % 232 would print 9 9Hi there 87Hi there 75Hi there 63Hi there The FOR. . .NEXT loop always executes once so FOR D=5 TO 3: PRINT D: NEXT D would print 5 and then stop. Examples 300 FOR X=1 TO 16 STEP 0.3: PRINT X: NEXT X 1040 FOR A% = 0 TO MAXIMUM% 560 FOR TEMPERATURE=0 TO 9 Description A statement initialising a FOR. . .NEXT loop. This structure always executes at least once. Any assignable numeric item may be used as the control variable but integer control variables are about three times faster than real variables. Note that when the step is non-integer, rounding errors may creep in and the value of the control variable may well diverge significantly from the true arithmetic value. Syntax FOR =T O [S T E P cnumerio] Associated keywords TO, STEP, NEXT GCOL 233 Purpose This statement sets the colour to be used by all subsequent graphics operations. In other words it selects the graphics foreground colour and the graphics background colour. It also specifies how the colour is to be placed on the screen. The colour can be plotted directly, ANDed, ORed or Exclusive-ORed with the colour already there, or the colour there can be inverted (reversed). The first number specifies the mode of action as follows: 0 Plot the colour specified 1 OR the specified colour with that already there 2 AND the specified colour with that already there 3 Exclusive-OR the specified colour with that already there 4 Invert the colour already there The second number defines the logical colour to be used in future. If the number is greater than 127 then it defines the graphics background colour. If the number is less than 128 then it defines the graphics foreground colour. See the keyword COLOUR for more information. Examples 100 GCOL 0,2 GCOL 3,129 Description This statement is used to select the logical colours used by graphics statements. Syntax GCOL, Associated keywords CLS, CLG, MODE, COLOUR, PLOT 234 GET Purpose This function waits for a key to be pressed on the keyboard and then returns with the ASCII number of the key pressed. See chapter 9 for a description of ASCII numbers. The GET function is used whenever the computer needs to wait for a reply from the user before continuing. Note that when using GET the character typed on the keyboard will not appear on the screen. If you wish it to appear you must then ask the computer to print it. Examples 1040 keyhit=GET 350 X=GET Description A function which waits for the next character from the input stream. The function then returns the ASCII value of the character. Syntax =G E T Associated keywords GET $ , INKEY, INKEY $ GET$ 235 Purpose The GET$ function waits for a key to be pressed on the keyboard and then returns with a string containing the character pressed. See the previous keyword, GET, for a similar function and for further explanation. For example, at the end of a game program you may wish the computer to ask the player whether or not he or she wants to go again. The demonstration program below shows how this can be done. Note that when using GET$ the character typed on the keyboard will not appear on the screen. If you wish it to appear you must ask the computer to print it. Example 1050 A$=GET$ Syntax =GE T $ Associated keywords GET, INKEY, INKEY $ Demonstration program PRINT "Do you want to play another game"; 2120 REM if user presses Y then 2130 REM go to line 100 2140 IF GET $ = " Y " THEN GOTO 100 2160 REM if it gets this far then the 2170 REM reply was not "Y" so give up! 2180 STOP 236 GOSUB go to a subroutine Purpose Often a group of lines in a program needs to be used in a number of different places within the main program. Instead of repeating the same piece of program several times you can make this small sub-section into a separate subroutine. This subroutine can then be ‘called’ from a number of different places in the main program by means of the statement GOSUB. The end of a subroutine is indicated by the word RETURN. This causes the program to return to the statement after the GOSUB statement. Beware of a subroutine calling itself too many times: a depth of 26 subroutines is the maximum that is allowed. As with GOTO, it is possible to GOSUB to a calculated line number. The same cautions that apply to GOTO apply to GOSUB in this case. Example 1020 GOSUB 4000 Description A statement used to call a section of program as a subroutine. One subroutine may call another subroutine (or itself) up to a maximum nested depth of 26. Syntax GOSUB cnumerio ONGOSUB{ , ) [ELSE] Associated keywords RETURN, ON Demonstration program First, here is a program to print out random phrases without using a subroutine: 100 REM A$ contains 7 words and each word 105 REM contains 5 characters - letters or spaces 110 A$="hand mouthear leg arm chestelbow" 120 FOR count = 1 TO 10 125 REM pick a random number 130 R = RND (7) 237 140 REM and use it to pick a random word 150 B$=MID$ (A$, 5*R — 4, 5) 160 REM print a message 170 PRINT "My " ; B$ ; " hurts" 180 REM get another random word 190 R = RND (7) 200 B$=MID$ (A$, 5*R — 4, 5) 210 REM and print out a second message 220 PRINT "Is your ";B$;" all right?" 230 NEXT count Now look at the same program using a subroutine and with the REMs (remarks) removed. 110 A$="hand mouthear leg arm chestelbow" 120 FOR count = 1 TO 10 150 GOSUB 810 170 PRINT "MY " ; B$ ; " hurts" 190 GOSUB 810 220 PRINT "Is your ";B$;" all right?" 230 NEXT count 240 END 810 B$=MID$ (A$, 5 *RND ( 7 ) - 4 , 5 ) 820 RETURN The 7 in line 810 is there to select one of the seven words in A$. In line 810 both the 5s are there because each word contains five letters or spaces. It is essential that all the words contain the same number of characters. Finally, here is the same program written with a string function, with REMs left out and generally cleaned up. 110 A$="hand mouthear leg arm chestelbow" 120 FOR count = 1 TO 10 170 PRINT "My ";FNword;" hurts" 220 PRINT "Is your ";FNword;" all right?" 230 NEXT count 240 END 800 DEF FNword= MI D $ ( A$ , 5 *RND ( 7 ) -4 , 5 ) 238 GOTO g o to a line number Purpose This statement makes the computer jump to a specified line number instead of continuing to the next one in the program. It changes the order in which the computer executes a program. Although GOTO is simple to use, do so with caution! It is all too easy to make a program difficult to follow by using too many GOT Os. Following a program full of GOTOs is like trying to disentangle a plateful of spaghetti and arrange it in a straight line! Adherents of ‘structured programming’ encourage program writers to use structures like REPEAT. . .UNTIL and FOR. . .NEXT and to avoid most (but not all) GOTO statements. It is possible in this version of BASIC to GOTO a variable. In the following example the destination variable is called ‘somewhere’: 10 s ome whe re= 1 0 0 5 20 GOTO somewhere but this feature must be used with great care since the program cannot be renumbered using the RENUMBER command. Note that if the destination line number is to be calculated using a mathematical expression then that expression must be in parentheses. GOTO can be used as a command to start a program without destroying the values assigned to the variables. Examples GOTO 330 100 IF X> 5 THEN GOTO 2000 100GOTO (starts*55+14) Description A statement used to transfer control to a specified or calculated line number unconditionally. 239 Syntax GOTO ONGOTO{ , } [ELSE] 240 HIMEM highest memory location Purpose BASIC uses the computer’s random access memory (RAM) to store the user’s program, all the variables that the program uses, and memory for high-resolution graphics displays. In the absence of other instructions the computer divides the available memory up logically. However there are occasions, particularly when changing display modes and when writing machine code programs, when you may wish to tell BASIC how to divide up the available memory. One way of changing the allocation is by altering the value of the variable HIMEM. This variable contains the address of the highest memory location that BASIC uses for your program and variables. It is automatically set to just below the memory used for the screen when the MODE is selected. Addresses above HIMEM are not used by BASIC. If it is manually altered then locations above HIMEM may be used by the programmer for other things, for example for machine code subroutines. If you wish to change the value of HIMEM you should normally do so very early in your program - preferably right at the beginning. The beginning of the program is also the place to select the display mode that you will be using. Other important boundaries are PAGE, TOP and LOMEM. The memory map in Appendix J gives an indication of their relative positions. Note that in the ‘shadow screen’ mode, HIMEM always returns a value of &8000 (see chapter 42 for more details). Examples 100 HIMEM=HIMEM-40 100 PRINT HIMEM 100 HIMEM = &2800 Description HIMEM contains the address of the first byte that BASIC does not use. This pseudo-variable must not be altered while executing a function or a procedure. Alter it with great care! When using a second processor, or if the computer is being used in shadow mode, HIMEM will not be altered when changing MODE . 241 Syntax HIMEM= or =H I ME M Associated keywords LOMEM, PAGE, TOP 242 IF Purpose This sets up a test condition which can be used to control the subsequent action of the computer. Examples 100 IF mont h= 1 2 THEN PRINT "December" 100 IF A= 1 THEN PRINT "One" ELSE PRINT "Not one" 100 IF answer$=" BANANA " THEN PROCfruit 100 IF he i ght <1.94 OR age<18 THEN GOTO 1030 100 IF length <>5 THEN 2140 100 IF RATE=5 THEN Y=6:Z=8 ELSE PRINT "Wrong rate" 100 IF month= 1 1 THEN IF day=5 THEN PRINT "Guy Fawkes " 100 IF month= 1 AND day=l THEN PRINT "New Year" 100 IF X THEN Y = 0 Description A statement forming part of the IF. . .THEN. . .ELSE structure. The word THEN is optional as is the E L S E section. Syntax I F [THEN][ELSE=I NKE Y () Associated keywords GET, GET $ , INKEY $ 245 Key Number Key Number ft) -33 1 -49 fl -114 2 -50 £2 -115 3 -18 f3 -116 4 -19 f4 -21 5 -20 f5 -117 6 -53 f6 -118 7 -37 f7 -23 8 -22 f8 -119 9 -39 f9 -120 0 -40 A -66 - -24 B -101 A -25 C -83 \ -121 D -51 0 -72 E -35 [ -57 F -68 - -41 G -84 > -88 H -85 -73 I -38 ] -89 J -70 j -103 K -71 -104 L -87 / -105 S -82 Space bar -99 M -102 ESCAPE -113 N -86 TAB -97 0 -55 CAPS LOCK -65 P -56 CTRL -2 Q -17 SHIFT LOCK -81 R -52 SHIFT -1 T -36 DELETE -90 U -54 COPY -106 V -100 RETURN -74 W -34 t -58 X -67 1 -42 Y -69 <- -26 Z -98 -122 246 INKEY$ input the character pressed Purpose This function waits for a specified time while constantly testing to see if a key has been pressed on the keyboard. If a key is pressed before the time runs out then the letter or number pressed is placed in the string variable. If no key is pressed in the given time then an empty string is returned and the program continues. Note that a key can be pressed at any time before INKEY $ is used. All keys pressed are stored in a buffer in the computer and a character is removed from the buffer by, for example, the INPUT statement. You can clear the buffer of all characters by giving the command *FX 15,1 The number in parenthesis, after the word INKEY $ , gives the amount of time that the computer must wait before giving up. The time is given in hundredths of a second. Examples 120 let t e r $ = INKE Y$ (0) 384 result$ = INKEY$ (100) 920 X $ = INKEY $ (Y) Description A function which waits for a key to be pressed within a specified period of time. The function returns a null string if no key is pressed in the specified time. If a key is pressed the string returned consists of the single character pressed. The argument is the maximum time in hundredths of a second. Syntax =INKE Y $ () Associated keywords GET, GET $ , INKEY to put information into the computer 247 Purpose When a computer program is running there is often a need to get numbers or words from the outside world into the computer so that it can do calculations on these numbers or words. The statement INPUT is used for this purpose. There are a number of options: 100 INPUT X will print a question mark on the screen and wait for the user to type in a number. This is not very ‘friendly’ - often it would be helpful to print a message on the screen before waiting for the user to type his/her reply. This can be done in two ways: 340 PRINT "How old are you"; 350 INPUT AGE or more simply 340 INPUT "How old are you", AGE If you do not wish the computer automatically to print a question mark then omit the comma between the message to be printed out and the variable to be filled in. 340 INPUT "How old are you" AGE Often you may want to input several values one after the other. This can be done by placing the variables after each other, but separated by commas, thus: 560 INPUT "Pick three numbers ", X , Y , Z When replying the user separates the values entered either with commas or by pressing the RETURN key after entering each value. The numbers that are typed in are placed in the appropriate variables - X, Y and Z in the example above. The above examples all required numbers to be supplied by the user. You can INPUT words as well. 250 INPUT "What is your name", NAME $ You can INPUT more than one string at a time if you wish by using 200 INPUT "Town , A$ , "Country" , B$ 248 INPUT LINE A$ will accept everything that is typed in including leading spaces and commas, and will place everything into A$ . INPUT may be used with TAB in the same way that PRINT can be. For example 300 INPUT TAB (3, 12) "number please" X SPC can be used also, to insert spaces. A semi-colon can replace a comma. Either a semi-colon or a comma will cause the computer to print a question mark when waiting for the variable to be filled. Description A statement to input values from the current input stream. The question mark prompt may be suppressed by omitting the comma following the prompt string. INPUT strips leading spaces off strings. Syntax INPUT[][; | , ] | { , | } Associated keywords INPUT#, LINE, TAB, SPC 249 INPUT# put information into the computer from cassette or disc Purpose It is possible to record data (numbers and words) on cassette or disc where they can be stored for later use. The statement INPUT# is used to read the data back into the computer from the cassette or disc. See chapter 31 on file handling for more information. Examples 1200 INPUT# channel, date, name$, address$ 3400 INPUT#X, U, V, W$ Description A statement which reads data in internal format from a file and places the data in the stated variables. Syntax INPUT#, | { , | } Associated keywords OPENIN, OPENUP, OPENOUT, EXT#, PTR# , PRINT#, BGET# , BPUT#, CLOSE# 250 INSTR in string Purpose To search one string for any occurrence of another string, for example to see if one word contains another specific word. The search normally starts from the beginning of one string but as an option the search can start from a specified point along the string. The number returned is the string position of the second string in the first string. The leftmost character position is position number 1. If no match is found then zero is returned. A search for a null string X=INSTR ( " Sunday" , " " ) will always return 1. Examples 240 X=INSTR(A$,B$) puts the position of B $ in A$ into X . 180 Y=INSTR(A$,B$, Z) starts search at position Z . PRINT INSTR ( "HELLO" , "L" ) would print 3 . Description A function which returns the position of a sub-string within a string. The starting position for the search may be specified. There must be no space between INSTR and the first parenthesis. Syntax =INSTR(, [, ]) Associated keywords LEFT$,MID$,RIGHT$,LEN$ 251 INT integer part Purpose This converts a number with a decimal part to a whole number. This function always returns a whole number smaller than the number supplied. Thus INT(23.789) gives 23 whereas INT (- 1 3 . 3 ) returns -14. Examples 200 X = INT(Y) 1050 wholenumber=INT ( de cimalnumbe r ) 330 pence = INT ( cost * mar kup / quant it y ) Description INT is a function converting a real number to the lower integer. Syntax =I NT 252 LEFT$ left string Purpose To copy part of a string starting at the left of the source string. For example if A$=" CATASTROPHE" then PRINT LEFT $ (A$ , 3) would give CAT , namely the left three characters of A$ . Examples 100 INDEX$=LEFT$ (WHOLEname$, 4) 3000 U$ = LEFT$ (H4$ , value) Description A string function which returns the left n characters from a string. If the source string is too short then the function returns with as many characters as there are in the source string. There must be no space between LEFT $ and the first parenthesis. Syntax =LE F T $ ( , ) Associated keywords RIGHT$, MID$, LEN, INSTR Demonstration program This prints out letters in a pattern. 10 PRINT "What is your full name"; 20 INPUT name $ 30 FOR X = 1 TO LEN (name $ ) 40 PRINT LEFT$ (name$, X) 50 NEXT X >RUN 253 What is your full name? JOHN J JO JOH JOHN JOHN JOHN A JOHN A JOHN A C JOHN A CO JOHN A COL JOHN A COLL COLL 254 LEN length (of a string) Purpose This function counts the number of characters in a string. For example K=LEN ( "FRIDAY ") would give K=7 since there are six letters in "FRIDAY " and it is followed by a space. This function is often used with aFOR. . .NEXT loop to do something once for each letter in a string. For example, we might wish to encode a word by replacing each letter with its successor in the alphabet so that, for example, "FRIDAY" would become " G S JE B Z " . See the demonstration program. Examples 100 X=LEN(A$) 2350 length=LEN (main$ ) Description This function returns the length of the string given as the argument. Syntax =L E N () Associated keywords LEFT$,MID$,RIGHT$, INSTR Demonstration program 300 PRINT "Type in your word"; 310 INPUT A$ 320 Length=LEN (A$ ) 3 2 5 C $ = " " 330 FOR V=1 TO length 340 B$=MID$ (A$,V, 1) 350 C$ = C$ + CHR$ (ASC (B$ ) +1 ) 360 NEXT V 370 PRINT "The coded version is " ; C$ In the above program each letter is copied one at a time into B $ . Then its ASCII 255 value is calculated, 1 is added to the ASCII value and the new ASCII value is converted back into a character which is then added onto C $ . See the keyword ASC for more information about the ASCII code. 256 LET Purpose In BASIC we often write things like X= 6 meaning put 6 into the box labelled X in the computer. The fact that we are changing the contents of the variable X can be made clearer by writing LET X= 6 The statement X=X + 6 is impossible in mathematical terms. How can something be the same as itself plus 6? In BASIC though it is quite legal to say LET X=X + 6 since the instruction simply means ‘store in the variable X whatever is already there plus 6’, or ‘increase the value of X by 6’. The word LET is optional, but its use makes the program more readable. Examples 100 LET length= 1 5 980 LET DAY $ ="Tuesday" 210 IF A= 6 THEN LET length = 12 Description LET is an optional assignment statement. Note: LET may not be used during the assignment of the pseudo- variables LOMEM, HIMEM, PAGE, PTR# , TIME. Syntax [LET ]= LIST 257 Purpose This command makes the computer list whatever program it has in its memory. It is often used before typing RUN to ensure that there aren’t any typing errors in the program just entered. You can list a single line LIST 280 or a range of lines LIST 100,450 or the whole program LIST LIST , 400 will list all lines up to and including line 400. LIST 400, will list all lines beyond line 400. If you have a very long program you may see the whole listing whiz past before you have time to read it. To stop it, and to make the computer stop at the bottom of each page you can type CTRL N (while holding down the key marked CTRL press the letter N). Then type LIST. This is called ‘page mode’ and the computer stops at the bottom of each page. The next page will be printed when the SHIFT key is pressed. To return to ‘scroll mode’ type CTRL O (hold CTRL down while briefly pressing O). Pressing CTRL and SHIFT together immediately pauses a listing, and pressing ESCAPE will stop a listing so that corrections can be made to the program. If you want a listing on the printer then you can turn the printer on by typing CTRL B before typing LIST. To turn the printer off afterwards type CTRL C . LIST is a command and cannot be used as part of a program or as part of a multiple statement line. The layout of programs as listed can be controlled by the command LIS TO (see next entry). As an option, the computer can be instructed to insert spaces for the duration of all FOR . . .NEXT and REPEAT. . .UNTIL loops. 258 Examples LIST LIST 400 LIST 400,500 LIST , 900 LIST 900, Description A command which lists the current program. Syntax L I S T [ , ][][, ][] Associated keywords NEW, OLD, LISTO LISTO list option 259 Purpose When a program is listed on the screen or the printer, it is often convenient to show all loops within the program indented. LISTO can be used to control the way that the LIST command displays a program on the screen. It can cause the computer to insert spaces in three situations: - After the line number. -During FOR. . .NEXT loops. - During REPEAT . . .UNTIL loops. The number following LISTO should be in the range 0 to 7. 0 Implies no inserted spaces. 1 Implies a space after the line number. 2 Implies spaces during FOR . . . NEXT loops. 4 Implies spaces during REPEAT . . .UNTIL loops. The numbers which select each option (1, 2 or 4) can be added together to select multiple options. If spaces were required during FOR. . .NEXT and REPEAT. . .UNTIL loops then L I S TO 6 would be selected. L I S TO 7 puts a space after the line number and double spaces for FOR. . .NEXT and REPEAT. . .UNTIL loops. The most common options are L I S T O 0 and L I S T O 7 . When editing programs using the cursor editing keys it is strongly advised that you use the L I S T O 0 option or else you will COPY in a lot of extra space. Description LISTO affects the print format produced by subsequent L I S T commands. Bit 0 of the argument controls the single space after the line number; bit 1 the double space in FOR . . . NEXT loops; bit 2 the double space in REPEAT . . . UNTIL loops. Syntax LISTO Associated keywords LIST 260 LN natural logarithm Purpose A mathematical function to calculate logarithms to the base e — usually called ‘natural logarithms’. Examples 100 X = LN ( t emp ) 3000 H5 = LN (REDOXpotential) Description A function returning the natural logarithm of its argument. Inverse logarithms (anti-logarithms) can be calculated by using antilog = EXP (log) Syntax =L N Associated keywords LOG, EXP LOAD 261 Purpose To load a program into the computer from cassette tape, disc or Econet, whichever is the current filing system. For example LOAD " GAME 1 " Once the program has been loaded, type RUN to start it. When you use the word LOAD, the computer forgets any previous program it had in memory and also the values of all variables. If you are loading from disc then the file name (enclosed in quotes) must be a string of not more than seven characters in length (or ten characters for the Advanced Disc Filing System). If a disc directory is specified then you do this by putting the directory character before the file name, like this: LOAD " B . GAME 1 " If you wish to load from a drive other than the one currently selected then the drive number also is included in the quotes preceded by a colon. For example LOAD " : 0 . D . GAME 1 " will load a file called GAME 1 in directory D from drive 0 . If you are loading from cassette, then the computer will show the name of each section of the program as it finds it on the cassette. The file name (enclosed in quotes) may be up to ten characters in length. LOAD " " (with no file name) will load the next program found on cassette, whatever its name. This does not work on disc or Econet or other filing systems. LOAD does not run a program. It just loads a file into memory. It clears all variables except A% to Z% and @%. The command LOAD cannot be used in a program. The statement CHAIN can be used in a program (or as a command) to load another program and to start that program running automatically. Examples LOAD "STARWARS" LOAD " MYPROG 262 Description The command LOAD deletes the current program, clears all variables except the resident integer variables and then loads a new program from the current filing system. The program to be loaded must be in internal format. Since LOAD is a command it cannot form part of a multiple statement line. Syntax LOAD Associated keywords SAVE, CHAIN LOCAL 263 Purpose This informs the computer that the named variables are ‘local’ to the procedure or function in which they occur; their use in this procedure or function in no way affects their value outside it. See the keyword DEF for more information. Example 560 LOCAL X , Y , A$ , B $ Description A statement which can only be used inside a procedure or function definition. LOCAL saves the values of the external variables named and restores these original values when the function or procedure is completed. Syntax LOCAL I { , I } Demonstration procedure 780 DEF PROCdrawTRIANGLE (size) 790 LOCAL X1,X2,Y1,Y2 800 Xl=320-size 810 X2 = 32 0 + si ze 820 Yl=256-size 830 Y2=256+size 840 MOVE XI, Y1 850 DRAW X2,Y1 860 DRAW 320, Y2 870 DRAW XI, Y1 880 ENDPROC Associated keywords DEF, ENDPROC, FN, PROC 264 LOG logarithm Purpose A mathematical function to calculate the common logarithm of a number to base 10 . Examples 100 Y=LOG (y) 440 pre s sure=LOG ( speed ) Description A function giving the common logarithm to base 10 of its argument. Inverse logarithms (anti-logarithms) can be calculated by using antilog = 10 A log Syntax =L O G Associated keywords LN, EXP 265 LOMEM Purpose Different sections of the computer’s memory are used for different purposes. Normally BASIC makes an intelligent decision about where to store the numbers that the user calls X and Y etc. In fact it stores these variables immediately after the user’s program. You can change the place where it starts to store these variables by changing the value of LOMEM, but this must be done right at the beginning of the program. The variable LOMEM gives the address of the place in memory above which the computer stores all its variables (except for the resident integer variables @ % and A% to Z%. LOMEM is normally set to be the same as TOP which is the address of the top of the user program. See the keyword HI MEM and the memory map in Appendix J for more details. Do not accidentally move LOMEM in the middle of a program - the interpreter will lose track of all the variables that you are using. Examples 100 LOMEM= TOP + & 1 0 0 PRINT LOMEM PRINT -LOMEM Note: The ~ tells the computer to print the value in hexadecimal. Description A pseudo-variable which sets the place in memory above which the BASIC interpreter stores dynamic variables - those that are created and destroyed as required. Space is always set aside for the resident variables 0 % to Z%. Normally LOMEM is set equal to TOP which contains the address of the end of the user program. Moving LOMEM in the middle of a program will cause loss of all variables. Syntax LOMEM= or =LOMEM Associated keywords HIMEM, TOP, PAGE 266 MED$ Purpose To copy part of one string into another string. For example, if demo $=" DOGMATIC" then the middle part of demo $, starting at position four and going on for three letters, ie MID $ ( demo $ , 4,3) would equal MAT. In fact MID$ can be used to copy any part of a string - not just the middle part. Thus MID $ ( demo $ , 1,3) would equal DOG and MID $ ( demo $ , 5,4) would be ATIC. This string function is very useful for selecting one word out of a long line. There is a demonstration program under the keyword GO SUB and another under the keyword LEN. If the last number is omitted then the function returns with the rest of the string. Example RESTof LINE S =MID $ (main$, 10) Description A string function which returns a sub-section of the first argument’s string. The second argument gives the starting position and the third argument gives the number of characters to be copied. If the source string is too short then the function returns as many characters as possible from the starting position. Syntax =MID$(, [, ]) Associated keywords LEFT$,RIGHT$, LEN, INSTR 267 MOD modulus Purpose The function MOD gives the remainder after division. When doing division with whole numbers (I emphasise - with whole numbers) it is sometimes useful to know the remainder. For example 14 divided by 5 leaves a remainder of 4 (14=2*5+4). Similarly PRINT 14 MOD 5 would print 4 . The whole number part of the above division is given by the function D I V . Thus PRINT 14 DIV 5 would print 2 . Notice that the result of both D IV and MOD is always a whole number. In fact all numbers used in the calculation of the function are first converted to integers (using internal truncation) before the computer calculates the result. Thus 14 DIV 5 gives 2 14.6 DIV 5.1 gives 2 1 4 MOD 5 gives 4 14.6 MOD 5 . 1 gives 4 The second example (14.6 DIV 5.1) is really the same as the first. However 14.6 DIV 4.9 gives 3 and 14.6 MOD 4 . 9 gives 2 are quite different. In effect the computer sees them as 14 DIV 4 14 MOD 4 Examples 100 LET X= A MOD B PRINT length MOD 12 268 Description A binary operation giving the signed remainder of an integer division. MOD is defined such that A MOD B = A— ( (A DIV B)*B) Syntax =M O D Associated keywords DIV MODE graphics mode 269 Purpose This statement is used to select which display MODE the computer is about to use. Changing MODES clears the screen. Mode Graphics Colour Text 0 640x256 Two colour display 80x32 text 1 320x256 Four colour display 40x32 text 2 160x256 16 colour display 20x32 text 3 Two colour text only 80x25 text 4 320x256 Two colour display 40x32 text 5 160x256 Four colour display 20x32 text 6 Two colour text only 40x25 text 7 Teletext display 40x25 text MODE 7 uses the Teletext standard display characters. These cannot be changed by the user. Since these characters differ slightly from the standard ASCII set you will find that a number of characters on the screen do not correspond to those printed on the keys. For example a left hand square bracket will be displayed as an arrow. In MODES 0 to 6 the character set can be changed by the user. See VDU2 3 in chapter 34. You cannot change MODE inside a procedure or function. MODES 128 to 135 are the ‘shadow’ equivalents of MODES 0-7. See chapter 42 for more details. Examples 10 MODE 5 MODE 7 Description A statement used to select the display MODE , which may not be used in a procedure or function. MODE resets the value of HIMEM, except when a second processor is in use, or when the computer is operating in the shadow screen mode. 270 Syntax MODE Associated keywords CLS, CLG, HIMEM MOVE 271 Purpose This statement moves the graphics cursor to a particular absolute position without drawing a line. For example to move to a point 100 points across the screen and 300 points up the screen one would say MOVE 100,300 Examples 1050 MOVE 100,300 MOVE X , Y Description To move the graphics cursor to a new position without drawing a line. This statement is identical to PLOT 4 . Syntax MOVE, Associated keywords DRAW, MODE, GCOL, PLOT 272 NEW Purpose To ‘remove’ a program from the computer’s memory. In fact the program is still there but the computer has been told to forget about it. If you want to, you can usually recover the old program by typing OLD. This only works if you have not entered any part of another program. NEW is normally used as a command before typing in a new program - to ensure that the computer has forgotten all its previous instructions. NEW does not clear any of the resident integer variables A% to Z% or @ %. Example NEW Description A command which resets internal pointers to ‘delete’ all program statements. The program may be recovered with OLD provided no new statements have been entered and no new variables have been created. Since it is a command it cannot form part of a multiple statement line. Syntax NEW Associated keywords OLD NEXT 273 Purpose This is used in conjunction with FOR to make the computer loop around a set of statements a number of times. If the loop is opened with (for example) FOR speed= 1 0 TO 100 then the NEXT statement would normally be in the form NEXT speed but the word s p e e d is optional. Example 340 length=100 350 FOR X= 0 TO 640 STEP 2 360 Y=2 * lengt h+2 5 0 370 DRAW X , Y 380 NEXT Description A statement delimiting FOR . . .NEXT loops. The control variable (X in the last example) is optional. If a variable is given after NEXT then the computer will ‘pop’ other FOR . . .NEXT loops off the ‘stack’ until it finds a matching variable. If none is found, an error will be reported. Syntax NEXT[], [], ... Associated keywords FOR, TO, STEP 274 NOT Purpose This is normally used with an IF . . .THEN statement to reverse the effect of some test. Example 680 IF NOT (A=6 AND B=5) THEN PRINT "WRONG" If A = 6 and B = 5 then the computer will not print WRONG. Description NOT is a high priority unary operator equivalent to unary minus. Syntax =N O T or =NOT() 275 OLD Purpose To recover a program which has been recently deleted by NEW or by pressing the BREAK key. Programs can only be recovered if no program lines have been entered and if no new variables have been created since the program was deleted. If you get the message Bad program, then type NEW again. Typing NEW or pressing BREAK are quite drastic moves. OLD will do its best to recover your program but will not always succeed fully. In particular if the first line number in your program is greater than 255 then it will get that one line number wrong. The ESCAPE key provides a clean method of stopping a program. BREAK is much more violent and should be avoided. Example OLD Description A command which undoes the effect of NEW. Syntax OLD Associated keywords NEW 276 ON Purpose To alter the order in which BASIC executes a program by jumping to one of a selection of lines depending on the value of a particular variable. The word ON is used with three other keywords GOTO, GOSUB and ERROR. For example: ON value GOTO 800,920,100 ELSE 7300 ON result GOSUB 8000,8300,120,7600 ON ERROR GOTO 9000 ON ERROR GOSUB 2001 First: ON X GOTO 1100,1210,1450,1600,1950 If the value X is equal to 1 then the program will go to line 1100. If X=2 then the program will go to line 1210. If X=3 then line 1450 and so on. What is it used for? Suppose that you are counting coins put into a machine and you want to offer different things if one, two or three coins are put in. The program which follows illustrates, in outline, how ON GOTO will help. 450 REM the variable COINS gives the number 460 REM of coins inserted 500 ON COINS GOTO 550,600,650 550 PRINT "One coin buys a biscuit" 560 REM gives him a biscuit somehow 590 GOTO 1000 600 PRINT "Two coins can buy tea or coffee" 610 GOTO 1000 650 PRINT "Three coins can buy a piece of cake" 660 REM something else in here as well 690 GOTO 1000 1000 REM all the routines end up here Secondly: ON X GOSUB 2200,2300,2400,2500 ON can also be used with GOSUB instead of GOTO. See GOSUB for an explanation of subroutines. ON X GOSUB provides a neat way of using different subroutines in different situations. 277 An ELSE clause can be included at the end of ON GOTO and ON GOSUB to trap out of range values without causing an error. Thirdly: ON ERROR GOTO ON ERROR OFF If the computer detects an error in your program or in the disc drives or anything else that it can’t handle, then it produces an error. In other words it complains and stops. The complaint takes the form of a message on the screen - for example Too big. Sometimes it is vital that the computer looks after such situations without troubling the user. The statement ON ERROR GOTO 7000 ensures that if an error occurs the computer does not complain and does not stop. Instead it goes to a section of program at line 7000 (in this case) which has been specially written to get the computer out of the mess it is in. This section of program may have to give the user instructions like P 1 e a s e enter a smaller number or it may be able to sort out the problem in some other way. How well this ‘error trapping’ works depends on the skill of the programmer in thinking of every possible thing that can go wrong. You will soon re-discover Murphy’s Law: ‘If anything can go wrong, it will.’ Good error handling is vital in all programs for use by non-specialists - and that means most people! The statement ON ERROR OFF lets the computer deal with errors once again - cancelling the effect of ON ERROR GOTO. Examples 40 ON ERROR GOTO 9000 50 ON ERROR PRINT "The computer is confused" 10 ON ERROR GOSUB 2000 Description A statement providing multiple options in changing the order of execution of a program, and error trapping. 278 Syntax ONGOTO{ , } [ELSE] or ONGOSUB{ , } [ELSE] or ON ERROR or ON ERROR OFF Associated keywords GOTO, GOSUB, ELSE 279 OPENIN open file for input to computer (from cassette, disc or Econet) Purpose To tell the computer that the program wishes to read data (words and numbers). Reading data in is quite a complicated procedure for the computer and it needs advance warning when you wish to do so. The advance warning is given by the OPENIN keyword. One use of this facility is to store names and addresses on file (eg the cassette or disc) and to read the file in each time you want to use it. After you have corrected it you can then transfer it back to disc (using OPENOUT) where it will be saved for future use. Further information about cassette, disc and Econet files is provided in chapter 31. A typical example of the useofOPENIN is X=OPENIN ( "cinemas " ) This informs the computer that you will shortly want to read data in from a file which is recorded under the name ‘cinemas’. The file name is ‘cinemas’. In accepting this instruction the computer allocates a ‘channel’ to this operation. It is as if it said ‘OK that information will be provided on telephone number 6’. It makes X=6 (or whatever number it decides). In all future operations on that file you must refer to it as channel X (channel 6 in this example). You get the actual data into the computer by using either BGET#X or I NPUT #X as the demonstration program on the next page indicates. Example 230 f i 1 e=OPEN IN (" census " ) Description A function which attempts to open a file for input or random access. In a disc or Econet environment if a file already exists with the correct name it will be opened for reading. The function returns the channel number allocated by the computer’s filing system. If the file does not exist then zero is returned. 280 Syntax =0 P E N I N () Associated keywords OPENOUT, OPENUP, EXT#, P TR# , INPUT#, PRINT#, BGET# , BPUT#, EOF#, CLOSE# Demonstration program 10 REM to read in the names of 10 cinemas from 20 REM disc assuming of course that you put 30 REM them there sometime before! 50 REM dimension a string array of 10 slots 60 DIM cine $ (10) 90 REM open the file 100 channel=OPENIN ("CINEMA") 110 REM and read in the ten cinema names 120 FOR X=1 TO 10 130 INPUT# channel, cine$(X) 140 NEXT X 150 REM that's the information in 160 REM do whatever you want with it! 281 OPENOUT open file for output to cassette, disc or Econet Purpose This opens a cassette or disc file for output. Before you can record data (rather than programs) you have to open a file. More information about files is given in chapter 31. OPENOUT is used to inform the computer that you wish to record data on cassette or disc. The computer allocates a channel to the operation. When working with discs or over Econet then if a file already exists with that name it will be deleted. If no file exists then a new one will be created. Example 330 X=OPENOUT (" cinemas " ) Description A function which returns the channel number allocated to an output file. If a file of the same name exists then that file will first be deleted. If no file exists then one will be created. Syntax =0 P E N OU T () Associated keywords OP ENIN, OPENUP, PTR#, EXT#, INPUT#, PRINT#, BGET#, BPUT#, EOF#, CLOSE# 282 OPENUP open a file for update Purpose This statement can be used with disc or Econet systems to open a file for update - that is, simultaneous reading and/or writing. With Econet, only one user may open a file for writing at any one time and therefore OPENUP should only be used in single user environments. If a file of the given name exists already then that file will be opened without any changes taking place to the file. If no file of that name exists then OPENUP will fail to open the file requested. OPENUP is normally used with random access files on disc or on the Level 2 Econet filing systems. Example 500 Y % = OPENUP ( "DATA" ) Description A function which returns the channel number allocated to a file opened for both reading and writing. The file must exist before this function can be used. Syntax =0 P E N UP () Associated keywords OPENIN, OPENOUT, P TR# , EXT#, INPUT#, PRINT#, BGET#, BPUT#, EOF#, CLOSE# OPT option 283 Purpose This statement determines what output is produced on the screen when assembly language routines are processed by the BASIC interpreter. An understanding of the operation of assemblers is required to understand the following. During assembly two common errors can occur: Branch out of range andUnknown label. The latter will occur during pass one for all forward references. It is therefore often desirable to turn off assembler error messages during pass one. The statement OPT is followed by a number in the range 0 to 7, with the following results: 0 Assembler errors suppressed, no listing. 1 Assembler errors suppressed, listing. 2 Assembler errors reported, no listing. 3 Assembler errors reported, listing. Options 4, 5, 6 and 7 behave exactly as options 0, 1, 2 and 3 except that the code can be placed at a different location from that at which it is intended to execute. With options 4 to 7 the variable P% controls the program counter during assembly, and the variable 0% gives the memory location where the code is placed. The OPT statement can only occur inside the square brackets which enclose Assembly Language commands. OPT is set to 3 every time the BASIC interpreter finds a [ . Do not confuse it with *OPT which is described in chapter 43. Examples 200 OPT 1 350 OPT (pas s *2 +1 i st ) Description An assembler pseudo-operation controlling the output during assembly. OPT is followed by an expression as detailed above. 284 Syntax OPT Demonstration program 10 oswrch=&FFEE 20 DIM memory% 100 30 FOR Z=0 TO 3 STEP 3 35 P%=memory% 40 [OPTZ 50 .start LDA#ASC" ! " 60 LDX #40 70 .loop jsr oswrch 80 dex : BNE loop 90 rts : ] NEXT Z 100 CALL start 110 END OR 285 Purpose To enable one condition or another condition to determine what happens next. The OR operator can be used either as a ‘logical OR’ or as a ‘Boolean OR’. See the keyword AND for details of logical and Boolean operators. Example 75 IF X= 6 OR date>2 0 THEN PRINT "Good" Description An operator performing Boolean integer logical OR between two numerics. Syntax =0 R Associated keywords AND, EOR, NOT 286 OSCLI operating system command line interpreter Purpose It is very useful in a BASIC program to be able to send commands to the operating system. Such commands might include *FX commands followed by two numbers. When the program is written you do not always know which numbers are to follow the *FX statement. However, you cannot substitute variables such as X and Y directly after the *FX because these variables are not known to the command line interpreter but are only known to the BASIC language. Thus the statement X= 5 : Y = 3 : *FX X, Y would be meaningless to the operating system. The statement OSCLI provides a neat way of passing variables to the operating system in such cases. OSCLI is followed by a string variable which is set to contain the values to be passed to the operating system. Note that numbers must be converted to string form by using the STR$ function; the above example would work correctly with the following. 10 X = 5 20 Y = 3 30 A $ = " FX "+STR$X+" , "+STR$Y 40 OSCLI A $ Examples 10FN$="XYZ " : REM FILE NAME 20 S TART % = &4000 : REM START OF CODE 30 FINISH%= &6000 : REM END OF CODE 40 EXECADD % = &5000 : REM EXECUTION ADDRESS 300 OSCLI "SAVE "+FN$+" " + S TR$ ~ ( S TART % ) + " "+STR$~ (FINISH%) + " "+STR$~ ( EXECADD %) Note that no * is needed in the string. Description A statement which passes its string argument to the operating system command line interpreter. 287 Syntax O S C L I Associated keywords STR$, CHR$ 288 PAGE Purpose PAGE is a variable which gives the address in memory where BASIC has stored (or will store) the user’s program. This is usually automatically set to be the lowest available address in the computer’s Random Access Memory but can be changed by the user. PAGE can be used to enable the computer to store two different programs at the same time in different areas of memory. Use with care. Examples PRINT PAGE 10 P AGE = & 5 0 0 0 20 PRINT -PAGE 235 PAGE=TOP+1000 Description A pseudo-variable giving the address used by the interpreter for the start of the user program. The least significant byte of PAGE is always set to zero by the computer. In other words user programs always start on a ‘page’ boundary where one page is 100 bytes hex (256 bytes decimal). Syntax PAGE = or =P AGE Associated keywords TOP, LOMEM, HIMEM 289 Purpose PI has the value 3.14159265. It is used in the example to calculate the area of a circle radius R. Examples 100 ARE A = P I * R A 2 PRINT PI Description P I =3 .14159265 Syntax =P I 290 PLOT Purpose PLOT is the multi-purpose point, line and triangle drawing statement in BASIC. The first number which follows the keyword PLOT tells the computer what kind of point, line or triangle it is going to draw. The two following numbers give the X and Y coordinates to be used in plotting the point or drawing the line or triangle. PLOT K , X , Y plots to the point at X,Y in a manner determined by the value of K. The effect of each value of K will be: 0 Move relative to last point. 1 Draw line relative in the current graphics foreground colour. 2 Draw line relative in the logical inverse colour. 3 Draw line relative in the current graphics background colour. 4 Move to absolute position. 5 Draw line absolute in the current graphics foreground colour. 6 Draw line absolute in the logical inverse colour. 7 Draw line absolute in the current graphics background colour. Higher values of K have other effects which are related to the effects given by the values 0 to 7. 8- 15 As 0-7 but with the last point in the line omitted. 16- 23 As 0-7 but with a dotted line. 24- 31 As 0-7 but with a dotted line and without the last point on the line. 32- 63 Reserved. 64- 71 As 0-7 but only a single point is plotted. 72- 79 As 0-7 but to draw a horizontal line to the left and right of the point until a colour other than the current background colour is reached. 80- 87 As 0-7 but plot and fill a triangle. When filling solid triangles with colour the computer fills the triangle between the coordinates given and the last two points visited. 291 88- 95 As 0-7 but to draw a horizontal line to the right until reaching the current background colour. 96- 255 Reserved for future expansions. See chapter 34 on VDU drivers for an alternative interpretation of the numbers given above. Suppose that in the above example, PLOT K, X, Y, the value of X was 50 and the value of Y was 80 then ‘draw line relative’ would mean draw a line to the point on the screen 50 places to the right of the origin and 80 places up from the origin. ‘Logical inverse colour’ is explained next. In two colour MODES the logical inverse colour of logical colour 0 is logical colour 1 . In four colour MODES the following apply: Logical colour Inverse 0 3 1 2 2 1 3 0 In the 16 colour MODE logical colour 0 becomes 15, logical colour 1 becomes 14 and so on. When drawing lines the computer draws a line from the last point X,Y position given. Normally the origin is set at the bottom left of the screen, but its position may be moved to any point by using the VDU2 9 statement. See chapter 34 for more information. The graphics screen is 1280 points (0-1279) wide and 1024 (0-1023) points high. The most commonly used PLOT statements are PLOT 4 and PLOT 5, so these two have been given duplicate keywords; MOVE and DRAW. To print a string at a specific place on the screen use the TAB ( X , Y ) statement. As an alternative one can join the graphics and text cursor together with the statement VDU 5 so that the computer prints text at the graphics cursor position. Once that has been done then the graphics cursor can be moved with MOVE, DRAW andPLOT statements. 292 Examples 100 PLOT 3 , X , Y PLOT 6, 100, 220 Description A statement controlling the generation of points, lines and triangles on the screen. Syntax PLOT, , Associated keywords MODE, CLG, MOVE, DRAW, POINT, VDU, GCOL POINT 293 Purpose To find out the colour of a certain position on the screen. Suppose that you are playing a game involving moving a car around a race track. On the race track are pools of green oil. To find out if the place where your car is about to move to has oil on it (so that the car will skid) you need to be able to find out if the screen is coloured green at that point. The number returned is the logical colour of the screen at the graphics point specified. If the selected point is off the screen then the number returned will be -1. There must not be a space between the word POINT and the opening parenthesis. Examples 1340 colour=POINT (X, Y) 100 IF POINT (X , Y ) =2 THEN PRINT "SKID!!" Description A function returning a number representing the colour on the screen at the specified coordinates. If the point is off the screen then the function returns -1. Syntax =P O I N T ( , ) Associated keywords PLOT, DRAW, MOVE, GCOL 294 POS position Purpose This function finds out how far across the screen the flashing cursor is. The left hand side of the screen is position 0 and the right hand side is position 19, 39 or 79 depending on the MODE that has been selected. Examples 1005 X=POS 320 distance=POS Description A function returning the horizontal position of the cursor in the current text window. Syntax =P O S Associated keywords COUNT, TAB, VPOS Demonstration program To print spaces on the screen up to a certain horizontal position - for example to align columns. 100 column=12 110 REPEAT PRINT""; 120 UNTIL POS=column PRINT 295 Purpose This does not print anything on paper. It does, however, print words and numbers on the screen. Anything enclosed in inverted commas ( " " ) will be printed exactly as it is. Things not enclosed in inverted commas will be assumed to be variable names and the contents of the variable will be printed out. The exact layout of the numbers and figures on the screen will depend on the punctuation used in the PRINT statement. The items following the word PRINT are referred to as the ‘print list’. The screen display is divided into vertical strips (or fields) which are (initially) ten characters wide. A comma after an item in the print list will cause enough spaces to be printed to ensure that the next item will be printed in the next field. A semi-colon after an item in the print list will cause the next item to be printed on the same line and immediately following the previous item. If the print list does not end with a semi-colon then the next PRINT statement will print its output on a new line. PRINT by itself leaves a blank line. A new line can be forced at any stage in the print list by inserting an apostrophe. The table below gives examples as they would appear, except that commas have been inserted where spaces would be to aid counting. 296 Example Print position 12345678901234567890 PRINT 1,2 1 2 PRINT 10,200 10 200 PRINT ; PRINT 1 0 ; 2 0 0 10200 PRINT "Answer" ; A Answer 4 2 PRINT "Answer "A Answer 42 PRINT "Answer" , A Answer 42 PRINT 1/2 0 . 5 PRINT 1/3 0.333333333 PRINT 3 . 3 ' 2 . 25 3 . 3 2 . 5 The printer can be turned on at any time by typing CTRL B or by the statement VDU2 in a program. The output of all PRINT statements will then appear on the printer as well as the screen. CTRL C turns the printer output off. See chapter 38 for more information about the printer. Considerable flexibility has been built into the interpreter to enable it to print numbers in several different layouts. There is no need to learn to use these options at first but they will be invaluable when layout is crucial. A more detailed explanation of the advanced features is given below. It is possible to control the overall field width, the total number of figures printed and the number of decimal places printed. All these features are set with one variable called @ %. In brief, setting @% = 131594 will give two decimal places @% = 131850 will give three decimal places @ % = & 9 0 A will return to the normal output format. For a detailed understanding of the format it is best to consider 0 % as a four byte number (eg 0 = 501020903), each byte controlling one aspect of the print format. The most significant byte will be called B4. It has a value of 01 in the example above. The least significant byte is called B1 and has the value 03 in the example above. B4 is tested by the function STR$ to determine the format of strings created by that function. If B4=01 then strings will be formatted paying attention to the setting of 0 % otherwise 0 % will be ignored by S T R $ . Initially B4=00. 297 B3 selects the basic format thus: 00 General format (G format) 01 Exponent format (E format) 02 Fixed format (F format) In G format numbers that are integers will be printed as integers. Numbers in the range 0.1 to 1 will be printed as 0.1 etc. Numbers less than 0.1 will be printed in exponent format. Exponent format will always print numbers in scientific notation; 100 becomes 1E2, 1000 becomes 1E3 and 1200 becomes 1.2E3 Fixed format prints numbers with a fixed number of decimal places. If the number cannot be fitted into the selected field width it reverts to G format. The decimal points are aligned vertically which is ideal for scientific and accounting programs. B2 controls the total number of digits printed in the selected format. If B2 is too large or too small for the MODE selected then B2 is taken as 10. The number is rounded to fit in the B2 digit field. In G format B2 gives the maximum number of digits that can be printed before reverting to E format. Range 1-9. In E format B2 specifies the total number of digits to be printed before and after the decimal point - but not counting the digits after the E. Another way of looking at it is to say that (B2-1) digits will follow the decimal point. In E format three characters or spaces always follow the final E. Range of B2 in E format is 1- 10 . In F format B2 specifies the number of digits to follow the decimal point. Range 0 - 10 . B1 sets the overall print field width and may have any value in the range 0 to 255 which in hexadecimal is &00 to &FF. For example accounting purposes would often require fixed format two decimal places and ten character field width. The four bytes of 0 are built up thus: 0 %=& 00 00 00 00 B4 - zero 0 0 B3 - fixed format 0 2 B2 - two decimal places 0 2 B1 - character field 0A So 0% = &OOO2O2OA, the & indicating that the number is in hexadecimal. You can, of course, omit the leading zeros. 298 Here are some other formats: Format (G2) (G9) (F2) (E2) 0%=& 0000020A 000009 0 A 0002020A 0001020A 100 1E2 100 100.00 1.0E2 10 10 10 10.00 1.0E1 1 1 1 1.00 1.0E0 0.1 0.1 0.1 0.10 1.0E-1 0.01 IE— 2 IE— 2 0.01 1 . OE-2 0.0005 5E-3 5E-3 0.01 5 . OE-3 0.001 IE— 3 IE-3 0.00 1 . OE-3 0 0 0 0.00 0 . 0E0 -10 -10 -10 -10.00 -1 .0E1 Description A statement causing numeric and string values to be printed on the screen. Syntax PRINT {['][, ?; ]?} ['][;] Associated keywords PRINT#, TAB, POS,STR#, WIDTH, INPUT, VDU PRINT# 299 Purpose This records numbers and words on cassette or disc. In other words it stores data on a file. Numbers and strings are stored in a special internal format. Before this statement is used the file must have been opened using the OPENIN, OPENOUT or OPENUP statements. See chapter 31 on file handling for more information. Example PRINT# file, X, Y, Z , A$ , "Monday" , 33 Description A statement which writes data to files. All values are written in a special internal format: Integer variables are written as &40 followed by the twos complement representation of the integer in four bytes, most significant byte first. Real variables are written as &FF followed by four bytes of mantissa and one byte exponent. The mantissa is sent lowest significant bit (LSB) first. 31 bits represent the magnitude of the mantissa and 1 bit the sign. The exponent byte is in twos complement excess 128 form. String variables are written as &00 followed by a 1 byte ‘byte count’ followed by the characters in the string in reverse order. Syntax PRINT# { , ? } 300 PROC procedure Purpose This is used as the first part of a name to indicate that it refers to a procedure. See the keyword DEF for a fuller description. Example 10 DEF PROChello (X) 20 LOCAL Z 30 FOR Z=0 TO X 40 PRINT "Hello - how about this for BASIC!" 50 NEXT Z 60 ENDPROC Description A reserved word used at the start of all user declared procedures. There must not be a space between PROC and the rest of the procedure name. Syntax DEF PROC[( | { , | } ] PROC[( | { , | } ] Associated keywords DEF, ENDPROC, LOCAL Demonstration program 10 REM Tower of Hanoi problem 20 INPUT "Number of disks", F 30 PROChanoi (F , 1 , 2 , 3 ) 40 END 50 DEF PROChanoi (A, B, C, D) IF A=0 ENDPROC 60 PROChanoi (A-l , B , D , C) 70 PRINT "Move disk ";A; " from pile ";B; " to pile " ; c 80 PROChanoi (A-1,D,C,B) 90 ENDPROC 301 PTR# pointer Purpose This statement is not available on cassette based systems. It selects which item in a long file is to be read or written next. Strings and numbers are stored in a long line one after the other. Each integer number occupies five bytes, each real number occupies six bytes and each string takes up the number of letters in the string plus two. See the keyword PRINT# for more details of the file format. The file pointer can be moved up and down the file to point to any selected word or number. Note that you have to keep a careful track of where each word or number starts to use the function. The number immediately following the keyword PTR# is the channel number allocated to the file when it was opened. A file must be opened with the OP ENIN and OPENOUT statements. See chapter 31 for more information on file handling. Examples PRINT P TR# X 560 PTR#f ile=PTR#f ile+80 85 P TR# channe 1 = 0 Description A statement and function which allows the programmer to move a pointer to a serial file and thus enables random access. Syntax =P T R # or PTR# = Associated keywords INPUT#, PRINT#, BGET # , BPUT # , OP ENIN, OP ENUP, OPENOUT, EXT#, EOF# 302 RAD radian Purpose To convert an angle measured in degrees to radians. A radian equals approximately 57 degrees. Examples 1030 X = RAD (Y) PRINT RAD (45) Description A function converting an angular argument given in degrees to radian measure. Syntax =RAD Associated keywords DEG READ 303 Purpose To enable numbers or words that are required in a program to be made available every time the program is run. It does this by reading numbers or words into numeric or string variables from DATA statements in the program. Most often the data is read into an array. See the keyword DIM for more information on arrays. See the keyword DATA for a more detailed description. Example 100 READ name $ ( X ) , A Description A statement which copies the next item from a data list into the variable or variables which follow the keyword READ. The DATA must contain the correct sequence of string and numeric data for the string and numeric variables to be assigned. In other words numeric data must be supplied if a numeric variable is to be filled. Syntax READ | { , | < string- var > } Associated keywords DATA, RESTORE Demonstration Program 200 INPUT"How much can you spend" , AFFORD 210 PRINT"You can afford the following cars" 220 FOR X=1 TO 1 0 : READ NAME $ , PRICE 230 IF PRICE< = AFFORD THEN PRINT NAME $ 240 NEXT : END 500 REM British Leyland Cars 510 DATA AUSTIN METRO 1.0 HLE, 4699 520 DATA etc etc 304 REM remark Purpose To enable the program writer to put remarks and comments into the program to help remember what the various parts of the program do. The computer completely ignores anything that appears after a REM. When you first start writing small programs you can get away with having no REMs, but as your programs grow in complexity you will find it necessary to have them liberally sprinkled over your program. If you come back to a program six months after you wrote it and find no REMs you will have a real job trying to remember how it worked and why you used that variable name etc. Use lots of REMs - it will save you hours of time in the long run. Examples 10 REM this revision dated 2-8-84 100 REM 550 REM data for British Leyland cars Description This statement allows comments to be inserted in a program. Syntax REM RENUMBER 305 Purpose When you type in a program you give each instruction a line number. As the program develops you quite often have to insert extra lines between other lines. You might well need to insert 25 lines between line numbers 300 and 310 — difficult! The RENUMBER command will go through your program and renumber it automatically. It recalculates things like GOTO 220 - which might well become GOTO 18 0 etc. However if your program contains the statement GOTO 10 0 and there is no line 100 then the RENUMBER command will be unable to deal with the problem and will say Failed at line If you renumber a program containing an ON GOTO statement which contains a calculated line number, eg ON X GOTO 120 , 240 , 2*R, 1000 , 2000 references prior to the calculated line will be renumbered. However it will not recalculate a calculated line number or other line numbers in the same statement-ie 2 * R , 10 0 and 2 0 0 0 in the example given. The command RENUMBER will renumber your program giving the first line the number 10, the second 20 and so on. The command RENUMBER 2 00 will give the first line of your program the number 200, the second will become line 210 etc. The command RENUMBER 2 0 0,4 would renumber starting with line 200 and then using 204, 208 etc. RENUMBER is a command: it cannot be used in a program, or as part of a multiple statement line. Examples RENUMBER RENUMBER 100,20 RENUMBER 6000 306 Description RENUMBER is a command which renumbers a user’s program and will correct most of the cross-references within the program. Syntax RENUMBER [[ , ]] REPEAT 307 Purpose To make the computer repeat a set of instructions a number of times until some condition is met. If you jump out of a REPEAT . . . UNTIL loop with a GOTO statement (which is bad practice) you must jump back in. A single REPEAT may have more than one UNTIL. Example 10 REM print stars for 1 second 20 N OW = T IME 30 REPEAT PRINT 40 UNTIL T IME = NOW+ 1 0 0 Description A statement which is the start of a REPEAT. . .UNTIL loop. These loops always execute once and may be nested up to a depth of 20. Syntax REPEAT Associated keywords UNTIL 308 REPORT Purpose To get the computer to report in words what the last error was. Example 100 REPORT Description REPORT prints the error message appropriate to the last error condition. Syntax REPORT Associated keywords ERR, ERL, ON ERROR RESTORE 309 Purpose Sometimes it is useful to have several sets of data in one program. For example one might want information on British Leyland cars and on Lotus cars as in the example given in chapter 22. The RESTORE statement enables the data pointer to be moved from one set of data to the other. The word RESTORE by itself resets the data pointer to the first set of data in the program. Examples 230 RESTORE 100 RESTORE 6500 RESTORE apointer Description This statement can be used at any time to reset the data pointer to any selected line number. Syntax RE S TORE [] Associated keywords READ, DATA 310 RETURN Purpose The word RETURN - not the key marked RETURN - is used in a program a the end of a subroutine to make the computer return to the place in the program which originally ‘called’ the subroutine. See GOSUB for more details. There may be more than one RETURN statement in a subroutine - but preferably there should be one entry point and one (RETURN) exit point. You should try very hard to avoid leaving a subroutine with GOTO - you should always exit with RETURN. Why? Well you will soon discover in reasonable sized programs that you can get into an awful tangle and lose track of how a program works if you make the program jump all over the place. The importance of dividing your programs into clearly defined sections wherever possible, with one entry point and one exit point, cannot be over emphasised. Examples 200 RETURN 300 IF X> 4 THEN RETURN Description A statement which causes the program to branch to the statement after the one which contained the GOSUB which called the current subroutine. Syntax RETURN Associated keywords GOSUB, ON GOSUB RIGHT$ 311 Purpose To copy the right hand part of one string into another string. For example if ABCDE $ = " HOW ARE YOU" then RIGHT$ (ABCDE$, 3) would be "YOU" and RIGHT$ (ABCDE$, 7) would be "ARE YOU" Note that RIGHT$ (ABCDE$ , 100) would be "HOW ARE YOU" since there are only 11 characters in HOW ARE YOU. Examples A$=RIGHT$ ( B $ , 5 ) last$=RIGHT$ ( 1 a st $ , X ) Description A string function returning a specified number of characters from the right hand end of another string. Syntax =R I G H T $ ( , ) Associated keywords LEFT $ , MID $ 312 END random Purpose To generate a random number. What exactly this function does is determined by the number which follows the word RND. RND by itself generates a random whole number between -2147483648 and 2147483647. RND ( — X ) returns the value -X and resets the random number generator to a number based on X. RND ( 0 ) repeats the last random number given by RND ( 1 ) . RND ( 1 ) generates a random number between 0 and 0.999999. RND (X) generates a random whole number between (and possibly including) 1 and X. The parentheses are compulsory and must immediately follow the word RND with no intervening space. Examples PRINT RND (6) 340 largenumber %=RND 950 PRINT RND (1 ) Description A function generating a random number. The range of the number generated depends on the argument (if any). Syntax =RND [()] Associated keywords None RUN 313 Purpose To make the computer obey the statements in the program in its memory. All variables (except the resident integer numeric variables @ % and A% to Z%) are first deleted and then the program is executed. RUN is a statement and programs may therefore execute themselves. If you want to start a program without clearing all the variables then you can use the statement GOTO 100 or GOTO whatever line number you wish to start from, instead of RUN. Examples RUN 9000 RUN Description RUN is a statement causing the computer to execute the current program. Syntax RUN Associated keywords NEW, OLD, LIST, CHAIN 314 SAVE Purpose To save a program that is in the computer’s memory onto cassette or disc. The program must be given a name - usually called its file name. The file name can have up to seven letters and numbers for the Disc Filing System, or ten letters and numbers for the Cassette Filing System and the Advanced Disc Filing System. The name must start with a letter, and cannot contain spaces or punctuation marks. Examples SAVE "FRED" SAVE A$ Description A command which saves the current program area — that is the area between the address given in the variables PAGE and TOP . Syntax SAVE Associated keywords LOAD, CHAIN 315 SGN Purpose This determines whether a number is positive, zero or negative. The function returns -1 for negative number 0 for zero +1 for positive number Examples 100 X=SGN(Y) 230 re suit = SGN ( di f f er ence ) Description A function returning -1 for an argument which is negative, +1 for a positive argument and zero for an argument equal to zero. Syntax =SGN() Associated keywords ABS 316 SINsine Purpose This calculates the sine of an angle. The angle must be expressed in radians rather than degrees — but you can convert from degrees to radians using the function RAD. Examples 120 Y = S IN (RAD (45) ) 2340 value=SIN (1.56) Description A function giving the sine of its argument. The argument must be in radians. Syntax =S I N () Associated keywords COS, TAN, ACS, ASN, ATN, DEG, RAD Demonstration program To draw a sine wave on the screen. 10 MODE 4 20 FOR X = 0 TO 1280 STEP 4 30 DRAW X, 500 + 500*SIN (X/50) 40 NEXT X SOUND 317 Purpose This statement is used to make the computer generate sounds using the internal loudspeaker. The sound generator is capable of making four sounds at once. Each of the four sound channels can generate one note. The keyword SOUND must be followed by four numbers which specify: - Which sound channel is to be used. - The loudness of the note (or the envelope number). - The pitch of the note. - How long the note is to last. For example SOUND 1,-15,52,20 will play a note on sound channel 1, with a loudness of -15 (maximum volume). A pitch value of 52 gives middle C and a duration of 20 will make the note last for 1 second. SOUND C , A , P , D The channel number (C) can be 0, 1, 2 or 3. Channel 0 is a special channel that can produce various noises, whereas channels 1, 2 and 3 are used to produce single notes. Other values of C produce special effects which are explained further on. The amplitude or loudness (A) can have any whole number value between -15 and 4. Values -15 to 0 produce notes of fixed loudness throughout the whole note. A value of -15 is the loudest, -7 is half volume and 0 produces silence. Values of 1 to 4 enable the amplitude to be controlled while the note is playing. When you play a note on the piano the sound gradually fades away. Effects like this are selected by using one of the four user defined envelopes which are selected by setting A to be 1, 2, 3 or 4. Envelopes are explained in chapter 30 and under the keyword ENVELOPE . The pitch (P) is used to set the pitch or frequency of the note. The pitch can have any value between 0 and 255. The note A above middle C is selected with a value of 88. The table in chapter 30 shows the value of P needed to produce a particular note. You will see that to go up an octave P is increased by 48 and to go up a perfect 5th P must be increased by 28. Increasing the value of P by one will increase the note produced by a quarter of a semi-tone. 318 To play the chord of C major which consists of the notes C, E and G for two seconds you could enter 100 SOUND 1,-15,52,40 110 SOUND 2,-15,68,40 120 SOUND 3,-15,80,40 However to play a number of notes in succession you would enter 100 SOUND 1,-15,96,10 110 SOUND 1,-15,104,10 120 SOUND 1,-15,88,10 130 SOUND 1,-15,40,10 140 SOUND 1,-15,68,20 which plays a well-known film theme. The duration (D) can have any value between -1 and 254. Values in the range 0 to 254 give a note duration of that number of twentieths of a second. Thus if D=40 the note will last for two seconds. Setting D=-l means that the note will continue to sound until you actually take steps to stop it. You can either press the ESCAPE key or stop it by sending another note, to the same channel, which has ‘flush control’ set to 1 — see later in this section. As was mentioned earlier, channel number 0 produces ‘noises’ rather than notes and the value of P in the statement SOUND 0 , A , P , D has a different effect from that described for channels 1, 2 and 3. Here is a summary of the effects of different values of P on the noise channel: P Effect 0 High frequency periodic noise. 1 Medium frequency periodic noise. 2 Low frequency periodic noise. 3 Periodic noise of frequency determined by the pitch setting of channel 1. 4 High frequency ‘white’ noise. 5 Medium frequency ‘white’ noise. 6 Low frequency ‘white’ noise. 7 Noise of frequency determined (continuously) by the pitch setting of channel 1. Values of P between 0 and 3 produce a rasping, harsh note. With P set to 4 the noise is like that produced by a radio when it is not tuned to a station - a sort of ‘shssh’ effect. P=6 sounds like the interference found on bad telephone calls. When P is set to 3 or 7 then the frequency of the noise is controlled by the pitch setting of sound channel number 1. If the pitch of channel 1 is changed while 319 channel 0 is generating noise then the pitch of the noise will also change. The program below generates a noise on channel 0 and varies the pitch of the noise by changing the pitch of channel 1. Notice that the amplitude of channel 1 is very low (-1) so you will hardly hear it - but you will hear the noise on channel 0. 100 SOUND 0,-15,7,150 110 FOR P= 100 TO 250 120 SOUND 1, -1, P, 1 130 NEXT P Notice that we have not yet described how sounds can be affected by a superimposed envelope. An envelope can affect both the pitch and amplitude of a note as it is playing. Thus the statement SOUND 1,-15,255,255 merely plays a continuous loud note, but ENVELOPE 1, 1,-26, -36, -45, 255, 255, 255, 127, 0,0,- 127, 126, 0 SOUND 1,1,255,255 produces a complex sound controlled largely by the envelope See the keyword ENVELOPE for more details. As mentioned briefly at the start of the description of the SOUND statement, the channel number, C, can be given values other than 0,1,2 and 3. You do not need to understand exactly why the following works to use it! For C you can write a four figure hexadecimal number to achieve certain effects — for example: SOUND 41213,-15,52,40 The first parameter in the above example has the value & 1 2 1 3 . The ampersand (& ) indicates to the computer that the number is to be treated as a hexadecimal number. The four figures which follow the ampersand each control one feature. In this new expanded form the SOUND statement looks like SOUND & HSFC , A , P , D and the functions H, S, F and C will be explained in turn. In essence these numbers enable you to synchronise notes so that you can play chords effectively. The first number (H) can have the value 0 or 1. If H=1 then instead of playing a new note on the selected channel, the previous note on that channel is allowed 320 to continue. If a note were gently dying away then it might be abruptly terminated when its time was up. Setting H=1 allows the note to continue instead of playing a new note. If H=1 then the note defined by the rest of the SOUND statement is ignored. The second number (S) is used to synchronise the playing of a number of notes. If S=0 then the notes are played as soon as the last note on the selected channel has completed. (There is a slight simplification here; ‘completed’ means ‘has reached the start of the release phase’.) The user is referred to the keyword ENVELOPE for relevant detail. A non-zero value of S indicates to the computer that this note is not to be played until you have a corresponding note on another channel ready to be played. A value of S=1 implies that there is one other note in the group. S=2 implies two other notes (ie a total of three). If a note was sent to channel 1 with S set to 1 then it would not be played until a note was ready on another channel which also had S set to 1. For example: 110 SOUND 6101,-15,50,200 110 SOUND 2,-15,200,100 120 SOUND 6102,-15,100,200 When this program is run the note at line 100 will not play until channel 2 is free. Line 110 sounds a note immediately on channel 2 - and for five seconds (duration 100). When the note has completed then both the notes from lines 100 and 120 will sound together. The third number (F) can have the value 0 or 1. If it is set to 1 then the SOUND statement in which it occurs flushes (throws away) any other notes waiting in the queue for a particular channel. It also stops whatever note is being generated on that channel at present. The SOUND statement in which F=1 then plays its note. Setting F behaves like an ‘over-ride’. For example: 20 SOUND 2,-15,200,100 25 FOR X=1 TO 500: NEXT X 30 SOUND 612,-15,100,200 In the above situation line 20 will start a sound on channel 2 but this will be stopped almost immediately by line 30 which will generate a lower and longer note on channel 2. Line 25 just gives a short delay. Setting F=1 provides an easy way of stopping an everlasting note! Thus S OUND 6 13,0,0,1 stops the current note on channel 3 and instead plays one at zero loudness and of minimum length. This will stop channel 3 immediately. The last number (C) is the channel number described earlier. 321 Description The sound generator has four separately controlled synthesis channels. Each can sound at one of 16 amplitudes, including ‘off. The audio output is the sum of the channel outputs. Channels 1-3 each generate a square wave with programmable frequency. Channel 0 can produce noise (unpitched sound of pseudo-random structure) or a pulse waveform. The frequency of the pulsewave or period of the noise can be set to one of the three fixed options, or to the frequency of channel 1. The BASIC program generates each sound by initiating one or more ‘requests’ each of which may take the form of a musical note or a single effect and is directed to a specific channel. If the destination channel is idle when a request requires it, the sound starts playing immediately. If a previous request is still being handled the new one is placed on a queue, where it waits until the current event is over (or past a critical stage - see ENVELOPE). If the queue is full, the program waits. Separate queues are provided for the four channels, each of which can hold up to four requests, not counting the one currently being executed. The program can look at the state of any queue and flush any queue, but cannot find out or alter the state of the current event, except for flushing the whole queue. The SOUND keyword is followed by four parameters, the first of which consists of four hexadecimal digits. Thus SOUND & HSFC , A , P , D Range Function H 0 or 1 Continuation S 0 to 3 Synchronisation F 0 or 1 Flush C 0 to 3 Channel number A -15 to 4 Amplitude or envelope number P 0 to 255 Pitch D 1 to 255 Duration The ‘H’ parameter allows the previous event on that channel to continue and if this is 1 the amplitude and pitch parameters of SOUND have no effect. Because the dummy note is queued in the normal way, it can be used to ensure that the release segment of sound, which occurs after the duration is over and would otherwise be truncated by the next sounding event on the same channel, is allowed to complete. 322 The ‘S’ parameter allows requests to be queued separately and then executed at the same instant, for chords and multiple voice effects. The value initially determines the number of other channels that must receive requests with the same value of S before the group will play. For example, each note of a three note chord would be generated by a SOUND with the value of 2 for S. The system will read the value of S from the first one and then wait for two more requests with 2 as the value of S before playing the complete chord. Single requests use 0 for S so that they play as soon as they reach the end of the channel queue. The parameter ‘F’ will normally be zero, causing the request to be queued. If it is 1, the channel queue will be flushed first, so the request will sound immediately. The parameter ‘C’ determines the number of the sound channel to be used. The ‘A’ parameter controls the amplitude of the sound and can be used in two ways. Positive values up to 4 select the envelope (1 to 4) to be used. If the RS423 is unused then envelope numbers up to 16 may be defined and used. Zero and negative integers up to -15 directly set the amplitude of the sound, which is then fixed at this value for the duration of the note. -15 corresponds to the loudest, and 0 is ‘off. The ‘P’ parameter determines the pitch of the note. It can take values from 0 to 255. The ‘D’ parameter determines the total duration of sounds whose amplitude is determined explicitly by a negative or zero value of the A parameter. The duration is given in twentieths of a second. If an envelope has been selected, by a positive value of A, then the duration D determines the total of the attack, decay and sustain periods - but not of the release phase. Syntax SOUNDcnumerio, , , Associated keywords ENVELOPE, ADVAL 323 SPC space Purpose This statement is used to print multiple spaces on the screen. It can only be used as part of PRINT or INPUT statements. The number in parenthesis gives the number of spaces to be printed. Examples 120 PRINT "Name" ; SPC ( 6) ; "Age" ; SPC (10) ; "Hours" 4030 INPUT SPC(10), "Value", V Description A statement printing a number of spaces on the screen. Up to 255 spaces may be printed. Syntax PRINT SPC() or INPUT SPC() Associated keywords TAB, PRINT, INPUT 324 SQR square root Purpose This statement is used to calculate the square root of a number. Examples 10 X=SQR(Y) 300 X= (-B + SQR (B A 2-4*A*C) ) / (2*A) Description A function returning the square root of its argument. An attempt to calculate the square root of a negative number will produce the error message -ve root which is error number 21. Syntax =S QR() Associated keywords None STEP 325 Purpose This is part of the FOR . . . TO . . . STEP . . . NEXT structure. In the program shown below, STEP indicates the amount that the variable cost is to be increased each time around the loop. In this case the cost is to increase in steps of five units. The step may be positive or negative. STEP is optional. If omitted a step size of +1 is assumed - see the keyword FOR. Example 300 FOR X=100 TO 20 STEP -2.3 Description Part of the FOR . . .NEXT construct. STEP is optional. Syntax F O R=T O [S T E P ] Associated keywords FOR, TO, NEXT Demonstration program 230 FOR cost=100 TO 200 STEP 5 250 product ion = FNt aken ( cos t ) 260 PRINT production, cost 270 NEXT cost STOP Purpose This statement interrupts a program which is running and prints the message STOP at line on the screen; otherwise the effect is identical to END . STOP may occur as many times as is needed in a program. Examples 2890 STOP 3080 STOP Description Causes execution of the program to cease and a message to be printed out. Syntax STOP Associated keywords END STR$ string 327 Purpose This string function converts a number into the equivalent string representation. Thus S T R $ (4 . 6 ) would give 4.6. STR$ is affected by the field width and format constraints imposed by the variable @%. The default format is format 0, field width 10 . See chapter 10. The opposite function of converting a string into a number is performed by the functions EVAL andVAL. Examples 20 A$=STR$ (X) 5060 num$=STR$ (size) 10 PRINT STR$ ~ (10 0) Description A string function which returns the string form of the numeric argument as it would have been printed. Syntax =S T R $ () Associated keywords VAL, PRINT, EVAL, OSCLI 328 STRING$ Purpose This produces a long string consisting of multiple copies of a shorter string. Thus STRING$ (6, " — 0") would be — 0 — 0 — 0 — 0 — 0 — 0. This function is useful for decorative features. It should be used whenever the user needs to generate a long string from identical short strings. It is very important, to avoid wasting memory space, that strings are set to their maximum length the first time that they are allocated. This can easily be done by using STRING$. For example to set A$ to contain up to 40 characters one could write A$ = STRING$ ( 40 , " " ) A$ can then be set back to empty using A$ = " " before use. Examples 400 A$=STRING$ (x,pattern$) 560 B4$=STRING$ (5, "0+") PRINT STRING$ (10, "hello") Description A string function returning multiple concatenations of a string. Syntax =S T R I N G $ ( , ) tabulation 329 Purpose TAB can only be used with the keywords PRINT and INPUT. There are two versions: TAB(X) will print spaces up to a certain column position. If the flashing cursor is beyond the required position then the cursor will move to the next line down and space across to the required column. TAB ( X , Y ) will move the cursor directly to position X,Y on the screen. Note that once TAB (X, Y) has been used on a line TAB (X) may not move to the correct position on the line. The origin (for all text commands) is at the top left of the current text area of the screen. The left hand column of the screen is column number 0. The right hand column is column 19, 39 or 79 depending on the graphics MODE selected. The top line is line number 0, the bottom line is line number 31 or 24; If the text scrolling area of the screen is changed then the TAB command will still work as outlined above. Examples 340 PRINT TAB (10) ; name $ TAB ( 30 ) ; job$ 440 PRINT TAB (20 , 31); value 230 INPUT TAB (10 , 20) "How much" cost 875 INPUT TAB (30) , "Doctor's name", DOC$ Description TAB with a single argument prints spaces (and a new line if necessary) to reach the specified column. TAB with two arguments moves the cursor directly to the specified coordinates. 330 Syntax PRINT TAB([, ]) or INPUT TAB([, ]) Associated keywords POS.VPOS, PRINT, INPUT tangent 331 Purpose This mathematical function calculates the tangent of an angle. The angle must be given in radians but may be converted to radians from degrees using the function RAD . A radian is about 57 degrees. Examples PRINT TAN ( RAD (45)) 10 Y=TAN (X) 1030 droop=TAN ( load) Description A function returning the tangent of the argument. The argument must be given in radians. Syntax =T AN Associated keywords COS, SIN, ACS, ATN, DEG, RAD 332 THEN Purpose A keyword used with IF to decide on a course of action as the result of some test. Examples 780 IF X= 6 THEN PRINT "good" ELSE PRINT "bad" 200 IF A$=B$ THEN PROCgood ELSE PROCbad Description Optional part of the IF. . .THEN. . .ELSE structure. Note that it is not optional if used when the condition assigns to a pseudo variable, eg 300 IF X THEN TIME=0 Syntax I F [T HEN ][E L S E ] Associated keywords IF, ELSE TIME 333 Purpose This can be used to set or read the internal timer. The timer counts in one hundredth of a second intervals. It is not a clock providing true time of day readout. However, once set, the internal clock will keep good time. Pressing the BREAK key does not reset the clock. To convert TIME to a 24 hour clock use the following routines: 1000 SEC= (TIME DIV 100) MOD 60 1010 MIN= (TIME DIV 6000) MOD 60 1020 HR = ( TIME DIV 360000) MOD 24 Examples 205 TIME= (Ho*60+Mi) *60+Se) *100 400 nowtime=TIME Description A pseudo-variable which sets or reads the lower four bytes of the internal elapsed time clock. Syntax TIME = or =T I ME Demonstration program 1070 f ini sht ime =T IME+ 1 0 0 0 1080 REPEAT 1090 REM wait for 10 seconds 1100 UNTIL TIME>=f inishtime 334 TO Purpose Part of the FOR. . .TO. . .STEP. . .NEXT statement. The final terminating value of the loop is given after the word TO. See chapter 15 for further information. Description Part of the FOR . . .NEXT construct. Syntax F O R=T O [S T E P ] Associated keywords FOR, STEP, NEXT Demonstration program 10 MODE 5 20 FOR C=1 TO 3 30 GCOL 3 , C 40 FOR X = 0 TO 1200 STEP 5*C 50 MOVE 600,750 60 DRAW X, 0 70 NEXT X 80 NEXT C TOP 335 Purpose The function TOP returns the address of the first free memory location afer the user’s program. The user’s program is normally stored from the bottom of the available Random Access Memory upwards. Thus the length of the user’s program in bytes is given by TOP-PAGE. Examples PRINT- (TOP-PAGE ): REM length in hex 2340 PRINT TOP 5460 X=TOP Description A function returning the first free location above the user’s program. Syntax =T O P Associated keywords PAGE, HIMEM, LOMEM 336 TRACE Purpose TRACE makes the computer print out the line number of each line of the program before execution. There are three forms of TRACE : TRACE ON Causes the computer to print line numbers. TRACE OFF Turns off the trace facility. TRACE 67 8 0 Would cause the computer to report only line numbers below 6780. With well-structured programs which have subroutines at high line numbers this will enable the user to trace through the structure of the program without being bothered with line numbers in procedures, functions and subroutines. Note that the interpreter does not execute line numbers very often. 10 FOR Z=0 TO 100 20 Q = Q * Z : NEXT Z 30 END would print [10] [20] [30] but 10 FOR Z=0 TO 100 20 Q=Q*Z 25 NEXT Z 30 END would print [10] [20] [25] [25] [25] [25] [25] etc. (Of course in MODE 7 the [ appears as <— and ] appears as — > .) TRACE is also turned off after an error, or by pressing ESCAPE or BREAK. Examples TRACE ON TRACE OFF TRACE X TRACE 3000 337 Description TRACE ON causes the interpreter to print executed line numbers when it encounters them. TRACE X sets a limit on the size of line numbers which may be printed out; only numbers less than X will be printed. TRACE OFF turns trace mode off. Syntax TRACE ON | OFF | 338 TRUE Purpose TRUE is represented by the value -1 in this computer. Examples PRINT TRUE 300 UNTIL result = TRUE Description A function returning -1. Syntax =TRUE Associated keywords FALSE UNTIL 339 Purpose Part of the REPEAT . . . UNTIL construct. See the keyword REPEAT for more details. Example 450 UNTIL X<10 Description A program object signifying the end of a REPEAT . . .UNTIL loop. Syntax UNTILctestable condition> Associated keywords REPEAT 340 USR user subroutine Purpose The USR function provides the user with a means of calling sections of machine code program which are designed to return one value. When the machine code section is called the computer sets the processor’s A, X and Y registers to the least significant bytes of A%, X% and Y%. The carry flag (C) is set to the least significant bit of C%. On return from the machine code section, an integer number is generated from the four registers P, Y, X, A (most significant byte to least significant byte). Again it must be emphasised that USR returns a result while CALL does not. Therefore you must either assign the result to a variable, eg Regi ster s = USR ( & 3 0 0 0 ) or print the result, eg PRINT USR(&3000) Each individual register may be obtained as follows: 10 DIM registers 3 20 ! registers = USR(address) After these two lines are executed, Accumulator X Y Flags registers?0 registers?l registers?2 registers?3 Examples 1400 R=USR ( & 3 0 0 0 ) 670 re suit % =USR (pi ot 5 ) Description A function allowing machine code to return directly a value for problems which do not require the flexibility of CALL . 341 Syntax =U S R() Associated keywords CALL 342 VAL value Purpose This function takes a string which contains a number and produces the number. In other words it can convert a number represented by a string (eg A$ = " + 2 4 " ) into the number. The string must start with a plus (+) or minus (-) sign or a number. If not then the function will return zero. The opposite function is performed by S TR$ . Examples 450 x=VAL (length$) 1560 date=VAL (DATE$ ) Description A function which converts a character string representing a number into numeric form. If the argument is not a signed unary constant then zero will be returned. Syntax =VAL() Associated keywords STR$, EVAL 343 VDU Purpose The statement VDU is followed by one or more numbers and the ASCII characters corresponding to these numbers are sent to the screen. The function CHR$ can generate a single ASCII character from a given number. This character can be added to a string or printed. VDU on the other hand is used to generate a sequence of numbers that are then sent to the VDU drivers. VDU provides an easy way of sending, for example, control characters to the VDU drivers. See chapter 34 for a detailed list of the VDU control codes. Two examples will make the purpose of this statement clearer: when defining the text area of the screen four bytes have to follow the VDU 2 8 statement. These four bytes represent the left X, bottom Y, right X and top Y coordinates of the text area. In MODE 4 the range of X is 0-39 and of Y is 0-31. Thus VDU 28,0,5,39,0 would define a six line text window at the top of the screen. If a different MODE is selected then the maximum screen width may be either 19, 39 or 79. The graphics area of the screen, on the other hand, uses coordinates up to 1279 points horizontally. Thus when defining the graphics area double byte numbers must be sent to the VDU drivers since the largest number that can be sent as a single byte is 255. VDU 24, 0; 0; 1279; 830 will define a graphics area at the bottom of the screen and 830 points high. Each of the four coordinates is sent as a double byte pair. Note that the graphics origin is bottom left while the text origin is top left and that the graphics screen is always 1280 by 1024 regardless of MODE. VDU is equivalent to PRINT CHR$; except that it does not change the value of COUNT. Examples VDU 1 4 Turn auto-paging mode on. VDU 15 Turn auto-paging mode off. VDU 2 Turn printer on. 344 Description A statement which takes a list of numeric arguments and sends them to the operating system output character routine (OSWRCH). If the argument is followed by a semi-colon then that argument will be sent as two bytes. The least significant byte will be sent first, followed by the most significant byte. This is the order required by the VDU drivers. Syntax VDU { , | ; } [; ] Associated keywords CHR$ VPOS vertical position of the cursor 345 Purpose VPOS is used to find the vertical position of the text cursor on the screen. Examples 670 V=VPOS 100 PRINT VPOS Description A function returning the vertical position of the text cursor. Syntax =VP O S Associated keywords POS WIDTH Purpose WIDTH is used to set the overall ‘page width’ that the computer uses. Initially this is set to zero which the interpreter interprets as ‘unlimited width’. WIDTH n will cause the interpreter to force a new line after n characters have been printed by the PRINT statement. WIDTH also affects all output to the printer. Examples 670 WIDTH 60 WIDTH 35 Description A statement controlling the overall output field width. It is initially set to zero which disables auto new lines. Syntax WIDTH cnumerio Associated keywords COUNT 34 VDU drivers 347 The statement VDU X is equivalent to PRINT CHR$(X); and the statement VDU X,Y,Z is equivalent to PRINT CHR$ ( X ) ; CHR$ ( Y ) ;CHR$(Z) ; . However the VDU statement is used the most when generating ASCII control codes and a detailed description of the effect of each control code is given in this chapter. The control codes are interpreted by part of the Machine Operating System called the VDU driver. Programmers writing BASIC programs will need to refer to this summary of the VDU drivers if they want to use some of the more advanced facilities such as definition of graphics and text windows. Programmers writing other high level languages or machine code programs will also need to refer to this chapter. The VDU drivers are part of the Machine Operating System (MOS) software. All high level languages (including BASIC) use them to print and draw on the screen. Because they are so extensive and easily accessible to programmers it will be easy to ensure that all high level languages and smaller assembly language programs have access to the same graphics facilities. There is no need for the user to write special routines to handle the screen display. The BBC Microcomputer is designed so that it can be expanded in many ways. All expansions will be compatible with the current Machine Operating System and it is very important that those writing software use the facilities provided. In a ‘twin-processor’ machine the only access to the screen memory is via the ‘Tube’ and use of these VDU drivers and other Machine Operating System features will ensure that code will work correctly whether executed in the input/output processor or in the language processor. The VDU drivers interpret all 32 ASCII control character codes. Many of the ASCII control codes are followed by a number of bytes. The number of bytes which follow depends on the function to be performed. The VDU code table summarises all the codes and gives the number of bytes which follow the ASCII control code. 348 VDU code summary Decimal Hex CTRL ASCII abbreviation Bytes extra Meaning 0 0 0 NUL 0 Does nothing 1 1 A SOH 1 Send next character to printer only 2 2 B STX 0 Enable printer 3 3 C ETX 0 Disable printer 4 4 D EOT 0 Write text at text cusor 5 5 E ENQ 0 Write text at graphics cursor 6 6 F ACK 0 Enable VDU drivers 7 7 G BEL 0 Make a short beep 8 8 H BS 0 Backspace cursor one character 9 9 I HT 0 Forwardspace cursor one character 10 A J LF 0 Move cursor down one line 11 B K VT 0 Move cursor up one line 12 C L FF 0 Clear text area 13 D M CR 0 Move cusor to start of current line 14 E N SO 0 Page mode on 15 F 0 SI 0 Page mode off 16 10 P DLE 0 Clear graphics area 17 11 Q DC1 1 Define text colour 18 12 R DC2 2 Define graphics colour 19 13 S DC3 3 Define logical colour 20 14 T DC4 4 Restore default logical colours 21 15 U NAK 0 Disable VDU drivers or delete current line 22 16 V SYN 1 Select screen mode 23 17 W ETB 9 Reprogram display character 24 18 X CAN 8 Define graphics window 25 19 Y EM 5 PLOT k,x,y 26 1A Z SUB 0 Restore default windows 27 IB [ ESC 0 Does nothing 28 1C \ FS 4 Define text window 29 ID ] GS 4 Define graphics origin 30 IE A RS 0 Home cursor to top left 31 IF US 2 Move text cursor to x,y 127 7F DEL 0 Backspace and delete 349 Detailed description 0 This code is ignored. 1 This code causes the next character to be sent to the printer only and not to the screen. The printer must already have been enabled with VDU2. Many printers use special control characters to change, for example, the size of the printed output. For example the Epson FX-80 requires a code 14 to place it into double width print mode. This could be effected with the statement VDU1 , 14 or by pressing CTRL A and then CTRL N. This code also enables the ‘printer ignore’ character selected by * F X 6 to be sent to the printer. 2 This code turns the printer on which means that all output to the screen will also be sent to the printer. In a program the statement VDU2 should be used, but the same effect can be obtained by typing CTRL B . 3 This code turns the printer off. No further output will be sent to the printer after the statement VD U 3 or after typing CTRL C 4 This code causes text to be written at the text cursor, ie in the normal fashion. A MODE change selects VDU4 , normal operation. 5 This code causes text to be written where the graphics cursor is. The position of the text cursor is unaffected. Normally the text cursor is controlled with statements such as PRINT TAB (5,10) ; and the graphics cursor is controlled with statements like MOVE 7 0 0, 450 Once the statement VDU5 has been given only one cursor is active (the graphics cursor). This enables text characters to be placed at any position on the screen. There are a number of other effects: text characters overwrite what is already on the screen so that characters can be superimposed; text and graphics can only be written in the graphics window and the colours used for both text and graphics are the graphics colours. In addition the page no longer scrolls up when at the bottom of the page. Note however that POS and VP OS still give you the position of the text cursor. See chapter 29 for more information. 350 6 VDU 6 is a complementary code to VDU21. VDU21 stops any further characters being printed on the screen and VDU 6 re-enables screen output. A typical use for this facility would be to prevent a password appearing on the screen as it is being typed in. 7 This code, which can be entered in a program as VDU 7 or directly from the keyboard as CTRL G, causes the computer to make a short ‘beep’. This code is not normally passed to the printer. 8 This code (VDU 8 or CTRL H) moves the text cursor one space to the left. If the cursor was at the start of a line then it will be moved to the end of the previous line. It does not delete characters - unlike VD U 1 2 7 . 9 This code (VDU 9 or CTRL I or TAB) moves the cursor forward one character position. 10 This statement (VDU 10 or CTRL J) will move the cursor down one line. If the cursor is already on the bottom line then the whole display will normally be moved up one line. 11 This code (VDU 11 or CTRL K) moves the text cursor up one line. If the cursor is at the top of the screen then the whole display will move down a line. 12 This code clears the screen - or at least the text area of the screen. The screen is cleared to the text background colour which is normally black. The BASIC statement CLS has exactly the same effect as VDU12, or CTRL L. This code also moves the text cursor to the top left of the text window. 13 This code is produced by the RETURN key. However, its effect on the screen display if issued asaVDU13 or PRINT CHR$(13);isto move the text cursor to the left hand edge of the current text line (but within the current text window, of course). 14 This code makes the screen display wait at the bottom of each page. It is mainly used when listing long programs to prevent the listing going past so fast that it is impossible to read. The computer will wait until a SHIFT key is pressed before continuing. This mode is called ‘paged mode’. Paged mode is turned on with CTRL N and off with CTRL O. When the computer is waiting at the bottom of a page both the shift lock and caps lock lights will be illuminated. 15 This code causes the computer to leave paged mode. See the previous entry (14) for more details. 351 16 This code (VDU16 or CTRL P) clears the graphics area of the screen to the graphics background colour and the BASIC statement CLG has exactly the same effect. The graphics background colour starts off as black but may have been changed with the GCOL statement. VDU16 does not move the graphics cursor - it just clears the graphics area of the screen. 17 VDU17 is used to change the text foreground and background colours. In BASIC the statement COLOUR is used for an identical purpose. VDU17 is followed by one number which determines the new colour. See the BASIC keyword COLOUR for more details. 18 This code allows the definition of the graphics foreground and background colours. It also specifies how the colour is to be placed on the screen. The colour can be plotted directly, ANDed, ORed or Exclusive-ORed with the colour already there, or the colour there can be inverted. In BASIC this is called GCOL. The first byte specifies the mode of action as follows: 0 Plot the colour specified. 1 OR the specified colour with that already there. 2 AND the specified colour with that already there. 3 Exclusive-OR the specified colour with that already there. 4 Invert the colour already there. The second byte defines the logical colour to be used in future. If the byte is greater than 127 then it defines the graphics background colour (modulo the number of colours available). If the byte is less than 128 then it defines the graphics foreground colour (modulo the number of colours available). 19 This code is used to select the actual colour that is to be displayed for each logical colour. The statements COLOUR (and GCOL) are used to select the logical colour that is to be text (and graphics) in the immediate future. However the actual colour can be redefined with VDU1 9. For example MODE 5 COLOUR 1 will print all text in colour 1 which is red by default. However the addition of VDU 19,1,4,0,0,0 orVDU 19,1,4;0; will set logical colour 1 to actual colour 4 (blue). The three zeros after the actual colour in the VDU1 9 statement are for future expansion. 352 In MODE 5 there are four colours (0, 1, 2 and 3). An attempt to set colour 4 will in fact set colour 0 so the statement VDU 19,4,4,0,0,0 orVDU 19,4,4;0; is equivalent to VDU 19,0,4,0,0,0 orVDU 19,0,4;0; We say that logical colours are reduced modulo the number of colours available in any particular MODE . 20 This code (VDU 2 0 or CTRL T) resets text and graphics foreground logical colours to their default values and also programs default logical to actual colour relationships. The default values are: Two colour MODEs 0=Black 1= White Four colour MODEs 0=Black l=Red 2=Yellow 3= White 16 colour MODE 0=Black l=Red 2=Green 3=Yellow 4=Blue 5=Magenta 6=Cyan 7= White 8=Flashing black/white 9=Flashing red/eyan 10=Flashing green/magenta ll=Flashing yellow/blue 12=Flashing blue/yellow 13=Flashing magenta/green 14=Flashing cyan/red 15=Flashing white/black 353 21 This code behaves in two different ways. If entered at the keyboard (as CTRL U) it can be used to delete the whole of the current line. It is used instead of pressing the DELETE key many times. If the code is generated from within a program by either VDU21 or PRINT CHR$ (21) ; it has the effect of stopping all further graphics or text output to the screen. The VDU is said to be disabled. It can be enabled with VDU 6 . 22 This VDU code is used to change MODE . It is followed by one number which is the new MODE . Thus VDU2 2 , 7 is exactly equivalent to MODE 7 (except that it does not change HIMEM). 23 This code is used to reprogram displayed characters. The ASCII code assigns code numbers for each displayed letter and number. The normal range of displayed characters includes all upper and lower case letters, numbers and punctuation marks as well as some special symbols. These characters occupy ASCII codes 32 to 126. If the user wishes to define his or her own characters or shapes then ASCII codes 224 to 255 are left available for this purpose. In fact you can redefine any character that is displayed, but extra memory must be set aside if this is done. ASCII codes 0 to 31 are interpreted as VDU control codes - and this chapter is explaining the exact function of each. The full ASCII set consists of all the VDU control codes, all the normal printable characters and a user defined set of characters. 354 For example if the user wishes to define ASCII code 240 to be a small triangle then the following statement would have to be executed: Character to be redefined VDU 23,240,1,3,7,15,31,63,127,255 Eight numbers giving the contents of each row of dots that makes up the desired character = l 2 + 1 = 3 4 + 2 + 1 = 7 8 + 4 + 2 + 1=15 16 + 8 . . . = 31 = 63 = 127 = 255 GO tJ* CM (C CO ’t N h CM CO CO h Note that you cannot define your own characters in MODE 7 . See chapter 29 for a more detailed explanation. As explained above the user may define any ASCII code in the range 224 to 255. To display the resultant shape on the screen the user can type PRINT CHR$ (240) or VDU 240 In the unlikely event of the user wishing to define more than the 32 characters mentioned above (ASCII 224 to 255) it will be necessary to allocate more RAM for the purpose. This is described in chapter 43. VDU 2 3 , 1 can be used to turn the flashing cursor off: VDU 23,1,0;0;0;0; will turn the cursor off and VDU 23,1,1;0;0;0; will turn it on again. Redefine character 355 A third use of VDU2 3 is to permit the advanced programmer to alter the contents of the 6845 CRTC circuit. If the user wishes to place value X in register R this can be done with the command VDU 23 , 0 , R, X, 0 , 0 , 0 , 0 , 0 , 0 The user is cautioned not to do this unless he or she understands how to program the 6845. Note however that when writing to register 7 (vertical synchronisation position) or register 8 (interlace) of the 6845, any offset that has been set up with the * TV statement (chapter 43) will be used to adjust the value sent to R7. 24 This code enables the user to define the graphics window - that is, the area of the screen inside which graphics can be drawn with the DRAW and PLOT statements. The graphics screen is addressed with the following coordinates. 1023 Y 0 Thus the coordinates of A would be approximately 1000,200. When defining a graphics window four coordinates must be given; the left, bottom, right and top edges of the graphics area. Suppose that we wish to confine all graphics to the area shown below. 700 300 0 150 1100 356 The left hand edge of the graphics area has an X value of (about) 150. The bottom of the area has a Y value of 300. The right hand side has X=1100 and the top has Y=700. The full statement to set this area is VDU 2 4, 150; 300; 1100; 700; Notice that the edges must be given in the order left X, bottom Y, right X, top Y and that when defining graphics windows the numbers must be followed by a semi-colon. For those who wish to know why trailing semi-colons are used the reason is as follows: X and Y graphics coordinates have to be sent to the VDU software as two bytes since the values may well be greater than 255. The semi-colon punctuation in the VDU statement sends the number as a two byte pair with low byte first followed by the high byte. 25 This VDU code is identical to the BASIC PLOT statement. Only those writing machine code graphics will need to use it. VDU2 5 is followed by five bytes. The first gives the value of K referred to in the explanation of PLOT in the BASIC keywords chapter. The next two bytes give the X coordinate and the last two bytes give the Y coordinate. Refer to the entry for VDU2 4 for an explanation of the semi-colon syntax used. For example VDU 25, 4, 100; 500; would move to absolute position 100,500. The above is completely equivalent to VDU 25, 4, 100, 0, 244 , 1 26 The code VDU2 6 CTRL Z) returns both the graphics and text windows to their initial values where they occupy the whole screen. This code repositions the text cursor at the top left of the screen, the graphics cursor at the bottom left and sets the graphics origin to the bottom left of the screen. In this state it is possible to write text and to draw graphics anywhere on the screen. 27 This code does nothing. 357 28 This code (VDU2 8 ) is used to set a text window. Initially it is possible to write text anywhere on the screen but establishing a text window enables the user to restrict all future text to a specific area of the screen. The format of the statement is VDU 28 , leftX, bottomY, rightX, topY where leftX sets the left hand edge of the window bottomY sets the bottom edge rightX sets the right hand edge topY sets the top edge 0 12 20 31 For the example shown the statement would be VDU 28,5,20,30,12 Note that the units are character positions and the maximum values will depend on the MODE in use. The example above refers to MODE 1 and MODE 4 . In MODES 2 and 5 the maximum values would be 19 for X and 24 for Y since these MODES have only 20 characters per line. 358 0 ► X 19 0| 1 Y 31 I I 29 This code is used to move the graphics origin. The statement VDU2 9 is followed by two numbers giving the X and Y coordinates of the new origin. The graphics screen is addressed as shown below: 1023 Y 0 0 ►X 1279 To move the origin to the centre of the screen the statement VDU 29, 640; 400; should be executed. Note that the X and Y values should be followed by semi- colons. See the entry for VDU2 4 if you require an explanation of the trailing semi-colons. Note also that the graphics cursor is not affected by VD U 2 9 . 30 This code (VDU 3 0 or CTRL A ) moves the text cursor to the top left of the text area. 31 The code VDU31 enables the text cursor to be moved to any character position on the screen. The statement VDU 3 1 is followed by two numbers which give the X and Y coordinates of the desired position. 359 To move the text cursor to the centre of the screen in MODE 7 one would execute the statement VDU 31,20,10 Note that the maximum values of X and Y depend on the MODE selected and that both X and Y are measured from the edges of the current text window not the edges of the screen. 32-126 These codes generate the full set of letters and numbers in the ASCII set. See the ASCII codes in the Appendices. 127 This code moves the text cursor back one character and deletes the character at that position. VDU1 2 7 has exactly the same effect as the DELETE key. 128-223 These characters are normally undefined and will produce random shapes (see below and chapter 43). 224-255 These characters may be defined by the user using the statement VDU 2 3 . It is thus possible to have 32 user defined shapes such as * VDU 23,224,8,28,28,107,127,107,8,28 * VDU 23,225,8,28,62,127,62,28,8,0 V VDU 23,226,54,127,127,127,62,28,8,0 * VDU 23,227,8,28,62,127,127,127,28,62 Note: You can use a *FX command which will then allow you to define characters 128 to 159 rather than 224 to 255. This has the advantage that you will then be able to use the new characters easily by holding down the SHIFT key while pressing one of the user definable (red) keys (see chapter 43). 360 35 Cassette files This chapter summarises the facilities available for file handling using a cassette recorder. Refer to chapter 5 for an introduction to loading and saving BASIC programs. Cassette motor control Some cassette recorders have a ‘remote’ or ‘automatic’ motor control socket. This can be used with a switch on the microphone to start and stop the tape. If your recorder is of this type then the computer will be able to start and stop the tape automatically at the start and end of each BASIC program or section of recorded data. If your cassette recorder does not have motor control then you will have to start and stop the tape manually. A light is provided on the keyboard to tell you when the tape should be running. This is labelled ‘cassette motor’. When it is on, the tape should be running. The description which follows assumes that you have automatic or remote motor control. If you don’t then you’ll have to start and stop the tape manually. Recording levels Many cassette recorders employ automatic record level. Recorders of this type do not have any ‘record level’ controls. If your recorder does not have automatic record level then you will have to set the record level yourself. Set the control so that the recording level indicator is slightly below the ‘OdB’ level or the red mark. Playback volume and tone It is important that the playback volume is set correctly. You will need to experiment to find the correct level for your machine. The tone control should normally be set to ‘maximum’ or ‘high’. Keeping an index of programs You will be able to record a large number of BASIC programs on a single cassette. However it is vital that the programs do not overlap on the tape. If they do then you will lose one of them. Beware of recording on the blank leader tape - always wind it on a little first. 361 If you forget what is on a tape then you can always use the command * CAT to obtain a ‘catalogue’ of the tape. When you give the command *CAT (and press the PLAY button on the recorder) the tape will play through, and the computer will print a catalogue of all the programs onto the screen. The catalogue gives the program name, the number of blocks (rather like pages in a book) used to record the program and lastly the length of the program (the number of letters in the book). It also checks that the recording is readable and reports any errors. As the catalogue is building up on the screen you will often see something like this SKETCH 02 This indicates that the computer has found a file called SKETCH and that it is currently checking block 2 of that file. The block number is given in hexadecimal not decimal numbers. Press ESCAPE at the end of the tape to get back control of the computer. Saving a BASIC program A program that you have typed into the computer’s memory can be saved onto cassette tape in the following way: 1. Insert the cassette into the recorder. 2. Type SAVE "PROG " and press RETURN. PROG is just an example of a file name; file names are explained later in this chapter. 3. The message RECORD then RETURN will appear. Now use the fast forward and reverse buttons to position the tape at the correct place. 4. Press the RECORD and PLAY buttons on the cassette recorder. 5. Press the RETURN key on the computer to let it know that everything is now ready. 6. The computer will then record your program. 7. The tape will automatically stop when the computer has finished recording your program. You can always abandon this process by pressing ESCAPE. 362 Saving a section of memory This will not be needed by most people writing BASIC programs. It is most often used to record sections of machine code programs. The process is very similar to that employed to record a BASIC program. 1. Insert the cassette in the recorder. 2. Type * SAVE PROG SSSS FFFF EEEE RRRR and press RETURN. 3. Continue as for saving a BASIC program, above. SSSS represents the start address of the data, in hexadecimal (hex). FFFF represents the end address of the data plus one, in hex. As an option the format +LLLL can be used in this position. The plus sign is followed by the length of the data, in hex. EEEE represents the (hex) execution address of the data. If the program is reloaded into the computer using the command * RUN PROG then once loaded the computer will jump to the specified execution address. The execution address is optional and if it is omitted the execution address will be assumed to be equal to the start address. RRRR represents the (hex) reload address. This is optional, but if used the file will reload (using * LOAD , see below) at address RRRR. If RRRR is omitted then the file will reload at address SSSS. Two examples may make the syntax clearer * SAVE patch 6000 6200 * SAVE match 4C00 + 0CE9 2A10 2000 Loading a BASIC program A BASIC program saved on cassette tape can be loaded into the computer’s memory in the following way: 1. Insert the cassette in the recorder. 2. Type LOAD PROG and press RETURN. 363 3. The message Searching will appear. 4. Now use the fast forward and reverse buttons to position the tape at the correct place. 5. Press the PLAY button on the cassette recorder. 6. The computer will give you the message Loading when it finds the correct program. It will then load it into its memory. 7. The tape will automatically stop when the computer has finished loading. When loading a program the usual catalogue-type display will appear. The message Loading will appear when the correct file is found. If the load should fail for any reason a message will appear. Loading a machine code program This will not be needed by most people using BASIC programs. It is used to load special purpose programs. The process is identical to that used to load a BASIC program except that the command is * LOAD PROG AAAA AAAA represents the absolute load address. It is optional but, if included, will force the program to load at the specified address. It therefore over-rides the reload address given when the program was saved. The program will load but not run; control will return to BASIC. Two examples may make the syntax clearer: *LOAD patch 100 MODE 7: * LOAD match 7E80 Loading and running a BASIC program The statement CHAIN allows a BASIC program to LOAD and RUN another program. It is particularly useful when there is a sequence of related programs. The command is used in exactly the same way as LOAD but with the word CHAIN substituted for the word LOAD . 364 1. Insert the cassette in the recorder. 2. Type CHAIN PROG and press RETURN. 3. The message Searching will appear. 4. Now use the fast forward and reverse buttons to position the tape at the correct place. 5. Press the PLAY button on the cassette recorder. 6. The computer will give you the message Loading when it finds the correct program. It will then load it into its memory. 7. The tape will automatically stop when the computer has finished loading and the computer will automatically run the program. Loading and running a machine code program A machine code program (not a BASIC program) can be loaded and run by using the statement * RUN PROG Using a cassette file to provide keyboard input It is possible to get the computer to accept input from a cassette file instead of from the keyboard. In this case the cassette file would contain a set of commands, or answers to questions which a BASIC program would need. The command to force the computer to accept input from a file called edit would be: *EXEC edit File edit above is known as an EXEC file. EXEC files can contain BASIC commands or operating system commands (or both). Some operating system commands are listed in chapter 41. A comment line can be included in an EXEC file using a BASIC REM statement or by beginning the comment with * | . To create a suitable cassette file you will need to use the BASIC statement BPUT# and not PRINT#, since the latter stores things in internal format. The command * SPOOL also creates suitable files - see chapter 37 for how to use it to merge programs. 365 Reading cassette data files Data, as well as programs, may be recorded on cassette tape. This facility enables the user to keep records of names and addresses (for example) on tape for later use. Since the cassette tape can be started and stopped by the computer it also enables it to record results from experiments over many hours. If the user wishes to read a data file then he or she must first open the file for input. In the process of opening a file the computer will allocate a channel number to the operation. If we wished to read in a list of names recorded on a data file called NAMES then the following statement would get the channel number into the variable X . 100 X=OPENIN ( "NAMES " ) Once a file has been opened for input data can be read in from the tape. This can be done in two ways: a chunk at a time (for example a whole name) or a single letter at a time. Data is read in a chunk at a time by using the INPUT#X , A$ statement. This will read the first entry into A$ . To read a single character in use the function A=B GET# (X) . Testing for end of file While reading data in it is useful to test to see if the end of the file has been reached. This is done with the function EOF#. For example: 100 DIM B $ (20) 110 Y=1 120 X=OPENIN ( "NAMES " ) 130 REPEAT 140 INPUT#X,A$ 150 PRINT A$ 160 B $ (Y) =A$ 165 Y= Y+ 1 170 UNTIL EOF #X 180 CLO S E # X 190 END The program above reads up to 20 names off tape, prints them on the screen and then stores them in an array in memory. Of course if there were more than 20 names on file then the program would fail because the array can only hold 20 entries. 366 Storing data on tape The data in the last example was read into an array in the computer. It could then be edited and the corrected version could be rerecorded on cassette. The process of recording the data on the cassette consists of three steps: open the file for output, write out the data and then close the file. The example program records 20 entries back to tape from the array in memory. 200 X=OPENOUT ( "NEWNAMES" ) 210 FOR Y= 1 TO 20 220 PRINT#X, A$ (Y) 230 NEXT Y 240 CLO S E # X 250 END Note that line 200 will make the computer issue the message RECORD then RETURN as it did when saving a BASIC program. The CLOSE# statement will record any remaining data and then stop the recorder automatically. Recording single characters on tape Single characters (bytes) can be placed on tape using the command BPUT#. The following program stores the alphabet on tape. Note that the letter A has an ASCII value of 65 and the letter Z has an ASCII value of 90. 100 X=OPENOUT ( "ALPHABET" ) 110 FOR D= 6 5 TO 90 120 BPUT #X , D 130 NEXT D 140 CLO S E # X 150 END File names File names on cassettes can be up to ten characters long and can include any character except space. 367 Responses to errors If an error occurs during any of the following operations SAVE LOAD CHAIN * SAVE * LOAD * RUN an error message and then the message Rewind tape will be printed on the screen. The user must then rewind the tape a short way and play it again. It is not usually necessary to replay the whole program, but only far enough to load the blocks causing errors. If an error is detected during BGET# BPUT# INPUT# PRINT# *EXEC * SPOOL the tape will stop and an error will be generated. This error can be trapped by BASIC in the usual way. The user can escape at any time by pressing the ESCAPE key. The error numbers generated by the cassette file system are as follows: 216 Data cyclic redundancy check (CRC) error. 217 Header CRC error. 218 An unexpected block number was encountered. 219 An unexpected file name was encountered. 220 Syntax error - for example an illegal *OPT statement. 222 An attempt was made to use a channel which was not opened. 223 End of file reached. 368 Changing responses to errors If the user wishes to change the way the computer behaves when it detects an error during a cassette file operation (eg LOAD or SAVE) then this can be done with the* OPT statement. * OP T by itself resets error handling and the ‘message switch’ to their default values. Default values are given later. * OP T 1 , X is used to set the message switch which controls the detail of the message. X=0 Implies no messages are issued. X=1 Gives short messages. X=3 Gives detailed information including load and execution addresses. *OPT2 , X is used to control the action that the computer takes when it detects an error during a cassette file operation. X=0 Lets the computer ignore all errors though messages may be given. X=1 The computer prompts the user to retry by rewinding the cassette. X=2 The computer aborts the operation by generating an error. *OPT3 , X is used to set the inter-block gap used when recording data using PRINT# and BPUT # . The gap on SAVE is fixed. The value of X determines the gap in 1/10 seconds. If X is set to greater than 127 then the gap is set to the default value. The default values for the *OPT command are: For LOAD, SAVE, CHAIN, *SAVE, * LOAD , *RUN * OP T , 1 Short messages. * OP T 2 , 1 Prompt for retry. *OPT3 , 6 0.6 second inter-block gap. For BGET#, INPUT#, BPUT#, PRINT# *OPT1,0 No messages. * OP T 2 , 2 Abort on error. *OPT3 , 2 5 2.5 second inter-block gap. Note: The effect of the *OPT command is different for each file system. When writing programs that are to run on cassette and disc the user must be fully aware of these different effects. The user is referred to the appropriate Disc Filing System User Guide. 369 Cassette tape format The format of each block of data stored on cassette is given here for those whowish to produce tapes on other machines that may be read into the BBC Microcomputer. - Five seconds of 2400Hz tone. - One synchronisation byte (&2A). - File name (one to ten characters). - One end of file name marker byte (&00). - Load address of file, four bytes, low byte first. - Execution address of file, four bytes, low byte first. - Block number, two bytes, low byte first. - Data block length, two bytes, low byte first. - Block flag, one byte. - Spare, four bytes, currently &00. - CRC on header, two bytes. - Data, 0 to 256 bytes. - CRC on data, two bytes. Notes: 1. Each data block has its own header as shown above. 2. Load and execution addresses should have the top two bytes set to &FF in 8 bit machines. 3. Bit 7 of the block flag is set on the last block of a file. 4. CRC stands for cyclic redundancy check. The header CRC acts on all bytes from the file name to the spare bytes inclusive. The data CRC acts on the data only. CRCs are stored high byte first and are calculated as follows. In the following C represents the character and H and L represent the high and low bytes of the CRC. H=C EOR H FOR X=1 TO 8 T=0 IF (bit 7 of H=l) THEN HL=HL EOR &0810:T=1 HL=(HL*2+T) AND &FFFF NEXT X The above algorithm is not a BASIC program! 370 36 Changing filing systems The previous section dealt in detail with cassette files because the BBC Microcomputer is fitted with all the circuitry to enable cassette recorders to be used to store and to retrieve data. Other filing systems can, of course, be used with the BBC Microcomputer. However, each of these will require additional hardware which will be accompanied by its own detailed instructions. Consequently, details are not given in this guide, for example, of the disc filing system. What is given below, however, is a list of the commands to enable the user to change from one filing system to another. *TAPE *TAPE3 * TAPE 1 2 * D I S C * ADF S ★ NET ★ROM Selects the cassette filing system running at the default speed of 120 characters per second (1200 baud). Selects the cassette filing system running at 30 characters per second (300 baud). Selects the cassette filing system at 1200 baud. This statement selects the disc filing system so that all future file operations (eg LOAD and SAVE ) work to the floppy disc units. Selects the advanced disc filing system is. Selects the Econet filing system for all future file operations. Selects the sideways ROM cartridge system. ★IEEE The IEEE 488 interface filing system. ★TELESOFT The Teletext filing system. 371 37 How to merge two BASIC programs There are a number of ways of merging two BASIC programs. Two methods are given below which will work for both disc and cassette. The line numbers in the two programs should not clash unintentionally. In order to merge two programs it is necessary to save one of them as an ASCII file rather than in the usual compressed format. This ASCII version of the program is then merged into the other program using the command *EXEC. Suppose we wish to merge the program SHORT into the program LONG. First, load in one of the programs LOAD "SHORT" and then create an ASCII version by typing in the next three lines * SPOOL SHORT2 LIST * SPOOL This produces an ASCII version of the program. The ASCII version is called SHORT2 . Now load in the big program by typing LOAD "LONG" and finally merge in the small program by typing *EXEC SHORT2 The command * SPOOL SHORT2 informs the computer that anything that it outputs to the screen is also to be sent to a file called SHORT2 . When the computer lists the file it therefore creates an ASCII listing. The command * SPOOL without a file name is used to end or close the spooled file. 372 Having created the ASCII version of SHORT called SHORT2 , the user then loads the file LONG. The command *EXEC " SHORT2 " tells the computer to read in the file SHORT 2 as if it were getting characters from the keyboard. If the file SHORT 2 contains line numbers (as it does), then these lines will just be added to the BASIC program. Of course, a line number read in from SHORT 2 will replace any line with an identical line number in LONG, so it is necessary to renumber the two programs SHORT and LONG so that their line numbers don’t clash. A quicker method is given below - but if you use this method you must make sure that the second program that you loaded in uses larger line numbers than the first program. You will get surprising results if not. Firstly load the program (with the lower line numbers) in the normal way. Then get the computer to print, in hex, the top-of-program address less two by entering PRINT -TOP-2 Call the resultant number XXXX. Then enter * LOAD SHORT XXXX to load the program SHORT (or whatever you have called it) in above the first program. Finally type END to get the computer to sort its internal pointers out. A typical dialog might look like this - assuming that one program is called ONE and the other is called TWO. >LOAD "ONE" >PRINT~TOP— 2 1 95F >*LOAD TWO 1 95F >END This method is very easy, but you must look after the line numbers. 373 38 Using printers The BBC Microcomputer can be used with the vast majority of printers available today. It is not possible, though, to drive old ‘Teletypes’ which operate at ten characters per second, or printers requiring a 20mA current loop connection. To use a printer with your BBC Microcomputer you will need to do three things: - Connect the printer to the computer. - Tell the computer whether your printer is plugged into the parallel or serial printer port. - Tell the computer to copy everything sent to the screen to the printer. Connect the printer to the computer As the above implies there are two possible sockets (ports) to which you can connect the printer. The parallel printer port is often called a Centronics compatible port. Printers that need to be connected to parallel ports usually have a 36-way Amphenol socket. To connect the printer to the computer you will need a cable made up as follows from a 26-way insulation displacement connector, about one metre of 26-way ribbon cable and one 36-way Amphenol plug. The 26-way cable should be inserted into the 36-way plug so that pins 1 to 14 and 19 to 32 are connected. The cable should be inserted into the 26-way plug so that pin 1 of the Amphenol 36-way plug is connected to pin 1 of the 26-way insulation displacement plug. Amphenol plug A parallel printer cable Both plugs viewed looking at the pins on the underside of the plug body. 375 Parallel printer connections 36-way plug 26-way plug Name pin number pin number Strobe 1 1 Data 0 2 3 Data 1 3 5 Data 2 4 7 Data 3 5 9 Data 4 6 11 Data 5 7 13 Data 6 8 15 Data 7 9 17 Acknowledge 10 19 Ground 19 to 32 All even numbers (except 26) The serial printer port is often referred to as an RS232 or V24 port. The standard connector plug is a Cannon 25-way D type and there is also a standard for the connections to the plug. Predictably however, different manufacturers disagree on the interpretation of the standard! The serial port on the BBC Microcomputer emerges via a 5-pin DIN plug and it will normally be possible to drive a serial printer with just three wires. The views of the two plugs are shown below from the outside of the case and looking into the respective sockets. 5-way DIN plug for computer 3 3 8 S o 5? -6? o * QO Oi CO ^ £ .<* 33 *$§’§ § 3 ucqQ o a; q; £ <5 mum 13 1211 10 9 8 7 6 5 4 3 2 1 fooooooooooo O 6 ) Voooooooooo Q Q J 25 24232221201918171615 14 v Data terminal ready 25-way D type plug for printer 376 Proceed as follows: - Connect a wire between GND and pin 7. - Connect another wire between data out and pin 3. - The position of the third wire varies from printer to printer. - For Qume printers connect CTS to pin 20. - If that doesn’t work then try CTS to pin 4. If you require a more complete RS232 connection try the following: - Data in to pin 2 (transmit data). - Data out to pin 3 (receive data). - CTS to pin 4 (clear to send). - RTS to pin 5 (request to send). - 5 GND to pin 7 (signal ground). Telling the computer whether you are using a serial or parallel printer * F X 5 is used to inform the computer whether it is to send printer output to the parallel or serial port. Type: *FX5 , 1 if you are using a parallel printer * F X 5 , 2 if you are using a serial printer * F X 5 , 4 if you are using the Econet printer The computer defaults to (assumes unless you tell it otherwise, that you are using) a parallel printer. If you have selected a serial printer then you will have to set the baud rate to match that on the printer. The default setting is 9600 baud but many printers run at other rates. Select one of *FX 8 , 1 75 baud * FX 8,2 150 baud *FX 8,3 300 baud * FX 8,4 1200 baud * FX 8,5 2400 baud * FX 8,6 4800 baud *FX 8,7 9600 baud * FX 8,8 19200 baud (this rate is not guaranteed) 377 Telling the computer to copy everything to the printer This is often called turning the printer on - but, of course, we are not referring to switching the printer on but to enabling and disabling its printing. To turn it on type CTRL B and to turn it off type CTRL C . In a BASIC program VDU 2 will turn the printer on and VDU 3 will turn it off. Note that all output will now appear on the screen and printer. To send output to the printer only you can use * F X 3 . For disc systems, * TYPE "filename" will print out a data file. Characters not sent to the printer Some printers automatically move the paper up one line when they receive a carrige return. Since the computer also moves the paper up one line (by using a line feed), the paper may move up two lines instead of one. It is possible to tell the computer not to send any particular character to the printer. The most common requirement is to ignore line feed which has an ASCII code of 10. *FX6 , 10 will set it to ignore line feeds. *FX6 should be set after *FX5 has been used to select the printer type. The default is to ignore line feeds. The complete set-up sequence for a serial printer running at 1200 baud and which does not require line feeds would be: * F X 5,2 Serial printer. *FX 8,4 1200 baud. * FX 6,10 No line feeds (default so it may be omitted). A few more detailed points: Data is transmitted using one start bit, eight data bits and one stop bit. Receive baud rates may be the same as, or may differ from, transmit baud rates. Receive baud rates are set by * F X 7 . The user may supply his or her own specialist printer driver routine. The address of the user routine should be placed at location &222 and the user defined routine can be selected as the printer output routine with *FX5 , 3 . If the user intercepts the operating system write character routine (OSWRCH) then all the VDU control codes must be dealt with. When a BASIC program executes DRAW 10,10 a string of six bytes is sent to the VDU driver via OSWRCH. In this case the bytes would be 25, 5, 10, 0,10,0, so beware! 378 39 Indirection operators Indirection operators are not normally available on personal computers. They enable the user to read and write to memory in a far more flexible way than by using PEEK and POKE. They are intended for programmers using the computer’s assembler or programmers wishing to create their own data structures. There are three indirection operators: Name Purpose Number of bytes affected ? query Byte indirection operator 1 ! shriek Word indirection operator 4 $ dollar String indirection operator 1 to 256 For the sake of illustration let us play with the memory around location &2000 (that is 2000 hex - an appreciation of hex numbers is essential. If you don’t understand hex then you will not need to use indirection operators). Let us set the variable M to &2000 M= &2000 ?M means the contents of memory location M, so to set the contents of &2000 to 40 we write ?M = 40 and to print the contents of memory location &2000 we write PRINT ?M Those familiar with other dialects of BASIC will realise that Y=PEEK(X) becomes Y = ? X and POKE X,Y becomes ?X = Y The query operator acts on one byte of memory only. The word indirection operator (shriek) acts on four successive bytes. For example ! M= & 12345678 would set location &2000=&78 &2001=&56 &2002=&34 &2003=&12 379 and PRINT ~!M (print in hex, shriek M) would give 12345678 Four bytes are used to store integers so the shriek operator can be used to manipulate integer variables. The last operator in this group is the string indirection operator called dollar. Do not confuse $M with M$ as they are quite different. M$ is the usual string variable. On the other hand $M can be used to read or write a string to memory starting at memory location M . Remembering that we have set M = & 2 0 0 0 then $M= " ABCDEF " will place the string ABCDEF and a carriage return (&0D) in memory from location &2000 on. Similarly PRINT $M will print ABCDEF And one last twist to the use of these operators. We have seen how query, shriek and dollar can be used as unary operators - that is with only one operand. We have used M as the operand in the example above - for example M= & 2 0 0 0 ?M= & 4 5 Y=?M PRINT ?M ! M= & 8A124603 Y = ! M PRINT ! M $M=" HELLO ZAPHOD" B$ = $M PRINT $M but in addition both query and shriek can be used as binary operators provided that the left hand operand is a variable (such as M 9 ) and not a constant. Thus M ? 3 means the contents of (memory location M plus 3) — in other words of location &2003. There is a simple routine to examine the contents of memory for 12 bytes beyond &2000. 10 FOR X = 0 TO 12 20 PRINT ~M+X, ~M? X 30 NEXT 380 Line 20 reads ‘print in hex (M plus X) and in the, next column, in hex, the contents of (M plus X)’. It is easy to write this into one of the user defined function keys and keep it for debugging - like this *KE Y 0 FOR X= 0 TO 12: P. ~M+X , ~M?X : NEXT |M but don’t forget that in MODE 7 it will be displayed as *KE Y 0 FOR X= 0 TO 12: P. -fM+X , -fM?X : NEXT ||m just to complicate matters! Here are some illustrations of some of the above. Set up function key 0 and use it to examine memory beyond &2000. > *KE Y 0 FOR X = 0 TO 12: P . ~M+X , ~M?X : N . | M >M= & 2 0 0 0 >FOR X= 0 TO 12 :P. ~M+X , ~M? X : N . 2000 FF 2001 FF 2002 FF 2003 FF 2004 FF 2005 FF 2006 FF 2007 FF 2008 FF 2009 FF 2 0 0 A FF 2 0 0B FF 2 0 0 C FF Use the byte indirection operator to change one byte. >M? 3 = & 3 3 >FOR X= 0 TO 12 :P. ~M+X , ~M? X : N . 2000 FF 2001 FF 2002 FF 2003 33 2004 FF 2005 FF 2006 FF 2007 FF 381 2008 FF 2009 FF 2 0 0 A FF 2 0 0B FF 2 0 0 C FF Use the word indirection operator to change four bytes. >M ! 2=&12345678 >FOR X=0 TO 12 : P . ~M+X , ~M? X : N . 2000 FF 2001 FF 2002 78 2003 56 2004 34 2005 12 2006 FF 2007 FF 2008 FF 2009 FF 2 0 0 A FF 2 0 0B FF 2 0 0 C FF Use the string indirection operator to insert a string into a known place in memory. >$M= " ABCDEFG " >FOR X=0 TO 1 2 : P . ~M+X, ~M?X:N. 2000 41 2001 42 2002 43 2003 44 2004 45 2005 46 2006 47 2 007 D 2008 FF 2009 FF 2 0 0 A FF 2 0 0B FF 2 0 0 C FF 382 Note that interesting structures can be generated using ! and $. For example you may need a structure containing a ten character string, a number and a reference to a similar structure. ! and $ together can do this. If M is the address of the start of the structure then $ M is the string Mill is the number M ! 1 5 is the address of the next structure The tools are therefore provided to enable you to manipulate general tree structures and lists very easily in BASIC. 40 HIMEM, LOMEM, TOP and PAGE These four pseudo-variables give the programmer an indication of the way the computer has allocated the available memory. PAGE and TOP give the bottom and top of the user’s program so PRINT TOP-PAGE can be used to find out how big a program is. HIMEM gives the top of memory so PRINT HIMEM-TOP will indicate how much space is left. When you run the program the computer will need some space to store variables so you cannot use up all the available memory just with your program. Random Access Memory extends from location 0 to location 32767 (in hexadecimal that is &7FFF). RAM is normally allocated in MODE 7 as shown in the diagram. As the user enters more program the program grows, increasing the value of TOP. Normally the computer stores program variables immediately above the user program but this can be changed by resetting the variable LOMEM. Again the computer normally expects to be able to use all the memory up to that set aside for the screen, but the user may move the position of the highest boundary by changing HIMEM. Note that in the ‘shadow screen’ mode, HIMEM always returns a value of &8000 (see chapter 42 for more details). The variable TOP is calculated by the computer on request. It does this by starting at PAGE and working through the program. The user cannot reset the value of TOP but can reset PAGE, HIMEM and LOMEM if needed for some special purpose. On a cassette system PAGE will be set to &0E00. On a disc system (DFS) PAGE would be &1900. On a system with an Econet interface (NFS) PAGE would be &1200. On a system with both disc and Econet interfaces fitted PAGE will be set to &1B00. If the Advanced Disc Filing System (ADFS) fitted, PAGE is &1D00. ADFS with DFS or NFS gives a PAGE value of &1F00, and ADFS with DFS and NFS gives a PAGE value of &2100. 384 The above notes, and the diagram, refer only to machines without a second processor fitted. 41 Operating system statements The BBC Microcomputer includes a large and powerful operating system. This can be accessed from user written machine code routines or from BASIC. If a BASIC statement starts with an asterisk then the whole of the rest of the statement is passed to the operating system directly. Note that the filing systems listed below can only be accessed if your BBC Microcomputer is fitted with the appropriate hardware. Full details of each filing system are contained in their related user guides. The operating system (OS) commands include: * LOAD * SAVE * RUN * CAT *KE Y * OP T *FX *TAPE *TAPE3 ‘DISC * ADF S *NET Loads a section of memory (not a BASIC program) (see chapter 35). Saves a section of memory (see chapter 35). Loads and executes a machine code program (see chapter 35). Displays a catalogue of files on the cassette, disc or Econet disc unit (see chapter 35). Programs one of the user defined keys (see chapters 25 and 43). Determines how the computer reacts to errors during loading of cassettes and the amount of detail given during cassette operations. Enables the user to control a large number of the computer effects such as flash rate (see chapter 43). Selects the cassette filing system running at 1200 baud. Selects the cassette filing system running at 300 baud. This statement selects the disc filing system so that all future file operations (eg LOAD and SAVE) work to the floppy disc units. Selects the advanced disc filing system for all future file operations. Selects the Econet filing system for all future file operations. 386 * SPOOL *EXEC *MOTOR Copies all screen output to a named file (see chapter 37). Uses a named file to provide input as if it had been typed in at the keyboard (see chapters 35 and 37). Used to turn the cassette motor on or off. * T V * ROM * SHADOW * IEEE This can be used to move the whole of the displayed picture up or down the screen and also controls the picture interlace (see chapter 43). The interlace on the BBC Microcomputer is off by default. Selects the sideways ROM filing system. Selects the shadow screen on a subsequent MODE change (see chapter 42). Selects the IEEE 488 interface filing system. *TELESOFT Selects the Teletext filing system. All these OS statements may, if required, have the contents of BASIC variables passed to them as parameters by using OSCLI (see chapter 33). 42 The shadow screen 387 There are two areas of memory in the computer that can be selected for use as display memory: from &3000 (variable, according to the display MODE selected) to &8000 in the main memory map, or an area outside the main memory map known as the ‘shadow RAM’. Using the shadow RAM leaves the whole of user memory (ie from PAGE to &8000) free for your use. Display MODES 128 to 135 use the shadow RAM as the display memory by default, and are the shadow screen equivalents of MODES 0 to 7 (respectively). To enter a shadow display MODE, simply type in its MODE number - for example: MODE 135 RETURN would enter the shadow screen equivalent of MODE 7 . Non-shadow mode may be re-entered by changing to a non-shadow MODE. The following command sequence illustrates the memory map changes which take place when switching between shadow and non-shadow MODEs: MODE 3 P. -HIMEM 4000 MODE 131 P. -HIMEM 8000 MODE 3 P. -HIMEM 4000 (The screen would be cleared by each MODE change). To force the operating system to enter shadow mode on subsequent MODE changes, (even if a non-shadow MODE is selected) type * shadow RETURN To return to non-shadow mode type * shadow 1 RETURN 388 followed by a MODE change to a non-shadow MODE. The following command sequence illustrates the memory map changes which take place when using the * SHADOW commands: MODE 3 P. -HIMEM 4000 * SHADOW P. -HIMEM 4000 MODE 3 P. -HIMEM 8000 MODE 131 would have the same effect as the second MODE 3 command above, although in that case there would be no need for the preceding * SHADOW command. Continuing from the above sequence we have: * SHADOW 1 P. -HIMEM 8000 MODE 3 P. -HIMEM 4000 Substituting MODE 131 for MODE 3 above would give: P. -HIMEM 8000 The * SHADOW commands are useful when switching between software which uses shadow RAM and software which doesn’t. Other shadow mode-related commands *FX1 1 4 produces the same effect as * SHADOW, and *FX1 1 4 , 1 produces the same effect as * S HAD O W 1 . VDU2 2 , <128+n> (used when in non-shadow mode) will select MODE n; shadow mode will not be entered and HIMEM will not be reset. Shadow mode is retained across a soft break, but is reset to non-shadow by a hard break. 389 43 The operating system and how to make use of it What is the operating system? The operating system (or the machine operating system (MOS) to give it its full title) is a program which controls the operation of the computer. The MOS for the BBC Microcomputer lives in ROM, and is activated automatically as soon as the microcomputer is switched on. The MOS runs constantly, performing repetitive tasks with such speed that you are not even aware that anything is going on; indeed, the MOS has already done a tremendous amount of work before you even press a key! The functions performed by the MOS include scanning the keyboard, updating the screen, performing analogue-to-digital conversions (for input from joysticks) and controlling the sound generator.The MOS is a large and complex machine code program (more about machine code later), consisting of a number of routines which perform functions such as those listed above. Like all computer programs, the MOS will not do anything until it is told to (although as already mentioned, just switching the computer on, and leaving it on, tells the MOS to do quite a lot). All the functions performed by the MOS can be made available to other machine code programs, and one machine code program which you will be familiar with is BASIC. BASIC provides an easy way of using the microcomputer’s computing power, and it makes constant use of the MOS routines to get input from the keyboard and reflect that input on the screen. How ever, the degree of control over the MOS provided by BASIC is limited. For example, typing colour 9 RETURN in MODE 2 will cause subsequent screen display to appear in flashing red/cyan, the ‘work’ being done by the MOS routines. However, BASIC provides no way of varying the flash rate. There are three principal ways of achieving fine control of this kind: 1. Using *FX commands 2. Calling operating system routines from BASIC 3. Calling operating system routines from assembly language Although the rest of this chapter covers all of the above three topics, you are referred to the next chapter for a detailed introduction to assembly language. 390 The *FX commands The *FX commands are a family of operating system commands which act directly on the MOS, causing its routines to carry out a variety of functions which are summarised below. The general form of a * F X command is *FXa , x, y where a, x and y are integers. (There can be a space between the X and the a.) For example, the MOS version number can be printed out on the screen by typing *FX0 RETURN Note that in the above command a=0, but x and y are absent. Another example, (which incidentally solves the ‘flashing colour’ problem which was referred to earlier) is *FX9 , 10 which causes the first named colour (red in our example) to be displayed for ten fiftieths of a second (*FX9 , 2 0 would display it for twenty fiftieths of a second, and so on). Note here that the default duration of the first named colour - ie the time for which it is displayed before a *FX9 command is typed in to change the time - is half a second, so restoring the default duration could be achieved by typing *FX 9 , 2 5 RETURN Incidentally, loading the default duration (plus many other default parameter values) into RAM is one of the jobs done by the MOS when the microcomputer is switched on. Typing *FX9 , 0 RETURN would set the duration to infinity, causing the first colour to be displayed continuously, and the second colour not to be displayed at all. Exactly the same effect could be achieved by typing *FX9 RETURN which illustrates the fact that if the x and y parameters are omitted they are taken to be equal to zero. As a final example, *FX1 4 4 , 2 , 1 would move the display two lines up the screen and turn the interlace off (after the next MODE change or soft break). A summary of all the *FX commands and their functions is given later in this chapter. 391 All the *FX commands can be typed in in immediate mode or can be included as a line of a BASIC program. The values typed in for x and y (if any) are copied by the MOS into registers known as X and Y, which reside in the microcomputer’s 6512 microprocessor (see next chapter for more details). Some *FX commands return values in X and Y, but unfortunately the MOS does not copy these returned values back into X% and Y% (it doesn’t even ‘know’ about X% and Y%). However, this drawback can be overcome through the use of a certain operating system call. An operating system call is a means of activating one of the routines which exist within the MOS. Several operating system calls exist, which perform a variety of functions (see chapter 45). The operating system call which we will examine first is OSBYTE. OSBYTE calls from BASIC OSBYTE has call address &FFF4, a memory location within the operating system ROM. This means that when OSBYTE is called, the instruction held in location &FFF4 is executed. This instruction loads an address which is held in a RAM location whose address is given in locations &FFF5 and &FFF6 (&020A for OSBYTE). OSBYTE is said to ‘indirect’ via &20A. Locations &20A and &20B contain the address of (or the ‘vector’ to) the routine within the MOS which is actually executed. This may seem a little complicated, but it does mean (for example) that OSBYTE’s call address will always be &FFF4, and its indirection vector will always be at &20A, no matter how much the MOS changes between different versions. OSBYTE can be called from within BASIC in two ways, one of these being with the CALL BASIC keyword. Try typing the following sequence: A% = 0 X% = 0 Y % = 0 CALL ( &FFF4 ) You will notice that this has exactly the same effect as the *FX0 command. This is because a *FX command and an OSBYTE call are very nearly one and the same thing: the MOS recognises any command beginning *FX as an OSBYTE call, in general, *Fxa, x, y is equivalent to OSBYTE with A=a, X=x and Y=y. A is another register in the 6512 (known as the ‘accumulator’, see chapter 45) whose contents are set equal to the value read by the BASIC interpreter from the resident integer variable A%. As another example, the following program would have the same effect as * FX 9 , 10 (see above): 392 10 MODE 2 20 COLOUR 9 30 A% = 9 40 X%=10:Y%=0 50 CALL ( &FFF4 ) Obviously, this is far more ponderous than simply typing * F X 9 , 10, and indeed the CALL BASIC keyword is mainly used in conjunction with assembly language programs (see chapter 45). A far more powerful method of making operating system calls is through use of the USR BASIC keyword. The above program could be re-written as 10 MODE 2 20 COLOUR 7 30 A% = 9 40 X%=10:Y%=0 50 PRINT —USR (&FFF4) (Note that the foreground colour has been changed to white - it’s a little difficult to read the output of this program in flashing red/cyan!). The effect of line 50 is to set the duration of the first flashing colour as before (which can be confirmed by using COLOUR to select a flashing colour combination), and also to print the following on the screen: 30191909 The string of figures are hexadecimal digits, which have come from registers in the 6512. They have the following meanings (reading from right to left): 09 This has come from register A (the accumulator), which was set up from A% (it is simply the OSBYTE call number; all OSBYTE calls preserve A). 19 This has come from the X register, and is the previous colour duration value. In this case the previous value is the default value, 25. (Remember the digit pairs are hexadecimal, & 19=25 decimal). 19 This has come from the Y register, and contains the same value as the X register. 30 This has come from the P register (the program status register, mainly of interest to those programming in assembly language). Of course, all OSBYTE calls do not have the same exit conditions. The next example performs a totally different function. Note also that the result of the USR call has been assigned to a set of variables, so that it can be printed out in a more readable form: 393 10 REM Uses OSBYTE with A=129 to wait one second for a key 15 REM to be pressed. 20 DIM registers 4 30 DIM A$ (2) : A$ (0) ="A=" : A$ (1) ="X=" :A$ (2) ="Y=" 40 A% = 12 9 50 X%=100 60 Y% = 0 70 ! registers=USR ( &FFF4 ) 80 FOR K= 0 TO 2 90 PRINT A$ (K) ; ~registers?K 100 NEXT K This program uses OSBYTE with A=129 to read a key, which must be pressed within a specified time limit (one second in the example). If it reminds you of BASIC’s INKEY keyword that’s because INKEY uses the same OSBYTE call. If a time limit of n centiseconds is required then X and Y should be set up as follows: X=n AND &FF Y=n DIV &100 The exit conditions vary according to what happens; if no key is pressed within the time limit, X is preserved, Y=&FF. If a key is pressed, X returns the hexadecimal ASCII code of the key character, Y=0. All the *FX calls and OSBYTE calls available on the BBC Microcomputer are detailed at the end of this chapter, preceded by a numerical and a functional summary. Other calls are available to various filing systems which may not be fitted to your machine; these are noted, but you are referred to the appropriate filing system user guide for details. OSBYTE calls from assembly language This section does not attempt to provide an introduction to assembly language and how to use it; you are referred to the next chapter for that. For those unfamiliar with the concept of assembly language, it is safe to assume for now that it is a sort of ‘half way house’ between BASIC and machine code, which is what the 6512 microprocessor actually executes. Programs written in assembly language are more difficult to write than BASIC programs, but they usually take up less memory and are faster than an equivalent BASIC program, which is why large, complex programs such as arcade games will always be written in assembly language. 394 Shown below is an assembly language equivalent of the ‘read a key within one second’ example which we have already seen in BASIC 10 OSBYTE=&FFF4 20 P%=&3500 30 [ 40 LDA# 129 50 LDX# 100 60 LD Y # 0 70 JSR OSBYTE 80 RTS 90 ] 100 CALL & 3 5 0 0 The first thing you will notice about the above program is that lines 10, 20 and 100 seem to be BASIC statements. Indeed they are, the program is really a ‘hybrid’. Line 30 tells BASIC ‘assembly language starts here’, line 90 says ‘assembly language finished’. Line 40 says ‘load the accumulator with decimal 129’. Similarly, lines 50 and 60 load the X register with decimal 100 and the Y register with 0. Line 70 calls OSBYTE, although the machine code assembled from lines 40 to 80 is not actually executed until the program reaches line 100. This description of the program is deliberately brief, but any questions that you may have will be answered in the next chapter. Try typing the program in and running it. You will see that six lines of text appear on the screen (more about that in the next chapter), there is a pause, and then the BASIC prompt reappears. If you run the program again, but press a key immediately after pressing RETURN, you will find that the same thing happens, except that the prompt reappears when you press the key. In both cases the program doesn’t appear to have done anything! In fact, it has carried out exactly the same processing as the BASIC example, but it hasn’t printed any results out on the screen. This illustrates a disadvantage of making an operating system call from assembly language: there is no direct way of printing out the contents of the 6512 registers. (This isn’t as bad as it sounds because real-life applications of assembly language programs, arcade games for example, rarely require register contents to be printed.) The following program gets around this problem (albeit in a limited and rather unsatisfactory way) by making use of another MOS routine, called OSWRCH: 10 OSBYTE =&FFF 4 20 OSWRCH=&FFEE 30 P%=&3500 40 [ 50 LDA# 129 60 LDX# 100 70 LD Y # 0 395 80 JSR OSBYTE 90 TXA 100 JSR OSWRCH 110 RTS 120 ] 130 CALL & 3 5 0 0 Of interest here is line 90, which turns an exit parameter from OSBYTE (in the X register) into an entry parameter for OSWRCH (supplied in the A register). Try running the program. If you don’t press a key after RETURN, then the first character on the last line to be printed out is d, which has decimal ASCII code 100, which was the value loaded into the X register. Pressing a key will cause the key character to be displayed followed immediately by the the BASIC prompt, indicating that the program has finished. What has happened is that OSWRCH has read the value in A, transformed it into a character and printed the character on the screen (without a line feed or carriage return). More details of OSWRCH and all the other operating system calls are given in chapter 45. The foregoing has probably left you somewhat unimpressed by the potentialities of assembly language programming, but hopefully the next chapter will change your views! The *FX commands and OSBYTE calls This section lists and details all the most useful *FX commands and OSBYTE calls. There are others which are recognised by the operating system but there is not sufficient space to document them here. You are referred to the many other publications which are available concerning the BBC Microcomputer for descriptions of these other, somewhat esoteric calls. The decimal and hexadecimal calls which appear in the following tables are the values of a for *FXa, or the values to be loaded into the accumulator for the related OSBYTE call. 396 Functional summary (alphabetical) heading brief description dec hex buffer flush selected class 15 F flush selected buffer 21 15 get buffer status 128 80 insert value into buffer 138 8A get character from buffer 145 91 examine buffer status 152 98 bus read from FRED 146 92 write to FRED 147 93 read from JIM 148 94 write to JIM 149 95 read from SHEILA 150 96 write to SHEILA 151 97 cursor enable/disable cursor edit keys 4 4 read text cursor position 134 86 read character at cursor 135 87 ESCAPE clear ESCAPE condition 124 7C set ESCAPE condition 125 7D acknowledge ESCAPE 126 7E events disable event 13 D enable event 14 E files close *SPOOL and*EXEC files 119 77 check end of file status 127 7F input/output select input device 2 2 select output device(s) 3 3 keyboard auto-repeat delay 11 B auto-repeat rate 12 C read CTRL/SHIFT key status 118 76 memory explode soft character RAM allocation 20 14 select shadow/non-shadow display memory 114 72 read high order address 130 82 read OSHWM address 131 83 read bottom of display RAM address 132 84 read lowest address for particular MODE 133 85 397 operating system print version number 0 0 paged ROM enter language ROM 142 8E printer select printer type 5 5 set printer ignore character 6 6 end of user print routine 123 7B RS423 receive baud rate 7 7 transmit baud rate 8 8 serial enable/disable 6850 ACIA IRQ 232 E8 soft keys enable/disable cursor edit keys 4 4 reset soft keys 18 12 cancel VDU queue 224 E0 set base number for function key codes 225 El set base number for SHIFT function key codes 226 E2 set base number for CTRL function key codes 227 E3 set base number for SHIFT CTRL function key codes 228 E4 sound sound on/off 210 D2 speech read from speech processor 158 9E write to speech processor 159 9F speech on/off 209 D1 return presence of speech processor 235 EB user user OSBYTE call 1 1 VDU read VDU status 117 75 VIA/6522 enable/disable user 6522 IRQ 231 E7 enable/disable system 6522 IRQ 233 E9 video set flash period of first colour 9 9 set flash period of second colour 10 A wait for field synchronisation 19 13 398 Numerical summary Decimal Hex Function 0 0 Prints operating system version number. 1 1 Reserved for application programs. 2 2 Selects input device. 3 3 Selects output devices. 4 4 Enable/disable cursor edit keys. 5 5 Select printer type. 6 6 Set printer ignore character. 7 7 Set RS423 receive baud rate. 8 8 Set RS423 transmit baud rate. 9 9 Set flash period of first colour. 10 A Set flash period of second colour. 11 B Set auto-repeat delay. 12 C Set auto-repeat period. 13 D Disable various events. 14 E Enable various events. 15 F Clear all or just input buffer. 16 10 Select number of ADC channels. 17 11 Force start of conversion on ADC channel. 18 12 Reset user defined function keys. 19 13 Wait for field synchronisation. 20 14 Explode soft character RAM allocation. 21 15 Clear selected buffer. 114 72 Control shadow/main memory selection 117 75 Read VDU status byte. 118 76 Read CTRL/SHIFT key status. 119 77 Close *SPOOL and *EXEC files. 123 7B End of user print routine. 124 7C Reset ESCAPE flag. 125 7D Set ESCAPE flag. 126 7E Acknowledge detection of escape condition. 127 7F Check end of file status. 128 80 Read ADC channel/fire buttons/last conversion. 129 81 Read key within time limit. 130 82 Read machine high order address. 131 83 Read top of operating system RAM address. 132 84 Read bottom of display RAM address. 133 85 Read lowest address for particular MODE . 134 86 Read text cursor position. 135 87 Read character at text cursor position. 399 137 89 Turn cassette motor on/off. 138 8A Insert character into specified buffer. 139 8B Set file options. 140 8C Select cassette file system and set speed. 142 8E Select sideways ROM. 144 90 Alter TV display position/interlace. 145 91 Remove character from buffer. 146 92 Read from I/O area FRED. 147 93 Write to I/O area FRED. 148 94 Read from I/O area JIM. 149 95 Write to I/O area JIM. 150 96 Read from I/O area SHEILA. 151 97 Write to I/O area SHEILA. 152 98 Examine specified buffer. 158 9E Read from speech processor. 159 9F Write to speech processor. 209 D1 Speech on/off. 210 D2 Sound on/off. 218 DA Read/write size ofVDU queue 239 EF Read/write shadow display mode state 224 E0 Cancel VDU queue. 225 El Set base number for function key codes. 226 E2 Set base number for SHIFT function key codes. 227 E3 Set base number for CTRL function key codes. 228 E4 Set base number for SHIFT CTRL function key codes. 229 E5 ESCAPE=&1B. 230 E6 Enable/disable normal ESCAPE key action. 231 E7 Enable/disable user 6522 IRQ. 232 E8 Enable/disable 6850 ACIA IRQ. 233 E9 Enable/disable system 6522 IRQ. 235 EB Return presence of speech processor. 253 FD Last reset type. 255 FF Write start-up option byte. Each * FX and OSBYTE call is now explained in detail. *FX0 prints a message giving the version number of the operating system. *FX1 , n sets the user flag to n. See OSBYTE with A=1 for more details. *FX2 selects the input device from which characters will be retrieved. *FX2 , 0 gets characters from the keyboard and disables the RS423 receiver. *FX2 , 1 gets characters from the RS423 port, and disables the keyboard. *FX2 , 2 gets characters from the keyboard and enables the RS423 receiver. 400 *FX3 is used to select whether output appears (or not) on different output streams. *FX3 is followed by one number, eg *FX3, 4. The second number should be considered in its binary form, because the value of each bit determines the effect of the command. Here is a list of the bits involved, and the effect of each bit when its value is 1. bit 0 enable RS423 driver bit 1 disable VDU driver bit 2 disable printer driver completely bit 3 enable printer driver (as long as bit2 at 0) bit 4 disable SPOOLd output bit 5 not used bit 6 disable printer driver unless character preceded by CTRL A or VDU1 (as long as bitl and bit2 at 0) bit 7 not used Here are some useful values in the table below Printer Screen RS423 *FX3 , 0 enabled on off *FX3 , 1 enabled on on *FX3 , 2 enabled off off *FX3 , 3 enabled off on *FX3 , 4 off on off *FX3 , 5 off on on *FX3 , 6 off off off *FX3 , 7 off off on *FX3 , 8 on on off *FX3 , 9 on on on *FX3 , 10 on off off *FX3 , 11 on off on *FX3 , 12 off on off *FX3 , 13 off on on *FX3 , 14 off off off *FX3 , 15 off off on *FX3,0 to*FX3,3 ‘enable’ the printer, which means that VDU 2 (or CTRL B) will send output to printer. Values from 0 to 3, 8 to 12, 16 to 19 etc are affected by * FX 5 . Values from 16 to 31 are equivalent to values from 0 to 15, except that *SPOOL output is turned off. 401 *FX4 , 0 resets the system so that the cursor editing keys COPY perform their normal cursor editing function. Hsmm and *FX4 , 1 disables the cursor editing and makes the cursor editing keys generate normal ASCII codes. The codes are shown below in both decimal and hex numbers. COPY 135 &87 — 136 &88 137 &89 T 138 &8A m 139 &8B *FX4 , 2 permits the and COPY keys to be programmed in the same way as the user defined function keys. In other words they may contain strings. In this case the * KEY numbers are COPY 11 12 13 7 ] 14 T] 15 * F X 5 is used to select the printer type. *FX5 , 1 selects output to the parallel (Centronics type) output connector. *FX5 , 2 selects the serial RS423 output. *FX5 , 3 selects a user supplied printer driver (see chapter 38). Note that none of these will actually cause output to appear on the printer. VDU2 (or CTRL B ) must be used to start output going to the selected printer channel. * F X 5 , 0 can be used to select a ‘printer sink’ where characters can be lost without the possibility of the system ‘hanging’ with a full printer buffer. *FX6 is used to set the character which will be suppressed by the printer driver routines. Some printers do an automatic ‘line feed’ when they receive a ‘carriage return’. It is therefore useful to be able to prevent the line feed characters from reaching the printer. The ASCII code for line feed is decimal code 10 so this can be achieved with the statement *FX6 , 10 402 Having set this mode it is still possible to get a line feed through to the printer by use of the VDU1 (send next character to printer only) statement. The character following VDU1 is not checked, thus to send a line feed one would use VDU1 , 10 * F X 6 should always be executed after the printer type has been set by * F X 5 . *FX6 , 0 provides no character filtering. Note that the default is that line feed characters are filtered out. *FX7 is used to select the baud rate to be used when receiving data on the RS423 interface, as follows: *FX7 , 1 75 baud receive *FX7 , 2 150 baud receive *FX7 , 3 300 baud receive *FX7 , 4 1200 baud receive *FX7 , 5 2400 baud receive *FX7 , 6 4800 baud receive *FX7 , 7 9600 baud receive *FX7 , 8 19200 baud receive (this rate is not guaranteed) *FX8 is used to select the transmit rate to be used on the RS423 interface. *FX8 , 1 75 baud transmit *FX8 , 2 150 baud transmit *FX8 , 3 300 baud transmit *FX8 , 4 1200 baud transmit *FX8 , 5 2400 baud transmit *FX8 , 6 4800 baud transmit *FX8 , 7 9600 baud transmit *FX8 , 8 19200 baud transmit (this rate is not guaranteed) Note: The standard receive/transmit format adopted for RS423 is one start bit, eight bits, one stop bit. 403 *FX9 and *FX10 are used to set the flash rate of flashing colours. Colours always flash between one colour and its complement. Thus in MODE 2 , COLOUR 9 selects flashing red/cyan. (See chapter 34 for more information on COLOUR.) *FX9 is followed by a number which gives the duration of the first named colour. The duration is given in fiftieths of a second. Thus to select one fifth of a second one would use the statement *FX9 , 10 The default is * F X 9 , 2 5 *FX1 0 is used to set the period of the second named colour. See the entry above for *FX 9 . Note that values of 0 for the duration will eventually force the selected state to occur full time. If neither counter is set to zero then setting the other counter to zero will immediately force the latter colour to appear. The default is *FX10, 25. *FX1 1 If a key is held depressed then after a short delay (the auto-repeat delay) the key will repeat automatically every so often. The delay is given in hundredths of a second. *FX1 1 sets the delay before the auto-repeat starts. * F X 1 1 , 0 turns off the auto-repeat all together. *FX1 1 , 5 would set the auto-repeat delay to five hundredths of a second, etc. *FX1 2 sets the auto-repeat period, that is the amount of time between the generation of each character. * F X 1 2 , 0 resets both the auto-repeat delay and the auto-repeat to their normal values. *FX1 2 , 10 would set the auto-repeat period to ten hundredths of a second thus producing ten characters per second. *FX13 and *FX14 are used to disable and enable certain events. These *FX calls will only be used by programmers writing in Assembly Language and more information on events is given in chapter 44. The summary which follows should be read in conjunction with that chapter. * F X 1 3 , 0 disables output buffer empty event. * F X 1 3 , 1 disables input buffer full event. * F X 1 3 , 2 disables character entering input buffer event. * F X 1 3 , 3 disables ADC conversion complete event. 404 *FX1 3 , 4 disables start of vertical synchronisation event. * F X 1 3 , 5 disables interval timer crossing zero event. *FX1 3 , 6 disables ESCAPE pressed event. *FX1 3 , 7 disables RS423 receive error event. *FX1 3 , 8 disables service/Econet error event. * F X 1 4 , 0 enables output buffer empty event. * F X 1 4 , 1 enables input buffer full event. * F X 1 4 , 2 enables character entering input buffer event. *FX1 4 , 3 enables ADC conversion complete event. * F X 1 4 , 4 enables start of vertical synchronisation event. *FX1 4 , 5 enables interval timer crossing zero event. *FX1 4 , 6 enables ESCAPE pressed event. *FX1 4 , 7 enables RS423 receive error event. *FX1 4 , 8 enables service/Econet error event. The initial condition is that all are disabled. *FX1 5 enables various internal buffers to be cleared (or emptied). * F X 1 5 , 0 clears all internal buffers. *FX1 5 , 1 clears the currently selected input buffer. See also *FX21. As an OSBYTE call this typically executes in 370 microseconds. *FX1 6 is used to select the number of analogue to digital converter (ADC) channels that are to be used. Each ADC channel takes 10ms to convert an analogue voltage into a digital value. Thus if all four ADC channels are enabled then new values for a particular channel will only be available every 40ms. It is, therefore, often wise to enable only the number of channels actually required. *FX1 6 , 0 will disable all ADC channels. * F X 1 6 , 1 will enable ADC channel 1 only. *FX1 6 , 2 will enable channels 1 and 2. *FX1 6 , 3 will enable channels 1, 2 and 3. * F X 1 6 , 4 will enable all four ADC channels. 405 *FX1 7 forces the ADC converter to start a conversion on the selected channel. Thus *FX1 7 , 1 will start a conversion on channel 1. Channels are numbered 1, 2, 3 and 4. OSBYTE call with A=&80 can be used to determine when the conversion is complete as can ADVAL ( 0 ) . * F X 1 8 resets the user defined function keys so that they no longer contain character strings. * F X 1 9 causes the machine to wait until the start of the next frame of the display for animation. *FX2 0 when the machine starts up, space is allocated at &C00 for the redefinition of 32 displayed characters using the VD U 2 3 statement. In the initial state, or after issuing a *FX2 0,0 command, the character definitions are said to be ‘imploded’. This means that an attempt to print any character with ASCII code greater than 128 (&80) will be mapped on to the characters stored at memory &C00. Initially this will produce garbage. Once a user defined character has been defined, the effect of the mapping is to produce the same character for four ASCII codes, each code being offset from its neighbour by 32 (decimal). Therefore, for example, ASCII codes &80, &A0, &C0 and &E0 will all show the same character. Redefining any one of the above codes would also identically redefine the other three. Note that code &80 (or &A0. &C0 or &E0) will be mapped to memory location &C00; code &81 (or &A1, Cl or &E1) will mapped to memory location &C08, etc. Character codes in the range &20 to &7F can also be redefined, but will map (in the *FX2 0 , 0 state) to the same memory locations, thus overwriting what is already there. After a * F X 2 0 , 6 command the character definitions are said to be fully ‘exploded’ and all printing characters (codes &20 to &FF) can be redefined, each group of 32 characters in the range having its own memory allocation (see below). However, until any character in a block is redefined, the block will map on to either the preset ROM (&20 to &7F) or on to memory at &C00 (&80 to &FF). The memory allocated to each block of ASCII codes in the fully exploded state is: ASCII code Memory allocation &20 to &3F &40 to &5F &60 to &7F &80 to &9F &A0 to &BF &C0 to &DF &E0 to &FF OSHWM + &300 to OSHWM + &3FF OSHWM + &400 to OSHWM + &4FF OSHWM + &500 to OSHWM + &5FF &C00 to &CFF OSHWM to OSHWM + &FF OSHWM + &100 to OSHWM + &1FF OSHWM + &200 to OSHWM + &2FF 406 The character definitions may also be partially exploded by using a parameter between 0 and 6. For example, *FX2 0, 1 will allow character codes &80 to &BF to be redefined. Each increment in the parameter allocates another page (256 bytes) of memory above OSHWM to the soft character definitions. *FX2 0 , 4 will allow redefinition of character codes &80 to &FF and &20 to &3F. The following command sequence should serve to make the situation clearer. (Press RETURN after each line.) VDU23, &E2, 1,3, 7, 15, 31, 63, 127, 255 *FX2 0 , 3 VDU23 , &E2, 255, 127, 63, 31, 15, 7, 3,1 VDU&E2 *FX2 0 , 0 VDU&E2 Although the fourth and sixth lines invoke the same character code, a different character is displayed each time due to the effect of the *FX2 0 commands. (Note that *FX2 0 could be typed in instead of *FX2 0 , 0 .) If this is done, and the programmer redefines characters in the range &20 to &3F, he or she must leave memory up to OSHWM + &3FF clear of his or her program. This is done by first finding the value of OSHWM (for the particular configuration in use) by using OSBYTE call 131 and then setting PAGE to a value &400 above this value. Of course, if the programmer only wishes to redefine ASCII codes in the range &80 to &9F (128 to 159) then he or she need not alter PAGE. Codes in the range &80 to &9F are of particular importance since the programmer can generate them directly from the keyboard by holding down SHIFT at the same time as pressing one of the user definable function keys. See later in this chapter for more information on Operating System High Water Mark. *FX2 1 allows any internal buffer to be cleared (or flushed). *FX2 1 , 0 flushes the keyboard buffer. *FX2 1 , 1 flushes the RS423 serial input buffer. *FX2 1 , 2 flushes the RS423 serial output buffer. *FX2 1 , 3 flushes the printer output buffer. *FX2 1 , 4 flushes the sound channel number 0 (noise). * FX2 1 , 5 flushes the sound channel number 1. * FX2 1 , 6 flushes the sound channel number 2. 407 *FX2 1 , 7 flushes the sound channel number 3. *FX2 1 , 8 flushes the speech synthesis buffer. See also *FX15, OSBYTE &80 and OSBYTE &8A. *FX1 1 4 is used to select shadow/main display memory at the next MODE change. *FX 114 selects shadow memory, *FX114 , 1 selects main memory. Note that a MODE change is required before HIMEM will be reset. (See chapter 42 for more details of the shadow screen facility.) *FX119 causes any open files being used as *SPOOLed output or *EXECed input to be closed. * F X 1 2 4 clears the ESCAPE pressed flag (bit 7 of location &FF). *FX12 5 sets the ESCAPE pressed flag. It must be set with this call and not directly. It has a similar effect to pressing the ESCAPE key. (Conditional on any OSBYTE 229 call.) * F X 1 2 6 must be used to acknowledge the detection of an escape condition if the user is intercepting characters fed into the computer through the OSRDCH (Operating System Read Character) routine. * F X 1 4 2 can be used to select one of the six sideways ROM sockets (note that only language ROMs can be selected). The sockets are numbered, counting anticlockwise from the MOS/BASIC socket at the top right, 14/15, 10/11, 8/9, 2/3, 4/5, and 6/7. Each socket is capable of accepting a 32K ROM. The paired socket numbers correspond to the two 16K ‘logical ROMs’ which may exist within the single 32K ‘physical ROM’. If a socket contains a 16K ROM then it can be selected by using either of the two socket numbers. For example, a (16K) language ROM in the socket immediately to the left of the MOS/BASIC socket could be selected by typing *FX142,10 or*FX142,ll. * F X 2 0 9 will turn speech on or off. *FX209, 80 is on, *FX209,1 is off. *FX210 will turn the sound on or off. * F X 2 1 0 , 0 is on, *FX2 1 0 , 1 is off. *FX2 1 8 , 0, 0 will cause the VDU software to forget the bytes it has received so far as part of an (incomplete) VDU (with parameters) statement. *FX2 5 5 will set the keyboard start-up option byte. This value will only take effect on a soft reset. Any other sort of reset (hard or power-on) and the value will be taken direct from the wiring on the keyboard. The links on the keyboard are all ‘unmade’, which ensures that the RAM location which can be accessed by this call (&28F) has all its bits set on start-up or by a hard break. The value contained in &28F and the corresponding keyboard link numbers are as follows: 408 Links 1 and 2 (bits 7 and 6 respectively) are unused. Links 3 and 4 (bits 5 and 4 respectively) set the disc drive timings. The default settings give the slowest disc access - this ensures that the computer can be used with any make of disc drive. Link 5 (bit 3) selects auto or manual boot on (default setting gives manual boot). Links 6, 7, and 8 (bits 2, 1, and 0) select the screen MODE (default settings give MODE 7). For example, *FX255, 203 would give fast discs, manual boot, and MODE 3 on soft reset. OSBYTE calls OSBYTE call with A=&0 (0) Read operating system version number If X=0 on entry then the operating system version number will be returned as an error message. If X is non-zero on entry then on exit X will contain the opeating system series number - for example OS 2.00 would return X=2. On exit A and Y are preserved, C is undefined. OSBYTE call with A=&1 (1) Read/write the user flag On entry, Y=0 for a write operation or Y=&FF for a read operation. For a write operation, X=new value of user flag. On exit, X=previous user flag value, Y=&FF. This call uses OSBYTE with A=&F1 (241), and is not used by the operating system, being left free for user applications. The user flag is stored at location &281 and has default value 0. OSBYTE call with A=&2 (2) Select input device On entry, the value in X determines the input device(s), as follows: X=0 Keyboard selected, RS423 disabled. X=1 RS423 selected, keyboard disabled. X=3 Keyboard selected, RS423 enabled (but not selected.) On exit, X=0 if previous input was from the keyboard, X=1 if previous input was from the RS423. A is preserved, Y an C are undefined. OSBYTE call with A=&3 (3) Select output device On entry, the value in X determines the output device to be selected, as follows: Bit 0 set - Enables RS423 driver. Bit 1 set — Disables VDU driver. Bit 2 set - Disables printer driver. Bit 3 set - Enables printer, independently of CTRL B or CTRL C . Bit 4 set Bit 5 Bit 6 set 409 - Disables spooled output. -Not used. — Disables printer driver unless the output character is preceded by a VD U 1 command (or equivalent). Bit 7 -Not used. The default options (X=0) are: RS423 disabled VDU enabled Printer enabled (if selected by VDU2) Spooled output enabled (if selected by * SPOOL) This OSBYTE call uses OSBYTE with A=&EC (236). On exit, A is preserved, X contains the previous *FX3 status, Y and C are undefined. OSBYTE call with with A=&4 (4) Enable/disable cursor editing keys On entry, the value in X determines the editing keys’ status, as follows: X=0 Cursor editing enabled (default setting). X=1 Cursor editing disabled. The cursor control keys will return normal ASCII codes, see *FX4 , 1 (later in this chapter) for details. X=2 Cursor editing disabled. The cursor control keys act as soft keys with soft key association numbers as detailed under *FX4 , 2 (see later in this chapter). On exit, A is preserved, X contains the previous *FX4 setting, Y and C are undefined. OSBYTE call with A=&5 (5) Select printer type/output channel On entry, the value in X determines the print destination, as follows: X=0 Printer sink (printer output ignored). X=1 Parallel printer (default setting). X=2 Serial printer (RS423 output). This setting will produce the effect of a printer sink if the RS423 is enabled using OSBYTE with A=3. X=3 User supplied printer driver routine, the address of which should be placed starting at location &222. X=4 Econet printer. X=5-255 User supplied printer driver routine (see X=3 above). On exit, A is preserved, X contains the previous *FX5 setting, Y and C are undefined. This call enables interrupts, and is not reset to default by a soft break. 410 OSBYTE call with A=&6 (6) Select character to be ignored by printer On entry, X contains the decimal ASCII code of the character to be ignored. The effect of this call can be suppressed through the use of an appropriate VDU1 statement. The default setting is X=10 (line feed). On exit, A is preserved, X contains the previous *FX6 setting, Y and C are undefined. OSBYTE call with A=&7 (7) Set RS423 receive baud rate On entry, the value in X determines the baud receive rate, as follows: X=1 75 baud X=2 150 baud X=3 300 baud X=4 1200 baud X=5 2400 baud X=6 4800 baud X=7 9600 baud X=8 19200 baud (this rate not guaranteed) On exit A is preserved, X and Y contain the old serial ULA register contents, C is undefined. OSBYTE call with A=&8 (8) Set RS423 transmit baud rate On entry, the value in X determines the baud transmit rate, as follows: X=1 75 baud X=2 150 baud X=3 300 baud X=4 1200 baud X=5 2400 baud X=6 4800 baud X=7 9600 baud X=8 19200 baud (this rate not guaranteed) On exit A is preserved, X and Y contain the old serial ULA register contents, C is undefined. OSBYTE call with A=&9 (9) Set flash rate of flashing colours (first colour) On entry, the value in X determines the duration in centiseconds of the first named colour, X=0 sets the duration to infinity; X=25 is the default setting. On exit, A is preserved, X and Y contain the value of the previous duration setting, C is undefined. OSBYTE call with A=&A (10) Set flash rate of flashing colours (second colour) On entry, the value in X determines the duration in centiseconds of the second named colour. X=0 sets the duration to infinity; X=25 is the default setting. 411 On exit, A is preserved, X and Y contain the value of the previous duration setting, C is undefined. OSBYTE call with A=&B (11) Set keyboard auto-repeat delay On entry, the value in X determines the delay in centiseconds before auto- repeating starts. X=0 disables the auto-repeat facility; X=50 is the default value. On exit, A is preserved, X contains the previous setting, Y and C are undefined. OSBYTE call with A=&C (12) Set keyboard auto-repeat rate On entry, the value in X determines the auto-repeat periodic interval in centiseconds. X=0 resets the auto-repeat delay and rate to their default values; X=8 is the default value. On exit, A is preserved, X contains the previous setting, Y and C are undefined. OSBYTE call with A=&D (13) Disable events On entry, X contains the event code, corresponding to n for the *FX13,n commands (see later in this chapter). Note that disabling an event means that a user-supplied event handling routine will not be activated should the disabled event occur. The default state is all events disabled. See chapter 44 for more information on events. On exit, A is preserved, X and Y contain the old enable state (0=disabled), C is undefined. OSBYTE call with A=&E (14) Enable events On entry, X contains the event code, corresponding to n for the *FX14,n commands (see later in this chapter). Note that enabling an event means that a user-supplied event handling routine will be activated should the enabled event occur. The default state is all events disabled. See chapter 44 for more information on events. On exit, A is preserved, X and Y contain the old enable state (>0=disabled), C is undefined. OSBYTE call with A=&F (15) Flush selected buffer class On entry, the value in X determines the class of buffer to be flushed, as follows: X=0 All buffers flushed X<>0 Input buffer flushed only On exit, the buffer contents are discarded, A is preserved, X, Y and C are undefined. OSBYTE call with A=&10 (16) Select ADC channels which are to be sampled On entry, X contains the number of channels to be sampled, corresponding to n for the *FX16,n commands (see later in this chapter). If n=0, sampling is disabled, if n>4 then n is reset to 4. On exit, A is preserved, X contains the previous setting, Y and C are undefined. 412 OSBYTE call with A=&11 (17) Force ADC conversion on specified channel On entry, X contains the required channel number. If X>4 then X is reset to 4. See also OSBYTE with A=&80 (128). On exit, A is preserved, X is preserved unless it is greater than 4 - in which case X is reset to 4, Y and C are undefined. OSBYTE call with A=&12 (18) Reset soft keys This call resets the user-defined function keys so that they no longer contain character strings. There are no entry conditions. On exit, A and Y are preserved, X and C are undefined. OSBYTE call with A=&13 (19) Wait for field synchronisation This call causes the machine to wait until the start of the next frame of the display. This occurs 50 times per second and can be used for timing or animation. User trapping of IRQ1 (see chapter 44) may stop this call from working. There are no entry conditions. On exit, A and Y are preserved, X and C are undefined. OSBYTE call with A=&14 (20) Explode soft character RAM allocation This call assigns memory blocks for use by soft character defenitions. On entry, the value in X determines the memory block(s) to be assigned, corresponding to n for the *FX20 , n commands (see later in this chapter). See also OSBYTE call with A=&B6 (182). On exit, A is preserved, X contains the new OSHWM (high byte), Y and C are undefined. OSBYTE call with A=&15 (21) Flush specified buffer On entry, the value in X determines the buffer to be flushed, corresponding to n for the *FX21 , n commands (see later in this chapter). See also OSBYTE calls with A=&0F (15) and &80 128). On exit, A and X are preserved, Y and C are undefined. OSBYTE call with A=&72 (114) Control shadow/main memory selection On entry, X=0 selects the shadow display RAM, X=1 selects the main display RAM. Note that the value of HI MEM is not reset by this command. On exit, X contains the previous display memory selection, A is preserved, Y and C are undefined. (See chapter 42 for details of the shadow screen facility). 413 OSBYTE call with A=&75 (117) Read VDU status byte This call returns the VDU status byte (which contains various status flags) in the X register. The bits in X are as follows: — Bit 0 setifVDU2 sent. Cleared by VDU 3. - Bit 2 set if paged mode on. Cleared if paged mode off. — Bit 3 set if software scrolling. Cleared if hardware scrolling. (Software scrolling is used when text windows have been defined whereas hardware scrolls are used when the whole screen scrolls.) - Bit 4 set if shadow display mode selected. — Bit 5 set when cursors are joined by VD U 5 . - Bit 7 set if VDU disabled by VD U 2 1 . OSBYTE call with A=&76 (118) Read CTRL/SHIFT key status This call returns with the carry bit set if the CTRL key is pressed, and with the negative bit set if the SHIFT key is pressed. Machine code routines may branch on these conditions. OSBYTE call with A=&77 (119) Close any SPOOL or EXEC files This call closes any open files being used as *SPOOLed output or *EXECed input to be closed. This call also performs a paged ROM call with A=&10. On exit, A is preserved, X, Y and C are undefined. OSBYTE call with A=&7B (123) End of user print routine This call is used by the user print routine to indicate to the MOS that it has finished its task. (Cancels a *FX5 , 3 command.) OSBYTE call with A=&7C (124) Reset ESCAPE flag This call clears any ESCAPE condition without any further action (ie no buffers are flushed and no open EXEC files are closed). The Tube (if it is active) is informed. The ESCAPE flag is stored as the top bit of location &FF, and should never be interfered with directly. This OSBYTE call can only be usefully entered from within an assembly language program, since BASIC itself resets the ESCAPE flag. There are no entry conditions; on exit, A, X and Y are preserved, C is undefined. OSBYTE call with A=&7D (125) Simulate ESCAPE condition This call partially simulates the ESCAPE key being pressed (conditional on any OSBYTE 229 call). The Tube (if it is active) is informed. An ESCAPE event is not generated. Note that if this OSBYTE call is made from BASIC, BASIC will reset the ESCAPE flag (see OSBYTE call A=125). There are no entry conditions; on exit, A, X and Y are preserved, C is undefined. 414 OSBYTE call with A=&7E (126) Acknowledge detection of an ESCAPE condition This call attempts to acknowledge the existence of an ESCAPE condition. It should be used if you are using OSRDCH (see chapter 44) to intercept characters being fed into the computer, and it is also useful as part of a user-supplied ESCAPE condition service routine. If an ESCAPE condition is detected, all active buffers will be flushed and any open EXEC files will be closed. There are no entry conditions; on exit, X=255 if the ESCAPE condition existed (X=0 otherwise), A is preserved, Y and C are undefined. OSBYTE call with A=&7F (127) (EOF#) This call returns the end of file status of a file which has been previously opened. On entry X should contain the channel number allocated to the file. On exit X will be zero if the end of file has not been reached and X will be non-zero if the end d OSBYTE call with A=&80 (128) Read ADC channel (ADVAL) This call returns the most recent value of a particular analogue to digital converter channel. It can also be used to detect an end of conversion and to see if the games fire buttons are pressed. On entry X contains the channel number to be read. If X is in the range 1 to 4 then the specified channel will be read and on exit the 16 bit value will be returned in X and Y. Y will contain the eight most significant bits and X the least significant bits. If on entry X=0 then on exit Y will contain a number in the range 0 to 4 indicating which channel was the last to complete. Note that *FX16 and *FX1 7 reset this value to 0. A value of zero indicates that no channel has completed. Also on exit the two least significant bits of X will indicate the status of the two fire buttons. The user should always AND X with 3 to mask out high order bits. If on entry X contains a negative number (in twos complement notation) then the call will provide information about various input buffers. On entry Y must contain &FF. 415 X on entry Buffer checked 255 Keyboard buffer 254 RS423 serial input buffer 253 RS423 serial output buffer 252 Printer output buffer 251 Sound channel 0 (noise) 250 Sound channel 1 249 Sound channel 2 248 Sound channel 3 247 Speech buffer On exit X contains a number giving, for input buffers, the number of characters in the buffers. For output buffers X contains the number of spaces still free in the buffer. OSBYTE call with A = &81 (129) Read key within time limit The call waits for a character from the current input channel until a time limit expires or it tests a particular key. The BASIC function INKEY uses this call. The programmer is reminded that this call will immediately obtain characters if the user has ‘typed ahead’. It is therefore often necessary to flush the input buffer with * F X 1 5 , 1 before using this call. The maximum time delay is passed to the subroutine in X and Y. The delay is measured in hundredths of a second and Y contains the most significant byte and X the least significant byte. The maximum delay is &7FFF hundredths of a second which is about five and a half minutes. On exit, Y=0 if a character was detected within the time limit. In this case X contains the character. Y=&1B indicates that ESCAPE was pressed. This must be acknowledged with *FX12 6. Y=&FF indicates a time-out. If on entry Y contains a negative number then the routine can be used to check for a specific key closure. See the BASIC keyword INKEY for more information. As an OSBYTE call a ‘check for character waiting with zero delay’ takes typically 130 microseconds. OSBYTE call with A=&82 (130) Read machine high order address The BBC Microcomputer uses a 6512 processor (a close relative of the 6502) which requires a 16 bit address. However a number of routines require a 32 bit address - for example most file system addresses are 32 bits wide to ensure compatibility with future products. A specific high order address - that is the top 16 bits of the total 32 bit address - is allocated to the present BBC Microcomputer. The high order address is returned in X and Y with Y containing the most significant byte and X the least significant byte. 416 OSBYTE call with A=&83 (131) Read top of OS RAM address (OSHWM) The Machine Operating System uses memory from page zero up to store operating system variables. The exact amount of RAM needed depends, for example, on whether or not the disc operating system is in use. This call is used to return the address of the first free location in memory above that required for the operating system. The value is returned in X and Y with Y containing the most significant byte and X containing the least significant byte. 1 0A% = 131 30PRINT ~USR(&FFF4) >RUN B10E0083 & 0 E 0 0 is the value returned, & 8 3 is the OSBYTE code. OSBYTE call with A=&84 (132) Read bottom of display RAM address This call returns, in X and Y, the lowest memory address used by the screen display or by special paged ROMs. It indicates the top of free RAM that BASIC can safely use. HIMEM is normally set to this value when using the MODE statement. As usual, Y contains the most significant byte and X the least significant byte of the result. If this call is made following a * SHADOW command, &8000 will only be returned following a MODE change. (See chapter 42 for details of the shadow screen facility). OSBYTE call with A=&85 (133) Read lowest address for particular MODE This call returns, in X and Y, the address of the start of memory that would be set aside if a particular display MODE were to be selected. Certain paged ROMs might also affect the value returned. The display MODE to be investigated is passed in X. This call does not change MODE s, it merely investigates the possible consequences of doing so. If this call is made following a *SHADOW command, it will return &8000 without the need for a MODE change. (See chapter 42 for details of the shadow screen facility). OSBYTE call with A=&86 (134) Read text cursor position This call returns in X and Y the X and Y coordinates of the text cursor. A similar function is performed in BASIC by POS and VPOS. As an OSBYTE call thistakes typically 100 microseconds. 417 OSBYTE call with A=&87 (135) Read character at text cursor position On exit X will contain the character at the text cursor’s position and Y will contain a value representing the current graphics display MODE . If the character cannot be recognised then X will contain zero. The following function could be used to read the character at position X,Y in a BASIC program. 2000 DEF FNREADCH (X, Y) 2010 LOCAL A% , LAS TX , LASTY , C 2020 LAS TX = PO S 2030 LAS T Y = VP OS 2040 VDU 3 1 , X , Y 2050 A% = 1 3 5 2060 C = USR (&FFF4) 2070 C=C AND &FFFF 2080 C=C DIV &100 2090 VDU 31, LASTX , LASTY 2100 = CHR$ (C) The call takes typically 120 microseconds. OSBYTE call with A=&89 (137) Motor control This call is similar to the *MOTOR statement in BASIC. If only one cassette recorder is in use then setting X=0 will turn the motor off X=1 will turn the motor on The cassette filing system (CFS) controls the motor using this OSBYTE call and sets Y as follows: Y=0 for write operations Y=1 for read operations As a result the user can easily implement a dual cassette system by trapping any OSBYTE call with A=&89 and activating (via his or her own hardware) the second recorder for, say, all write operations. The normal internal motor control could then be activated for all read operations. 418 OSBYTE call with A=&8A (138) Insert character into specified buffer This enables characters to be inserted into any buffer. On entry X contains the buffer number and Y contains the character to be inserted. Buffer numbers are as follows: 0 Keyboard buffer 1 RS423 serial input buffer 2 RS423 serial output buffer 3 Printer output buffer 4 Sound channel 0 buffer 5 Sound channel 1 buffer 6 Sound channel 2 buffer 7 Sound channel 3 buffer 8 Speech buffer Therefore to place the letter R (ASCII code 82) into the keyboard input buffer you would use *FX1 3 8 , 0,82 In machine code the X register must contain the buffer number and the Y register the character, eg 10 DIM GAP 20 20 OSBYTE = &FFF4 30 P % = GAP 40 [ 50 LDA #138 60 LDX #0 70 LDY #82 80 JSR OSBYTE 90 RTS 100 ] 110 120 CALL GAP >RUN 1B82 1B8 2 A 9 8 A LDA #138 1B8 4 A2 00 LDX #0 1B8 6 A0 52 LDY #82 1B8 8 20 F 4 FF JSR OSBYTE 1B8B 60 RTS (see next chapter for more details of machine code and assembly language.) 419 OSBYTE call with A=&8B (139) File options This call is directly equivalent to *OPT and it controls the computer’s response to errors during file operations such as LOAD and SAVE. See chapter 35 for more details. For example, with the cassette filing system *FX139, 1, 0 would issue no messages during file operations. *FX139,2,2 would make the computer abort if any error were detected. *FX1 3 9 , 3, 5 would result in 0.5 second inter-block gaps. On entry X contains the option number and Y contains the particular selected option. Thus LDA #139 LDX #1 LDY #0 JSR &FFF4 would ensure that no error messages were issued during file operations. OSBYTE call with A=&8C (140) Tape speed This call is directly equivalent to *TAPE which selects the cassette file system and the baud rate to be used. On entry X contains a number to set the baud rate. X=0 Default rate (1200 baud) X=3 300 baud X=12 1200 baud OSBYTE call with A=&90 (144) TV This call is functionally equivalent to *TV. The contents of the X register are used to control the vertical position of the screen display. For example, setting X=2 would move the display two text lines up the screen. Setting X=253 would move the display three lines down the screen. The contents of Y should be either 0 or 1 on entry. Y=0 gives an interlaced display and Y=1 gives a non-interlaced display. Note that the offset and interlace mode selected only come into effect at the next MODE change. The values set stay in force until a hard reset. Interlace is off on power up. 420 * TV is used as follows. * T V on its own is equivalent to *TV0 , 0 and will turn the interlace on at the next MODE change or BREAK. * T Vx will also turn the interlace on while giving a vertical offset of x. To maintain interlace off when giving an offset you must use *TVx, 1 (interlace on may make the picture flicker on some TVs or monitors). OSBYTE call with A=&91 (145) Get character from buffer This enables characters to be removed from various input buffers. On entry X indicates the buffer from which the character is to be extracted. On exit Y contains the character and C=0 if a character was successfully removed. If the buffer was empty then C will be 1. The buffer numbers are as follows: X=0 Keyboard buffer X=1 RS423 input buffer OSBYTE calls with A=&92 to &97 Read or write to memory mapped input/output This group of calls is used to read or write data from or to the various memory mapped input/output devices. It is vital that users use these routines rather than attempting to address devices directly. The use of these routines will ensure that programs will work whether they are executed in the input/output processor (the BBC Microcomputer) or in a second processor. If the user insists on addressing I/O ports directly (eg STA &FE 6 0) then he or she will have to rewrite programs when the system is expanded. Considerable effort has been expended to ensure that suitable routines are provided to enable the Assembly Language programmer to expand his or her system painlessly. Please learn to use the facilities provided! There are three memory mapped input/output areas and these are named FRED, JIM and SHEILA. SHEILA contains all the machine’s internal memory mapped devices, such as the analogue to digital converter, and should be treated with considerable respect. FRED and JIM, on the other hand, address external units connected to the 1MHz expansion bus. 421 Name Memory address range OSBYTE call Read Write FRED &FC00-&FCFF &92(146) &93Q47) JIM &FD00-&FDFF &94(148) &95Q49) SHEILA &FE00-&FEFF &96(150) &97Q51) On entry to the routines A contains the OSBYTE call number and X the offset within the page. If a byte is to be written it should be in Y. An application note entitled ‘BBC Microcomputer application note no. 1 - 1MHz bus’ explains suggested memory allocations for FRED and JIM. It can be purchased from Acorn Computers. SHEILA addresses the devices with the offsets shown in the table. The user should be aware that the computer expects to service interrupts from all except the user port on the B side of the 6522. The A side is used for the parallel printer interface. Routines are provided (such as OSBYTE and OSWORD calls, etc) to handle these devices. The only circuit that the user should need to handle directly is the 6522 user port. Information about other ports is given for information only — not to encourage you to access the circuits directly. You would be well advised to use the numerous routines provided wherever possible. Further details about some ports are given in the section in chapter 45 which deals with input/output. 422 SHEILA addresses (offset from &FEOO) Hex Integrated Register Description offset circuit destination Write Read 00 6845 CRTC Address register 01 6845 CRTC Register file 08 6850 ACIA Control register Status register 09 6850 ACIA Transmit data register Receive data register 10 Serial ULA Control register 20 Video ULA 21 Video ULA 30 LS161 Paged ROM/RAM ID 34 Shadow RAM select (top bit) 40 6522 VIA MOS input/output 60 6522 VIA ORB/IRB Output register ‘B’ Input register ‘B’ 61 6522 VIA ORA/IRA Output register ‘A’ Input register ‘A’ 62 6522 VIA DDRB Data direction register ‘B’ 63 6522 VIA DDRA Data direction register ‘A’ 6522 VIA T1C-L T1 low-order latches T1 low-order counter 65 6522 VIA T1C-H T1 high-order counter 66 6522 VIA T1L-L T1 low-order latches 67 6522 VIA T1L-H T1 high-order latches 68 6522 VIA T2C-L T2 low-order latches T2 low-order counter 69 6522 VIA T2C-H T2 high-order counter 6A 6522 VIA SR Shift register 6B 6522 VIA ACR Auxiliary control register 6C 6522 VIA PCR Peripheral control register 6D 6522 VIA IFR Interrupt flag register 6E 6522 VIA IER Interrupt enable register 6F 6522 VIA ORA/IRA Same as register 1 except no ‘handshake’ 80 8271 FDC Command register Status register 81 8271 FDC Parameter register Result register 82 8271 FDC Reset register 83 8271 FDC Illegal Illegal 84 8271 FDC Write data Read data 80 1770 FDC Drive select 84 1770 FDC Control Status 85 1770 FDC Track Track 86 1770 FDC Sector Sector 87 1770 FDC Data Data AO 68B54 ADLC CR1/SR1 Control register 1 Status register 1 A1 68B54 ADLC CR2/SR2 Control register 2/3 Status register 2 A2 68B54 ADLC TxFIFO/ RxFIFO Transmit FIFO, continue Receive FIFO A3 68B54 ADLC TxFIFO/ RxFIFO Transmit FIFO, terminate Receive FIFO CO |tPD7002 Data latch, A/D start Status Cl ADC High byte of result C2 423 A few examples will help to clarify the use of these calls. *FX1 4 7 , 5 , 6 would write to FRED+5 (&FC05) the value 6. Similarly LDA # & 9 7 LDX #&62 LDY # &FF JSR &FFF4 would write &FF into location &FE62. An OSBYTE call with A=&97 will write to SHEILA. The base address of SHEILA is &FE00 and to this is added the offset in X (&62). The value written is contained in Y. The net effect is to write to the 6522 data direction register and to cause all the PB lines to become outputs. OSBYTE call with A=&98 (152) Examine specified buffer This call examines a buffer. The buffer number (same as for OSBYTE calls 21 and 138) must be in X, and the call returns as follows: - Carry bit set if buffer empty. - Carry bit clear if characters present in buffer. - Y contains the next character which will be returned if the buffer is read. (Note that the character in Y has not actually been removed from the buffer.) OSBYTE call with A=&9E (158) Read from speech processor See Speech System User Guide. OSBYTE call with A=&9F (159) Write to speech processor See Speech System User Guide. OSBYTE call with A=&D1 (209) Enable/disable speech This call reads (or writes to) location &261, which contains the value sent to the speech processor when speech is output. A value of &50 is the default value, representing speech on. Writing to the location with X=1 will turn speech off. To write to &261, set X=new value, Y=0. To read the location, set X=0, Y=255. On exit, X=the contents of location &261 (if reading), or the previous value held (if writing). The contents of the next location (ie &262) are returned in Y. OSBYTE with A=&D2 (210) enable/disable sound This call reads (or writes to) location &262, which contains the sound status. A value of zero is the default value, representing sound on. Writing to this location with X=1 will turn sound off. To write to &262, set X=new value, 424 Y=0. To read the location, set X=0, Y=255. On exit, X=the contents of location &262 (if reading), or the previous value held (if writing). The contents of the next location (ie &263) are returned in Y. OSBYTE with A =&DA (218) Read/write number of items in VDU queue This call reads (or writes) the number of VDU parameters which are still expected. To read the location containing the value of this parameter (&26A), set X=0, Y=&FF. To write to the location, set X=new value, Y=0. Setting X=0, Y=0 gives a useful way of abandoning a VDU queue, otherwise writing to &26A is not recommended. On exit, X contains the 2’s complement negative number of bytes still required for the execution of a VDU command. The contents of next location (ie &26B) are returned in Y. OSBYTE call with A=&EO (224) Cancel VDU queue Many VDU codes expect a sequence of bytes. For example, VDU19should be followed by five bytes. This call signals the VDU software to throw away the bytes that it has received so far. On entry X and Y must contain zero. The next group of OSBYTE calls (&E1 to &E8) can be used to read or write status information. The calls read the current value of the status being investigated, AND the value with the contents of Y, EOR the result with the contents of X and then write the value back. If V represents the particular status in the computer then V becomes (V AND Y) EOR X. This sequence enables V to be read or be written to and enables any single bit, or group of bits of V, to be set, cleared or inverted. If, on entry, X=&00 and Y=&FF the net effect will be to read the value of V into X without altering V. On the other hand if Y=&00 then the contents of X will be written into V. To set a single bit of V without altering other bits set all the bits of Y to 1 except the specified bit. Clear all the bits of X to 0 except the specified bit. For example, to set bit 0 of V to 1 use Y=&FE and X=&01. To clear a single bit in V set all the bits of Y to 1 except the specified bit and set X=0. For example, to clear bit 0 of V use Y=&FE and X=&00. Of course, many bits may be set, cleared, inverted or examined at the same time. OSBYTE call with A=&E1 (225) Set base number for function key codes Normally the red function keys can be programmed to produce strings of characters by, for example, the statement * KEY 0 PRINT 425 As an alternative the keys can produce a single ASCII code. The statement *FX2 2 5 , 240 would set the base value for the function keys to 240 thus causing key fO to produce ASCII code 240, fl to produce 241 and so on to f9 which would produce 249. This enables these keys to produce ASCII codes for user defined characters. *FX2 2 5 , 1 returns the keys to their normal function of generating strings. *FX2 2 5 , 0 makes the keys have no effect. On entry Y must contain zero. OSBYTE call with A=&E2 (226) Set base number for SHIFT function key codes Pressing one of the function keys while the SHIFT key is pressed will normally produce ASCII codes in the range 128 to 137. These values were chosen with the Teletext codes in mind. Shift function key ASCII code Teletext effect fO 128 Nothing fl 129 Red alphanumeric f2 130 Green alphanumeric f3 131 Yellow alphanumeric f4 132 Blue alphanumeric f5 133 Magenta alphanumeric f6 134 Cyan alphanumeric f7 135 White alphanumeric f8 136 Flash f9 137 Steady These codes are said to have a ‘base value’ of 128 since key fO produces a code 128. If the user wishes, the base value of the ASCII codes can be changed by using this call: *FX2 2 6 , 144 This would set the SHIFT function key codes to produce equivalent graphics color Teletext codes. The default setting is *FX2 2 6 , 128 On entry Y must contain zero. 426 OSBYTE call with A=E3 (227) Set base number for CTRL function key codes See the entry for OSBYTE &E2. The default base value is 144. OSBYTE call with A=&E4 (228) Set base number for SHIFT CTRL function key codes See entry for OSBYTE &E2. The default is that these key combinations have no effect. Note: Remember that pressing CTRL and SHIFT together stops screen output while they are pressed. OSBYTE call with A=&E5 (229) ESCAPE key gives ASCII code This call can be used to make the ESCAPE key generate an ASCII code (27 or &1B) instead of interrupting a BASIC program. If X=0 then the ESCAPE key interrupts the BASIC program (*FX2 2 9,0). On the other hand, *FX22 9, 1(X=1) causes the key to generate its ASCII code. On entry Y must contain zero. OSBYTE call with A=&E6 (230) Enable/disable normal ESCAPE key action When a BASIC program is interrupted by pressing the ESCAPE key, or by *FX12 5, all internal buffers will be cleared. This call can be used to stop the flushing of all internal buffers when a program is stopped. On entry Y must contain zero. *FX2 3 0 , 0 will permit all buffers to be flushed. *FX2 3 0 , 1 will ensure that no buffers are flushed. OSBYTE call with A=&E7 (231) Enable/disable user 6522 IRQ This call sets a ‘mask’ byte which the operating system uses when servicing IRQs which may have originated in the 6522 which is used for the parallel printer and user port. The operating system ANDs the mask byte with the 6522 interrupt flag register AND the 6522 interrupt enable register. Setting the mask to zero would prevent the operating system from handling the 6522 interrupts thus leaving them available to be handled by user supplied routines. Additionally sideways ROMs may handle (via the operating system) interrupts generated from the B side of the 6522. The mask byte could hide those interrupts from the sideways ROMS. The mask byte defaults to &FF. 427 OSBYTE call with A=&E8 (232) Enable/disable 6850 AC I A IRQ This call sets a ‘mask’ byte which is used by the operating system when servicing IRQs which may have originated from the 6850 ACIA used for the RS423 and cassette interfaces. The operating system ANDs the mask byte with the 6850 status register. See the entry above (OSBYTE &E7) for further comments. The default value is &FF. OSBYTE call with A=&E9 (233) Enable/disable system 6522 IRQ As for &E7 but affects the system 6522. The system 6522 is used extensively in the normal operation of the machine and consequently this call should be used with extreme care. OSBYTE call with A=&EB (235) Presence of speech processor This call returns presence of speech processor. X=&FF if speech processor present X=0 if speech processor not present OSBYTE with A=&EF (239) Read/write shadow mode state This call reads (or writes to) location &27F, which contains the shadow mode flag. To read the flag, set X=0, Y=255. To write to &27F, set X=new value, Y=0. On exit, X=0 if in shadow mode, X=1 otherwise (reading). If writing, X returns the previous value of the flag. The contents of next location (ie &26C) are returned in Y. (See chapter 42 for details of the shadow screen facility.) OSBYTE call with A=&FD (253) Last reset type This call returns a number indicating what sort of reset last occurred. Y = 0 Soft break Y = 1 Power-on break Y = 2 CTRL BREAK 428 44 An introduction to assembly language Machine code and the assembler The heart of any computer, the part that actually does all the processing, is the central processor unit (CPU). It is important to realise that no matter what language is typed in at a computer’s keyboard (sometimes called the source language), the only language that the CPU understands is machine code. Of course, the exact form of the machine code depends upon the source language and also upon the type of CPU, but generally speaking any machine code instruction would look something like this: 1010100110000001 This hardly looks like an intelligible instruction, and even rewriting it as two pairs of hexadecimal digits hardly makes it look any better: A9 81 However, to the BBC Microcomputer’s 6512 microprocessor, A9 means ‘load the accumulator’, and the whole of the above instruction means ‘load the accumulator with hexadecimal 81’. (The accumulator is one of six registers which reside in the 6512 microprocessor.) The ‘A9’ part is known as the ‘operation code’ or opcode for short, and the 81 is the ‘operand’. A few more hex machine code instructions with their meanings are: A2 64 ‘load the X register with &64’ A0 00 ‘load the Y register with zero’ 20 F4 FF ‘jump to the subroutine at location &FFF4’ You will probably agree that whilst it would be possible to write programs using opcodes like those shown above, it would be extremely tedious. What’s more, since the 6512 only carries out one instruction at a time, a sequence of instructions like that shown above would have to be ‘poked’ into contiguous locations in RAM. As a final indignity, if you spent a long time writing a machine code program which turned out not to work, it would be extremely difficult to trace the errors - unlike BASIC, machine code does not generate error messages automatically. Fortunately, the BBC Microcomputer contains a program which will generate machine code instructions for you. It’s available as part of the BBC BASIC language, and is known as the assembler. The language that the assembler understands is simply known as assembly language, or 6502 assembly 429 language in the BBC Microcomputer’s case. (The 6512 is a close relative of the 6502, and uses the same assembly language.) The assembly language statements which would generate the machine code statements shown above are: LDA #12 9 (=A9 81, ‘load the accumulator with &81) LDX #10 0 (=A2 64, ‘load the X register with &64’) LD Y #0 (=A0 00, ‘load the Y register with zero’) JSR &FFF4 (=20 F4 FF, ‘jump to the subroutine at location &FFF4’) Clearly, an instruction like ‘LDX’ looks a lot more like ‘load the X register’ than ‘A2’ does. LDA, LDX, LDY and JSR are all known as ‘assembly code mnemonics’. 6502 assembly language has 56 mnemonics, some of which will be discussed in detail later in this chapter. Uses of assembly language BASIC is a very easy language to use; most of its statements look much like English, and it is very ‘friendly’, since it gives you helpful messages about mistakes that you may have made in your program. The price to be paid for this ‘friendliness’ is in memory usage and speed of execution. Assembly language is used where high speed is vital (such as in an arcade game) or where the minimum amount of memory must be used. BASIC is used where ease of programming is more important than either speed or memory usage. The main features of 6502 assembly language It is not the purpose of this chapter to teach you how to program in assembly language; you are referred to the large number of books that have been published which are concerned with the BBC Microcomputer and how to program it. This chapter does, however, describe the most important features of assembly language. Below is an example of a simple hybrid BASIC/assembly language program, which you may have already seen in the previous chapter: 10 OSBYTE=&FFF4 20 P%=&3500 30 [ 40 LDA #129 50 LDX #100 60 LDY #0 70 JSR OSBYTE 80 RTS 90 ] 100 CALL & 3 5 0 0 430 If you have not already seen this program, then what it does is to wait one second for a key to be pressed. If no key is pressed within one second then nothing happens (command mode is returned to). If a key is pressed within one second, its character is printed on the screen and command mode is returned to immediately. Line 10 simply assigns the value &FFF4 to the variable called OSBYTE. If you are familiar with operating system calls then you will recognise OSBYTE as the name of a MOS rotine and &FFF4 as its call address. Line 20 assigns the value &3500 (an address, as it happens) to the resident integer variable P%. When the program is run, the value in P% is transferred by the machine operating system (MOS) into a register in the 6512 microprocessor called the ‘program counter’ (abbreviated to PC). The program counter is one of six registers which reside in the 6512. The registers, their abbreviations and their uses are listed below. The 6512 registers Program counter - PC (PCL - low byte, PCH - high byte) A 16-bit register which contains the address of the next instruction to be executed. In the example shown above, the initial value of PC is taken from P%. The contents of the program counter are altered by ‘jump’ and ‘branch’ instructions, thereby diverting the flow of the program. Accumulator - A An 8-bit general purpose register used for all arithmetic and logical operations. X register - X An 8-bit general purpose register often used to contain entry and exit parameter values for MOS routine calls, also used to contain the ‘offset’ for indexed addressing modes (see later), or as a counter. Y register - Y An 8-bit general purpose register often used to contain entry and exit parameter values for MOS routine calls, also used to contain the ‘offset’ for indexed addressing modes (see later), or as a counter. 431 Program status register - P An 8-bit register set up and used by the microprocessor itself. Each bit has its own meaning, concerned with the results of arithmetic and logical operations, and with interrupt status. A detailed description of this register is beyond the scope of this manual. Of occasional interest is the state of bit 0, the carry flag (abbreviated to C). This is set if a carry occurs during an add operation, and is cleared if a borrow occurs during a subtract operation. Following an operating system call, the state of C has a significance which varies according to the particular call involved. Stack pointer - SP An 8-bit register which contains the least significant byte of the address of the next free stack location (the most significant byte is always &01). The stack is a portion of memory (&100-&1FF) used for the temporary storage of data (such as return addresses from subroutines). Data is ‘pushed’ onto the stack in sequence, then removed by ‘pulling’ it off again. The last byte to be pushed on is the first byte to be pulled off (this is often referred to as a ‘last in, first out’ queue). The assembler delimiters T and T, and general assembly language syntax rules Assembly language statements within a BASIC program must be enclosed between a pair of square brackets (see lines 30 and 90 in the above example). When the BASIC program is RUN, the assembly language statements between the square brackets are assembed into machine code, which is inserted into memory starting at the address specified by P%. An assembly language program consists of a number of assembly language statements, separated by new lines or colons (as in BASIC). Each assembly language statement consists of an optional label (which must always be preceded by a dot), followed by an instruction. An instruction consists of a three letter assembly language mnemonic followed by an operand (or an address) (both of the latter may be absent depending upon the mnemonic). If a label is included, it must be separated from the mnemonic by at least one space. There need not be any spaces between the mnemonic and the operand. Any character following the operand and separated by at least one space from it will be ignored by the assembler which will move on to the next colon or line for the next statement. A comment may be placed after the operand and should be preceded by a backslash (\). Any text following a backslash in an assembly language statement will be ignored by the assembler up to the next colon or end- of-line. 432 Line 40 of the example program could therefore be re-written as: .start LDA #129 \load accumulator with OSBYTE number or as .start LDA#129\load accumulator with OSBYTE number (Here, the label . start would have no fuction, but would not affect the processing.) Addressing modes Most assembly language instructions require data to work on, which must be provided in the operand field of the assembly language statement. Often, this data is an address. The assembler allows several different methods of providing these addresses or data to be used, these methods being known as addressing modes. Not all assembly language instructions can use all the addressing modes; see the table in the appendix for more details. Implicit addressing This is the simplest form of addressing, which does not require an address to be supplied at all; the address is implied by the instruction itself. For example an RTS instruction (see line 80 above) always causes the processor to jump to the bottom two bytes of the stack, which contain the return address to the main program. Immediate addressing and zero page addressing Line 40 of the example is LDA #129 which means ‘load the accumulator with decimal 129’ (hexadecimal 29 could be loaded by the statement LDA # & 2 9 ). Here the statement uses the data supplied in the operand field without having to look for it in memory, hence the name ‘immediate addressing’. The data can be supplied as a variable, hence LDX fvalue would load the X register with the value of the variable value . The role of the # character in the above examples is important, since it indicates to the assembler that immediate addressing is to be employed, using the data supplied immediately to the right of the # . The instruction LDA 129 means something quite different; it means ‘load the accumulator with the contents of memory location 129’. The computer’s main memory is divided into 433 256 ‘pages’ each of 256 bytes. Page 0 extends from location 0 to location 255 (ie any address with the two most significant bytes set to zero), hence the addressing mode exemplified above is known as ‘zero page addressing’. The assembler will automatically select zero page addressing mode (if appropriate) when the address supplied is less than &100 (decimal 256). Absolute addressing This addressing mode is very similar to zero page addressing, except that any address in memory can be specified. For example, LDA & 3 4 5 6 would load the accumulator with the contents of memory location &3456, LDA 8200 would load the accumulator with the contents of memory location 8200. Indirect addressing This addressing mode uses an address which is stored in memory. Only two assembly language mnemonics use this mode, ADC and JMP . For example, JMP ( & 2 0 1 0 ) means ‘jump to the location whose address is held in &2010 (least significant byte) and &2011 (most significant byte)’. Note that indirect addressing is indicated to the assembler by enclosing the address in brackets. Indexed addressing This addressing mode exists in several forms, all of which share the common feature that two addresses are given: a base address and an offset, or index. An example is shown below: LDA & IF 0 0 , X This means ‘load the accumulator from the value held at &1F00+X’. X is used here as the index register; Y could be used in the same way. This form of indexed addressing is known as absolute indexed addressing. Note that X (or Y) must be in the range 0-255. Zero page, X indexed addressing is similar except that the base address must be in page zero. For example, LDY & 7 4 , X means ‘load the Y register with the contents of (&74+X)’ The assembler automatically uses this mode, if available, if a page zero address is specified in the operand field. Note that the offset must be supplied in the X register, except for the LDX mnemonic when the Y register can (and must) be used. Note that the sum of the base address plus the offset in this mode will 434 always be taken as an address in page zero. If the address moves out of page zero the processor will perform a ‘wrap-around’ operation to take it back into page zero - for example an address of &102 would be wrapped around to &002. Another type of indexed addressing is pre-indexed indirect addresing. An example of the instruction format is: LDA (682, X) This instruction adds the address in the X register to &82 to give a new address; the contents of the location at this new address, and the contents of the location above it, together supply the full 16-bit address from which the accumulator is loaded. This addressing mode is designed for use with a table of addresses in zero page locations. For example, if we have ?&70=&00 ?&71=&20 ?&72=&FF ?&73=&21 then LDX #0 LDA (670, X) (671+X) LDX #2 \ set X to 2 LDA (670, X) \ A= ? 6 2 1 FF , ie address in (672), (673) Note that the base address, and the base address plus the offset must be in page zero (the wrap around operation described above still applies). The Y register cannot be used for this addressing mode. The mode is called pre-indexed because the index is first added to the base address to give the address of the pair of locations which hold the address loaded into the accumulator. In post-indexed indirect addressing, the 16-bit address held in the location pair given by the base address is extracted first, and the index is then added to this address to give the address of the location from which the accumulator is loaded. An example of the instruction format is: \ set X to zero \ A=?62000, ie address in (670+X), LDA (680) , Y 435 Note that in this addressing mode the Y register is used as the index (X cannot be). The ‘zero page’ and ‘wraparound’ comments given above still apply. An example of its use is shown below. The program shown sets 128 memory locations to &FF, starting at the address contained in locations &80 (low byte) and &81 (high byte): 10 ?&80=&00 : ?&81=&28 20 DIM GAP 50 30 P % = GAP 40 [ 50 LDY #0 60 LDA # &FF 70 . loop STA ( & 8 0 ) , Y in & 8 0 and & 8 1 80 INY 90 CP Y #128 100 BNE loop 110 RTS 120 ] 130 CALL GAP (Lines 20, 30 and 130 are explained below.) Relative addressing Relative addressing is the addressing mode used by assembly language branch instructions. In the above example, the mnemonic at line 100 is such an instruction. If the condition tested at line 100 is not satisfied, ie if Y=128, the next instruction to be executed will be the one being pointed to by the program counter - the instruction at line 110. If however the condition is satisfied, ie if Y<>128, the processor will decrement the program counter so that it points to the insruction labelled by loop, and this instruction will be the next one to be executed. In the above case, the program counter has to be decremented by 7 (this will become apparent from a careful study of the machine code printed on the screen when the above program is RUN). The address of the instruction labelled by loop is -7, relative to the address held in the program counter. This relative addressing is carried out by the assembler, and normally goes unnoticied by the programmer. There are, however, limits to the number of bytes which can be jumped forward or back; a branch of up to 127 bytes forward or 128 bytes back from the Program counter value (at the time the branch instruction is being executed) is allowed. Care should therefore be taken that labels are not too far from the instructions that branch to them, otherwise an ‘Out of range’ error will result. (Remember that each instruction may be 1, 2 or 3 bytes long.) \ set loop index to zero \ set value to be loaded \ ? (&2800+Y) =&FF, base addr . \ Y=Y+1 \ end of loop reached? \ if not, go to loop 436 Accumulator addressing Th final addressing mode used by the assembler is known as accumulator addressing. This is where the accumulator is addressed rather than a memory location, and is specified by placing A in the operand field. For example: ASL A means ‘shift the contents of the accumuator one bit to the left’. Note that this means that ‘A’ cannot be used as a variable name within an assembly language program. Finally, it must be re-emphasised that each assembly language mnemonic can only use some of the addressing modes detailed above; see the table in the appendix for the addressing modes used by each instruction. Placing machine code programs in memory When the assembler is creating a machine code program the code produced is placed in memory starting from the address in P% (unless 0% is used, see OPT below). The assembler updates the value in P% as it is assembling, and at the end of an assembly operation the value in P% represents the address of the first Tree’ memory location after the machine code program. In the example program shown above, the value of P% is allocated directly (at line 20). This method of setting up P% is somewhat dangerous, since you have to be sure that location &3500 and (in this case) the nine locations beyond &3500 do not contain anything important. A much safer way of setting up P% is to dimension a block of memory using a variation of the BASIC DIM keyword. The example program below illustrates this feature, and other features used with assembly language: 5 REM Uses assembly code to change to mode 4 and draw a triangle 10 OSWRCH=&FFE3 20 DIM GAP % 100 30 DIM data &1C 40 FOR opt % = 0 TO 3 STEP 3 50 P % = GAP % 60 [ 70 OPT opt % 80 .entry LDX#0 \ set data block offset to zero 90 .loop LDA data, X \ load VDU parameter from data block 100 JSR OSWRCH \ perform VDU command 110 INX \ increment offset 120 CPX #&1C \ has all data been loaded? 437 130 BNE loop \ 140 RTS \ 150 ] 160 NEXT opt % 170 !data=&04190416 180 data ! 4=&00C800C8 190 data! 8=&00C80119 200 data! &C=&01190000 210 data! & 1 0 = & 0 0 ADFF 9C 220 data! & 1 4 = & FF 9C 0 1 1 9 230 data! & 1 8 = & 0 0 0 0 FF 5 3 240 CALL entry This program uses OSWRCH to perform operations equivalent to BASIC’s VDU command, drawing a triangle on the screen in MODE 4 . Note the use of indexed addressing at line 90 to load values from a table. Extensive use is made of indirection operators (see chapter 39 for details). The first pass of the assembly language loop is equivalent to VDU & 1 6 , & 0 4 (see line 170, reading from the right) or VDU 2 2,4 (ie change to MODE 4). Note line 20. This use of the DIM statement causes (in this case) 100 bytes of memory to be reserved, the start address of the block being transferred to the variable GAP % . The start address will always be greater than the value of TOP and below the start of screen memory, and so is ‘safe’ (the allocation is made in the same way as the allocation of a BASIC variable). The value in GAP % is transferred to P% at line 50, and so gives the address where the machine code will be assembled. Line 30 uses DIM to reserve 28 memory locations for the data table to be used by the program, the start address of the table being stored in the variable data. Although for most applications machine code will be assembled and run at the same place in memory, it is possible to assemble code at one location and run it at another. Setting the resident integer variable 0% to an address (and at the same time setting the pseudo variable OPT to a certain value, see below) will cause the machine code to be assembled at that address but assembled to run at the address given by P%; in other words the program counter is still controlled by P% during assembly. 0% can be used, for example, to get a machine code program to run in the I/O processor which has been assembled in a second processor. See the description of the OPT keyword below for further details. if not, load next item return to BASIC 438 OPT, forward referencing and two-pass assembly Take a look at the following program: 10 DIM GAP 50 20 P % = GAP 30 addre s s = & 7 0 40 1 50 LDA address \ Load accumulator with contents of address 60 CMP #0 \ Compare accumulator contents with zero 70 BEQ zero \ Jump to label if accumulator contents zero 80 STA address+1 \ Store accumulator contents at addre s s + 1 90 . zero RTS 100 1 In this program, line 70 is known as a ‘forward reference’, since it refers to a label which doesn’t appear until line 90. If the above program is run, the error message No such variable at line 70 will appear. This is because the assembler has not allocated an address to the label zero yet. The reason for the error message is that a ‘two-pass assembly’ is required. The first pass allocates addresses to all labels; the second pass can then jump to the correct memory location when a ‘jump to label’ instruction is encountered. A two-pass assembly is controlled using the OPT command, as shown below: 10 DIM GAP 50 20 addre s s = & 7 0 30 FOR pass=0 TO 3 STEP 3 35 P % = GAP 40 1 45 OPT pass 50 LDA address \ Load accumulator with contents of address 60 CMP #0 \ Compare accumulator contents with zero 70 BEQ zero \ Jump to label if accumulator contents zero 80 STA address+1 \ Store accumulator contents at addre s s + 1 90 . zero RTS 439 100 ] 110 NEXT 120 CALL GAP Here, the assembly language is enclosed in a loop such that two passes are made, with the value of OP T as 0 on the first pass and 3 on the second pass. (The same device is employed in the triangle drawing example above.) The values which are assigned to OPT have the following effects: 0 Assembler errors suppressed, no listing 1 Assembler errors suppressed, listing 2 Assembler errors reported, no listing 3 Assembler errors reported, listing So, in the above example, OPT = 0 on the first pass so there will be no listing and no errors reported. This allows the forward referenced label to be identified without the assembly being interrupted. On the second pass, OPT = 3 and so a listing of the compiled code is produced, along with any programming errors. Note that the assignment statement P% = GAP must be enclosed within the loop so that it is reset before each pass. If the above program is RUN, its assembly and execution should now be successful. (The program is trivial; it merely loads a byte from the memory location specified by address, and, if the byte is non-zero, transfers it to the next memory location). Setting OPT equal to 4, 5, 6 or 7 has the same effect as setting it to 0, 1, 2 or 3 (respectively) except that the code is placed in memory starting at the address supplied in 0%, rather than that in P%. Note that OPT is not an assembly language mnemonic, but a so-called ‘pseudo- operation’, or assembler directive. It tells the assembler to do something, but is not an assembled instruction. The EQUate facility EQU is a pseudo-operation, which allows data to be incorporated into the body of an assembly language program. The EQU operations available are: EQUB equate byte reserves one byte of memory EQUW equate word reserves two bytes of memory EQUD equate double word reserves four bytes of memory equate string reserves memory as required EQUS 440 These operations set the reserved memory locations to the values specified in the operand field. The operand field may contain a string (in double quotes) or a string variable for the EQUS operation, or a number or a numeric variable for the other EQU operations. The assembler will use the least significant part of the value if too large a value is specified. As an example of the use of EQUD, lines 30 and 170 to 230 of the triangle drawing example could be replaced with: 141 .data EQUD &04190416 142 EQUD &00C800C8 143 EQUD &00C80119 144 EQUD &01190000 145 EQUD &00ADFF9C 146 EQUD &FF9C0119 147 EQUD &0000FF53 The following example program illustrates the effects of including each of the EQU pseudo-operations within an assembly language program: 10 P%=&3000 20 A$= " stringvar " 30 [ 40 EQUS "string" 50 EQUS A$ 60 EQUB 180 70 EQUW 12500 80 EQUD 6E6 90 ] >RUN 3000 3000 73 74 72 69 6E 67 EQUS "string" 3006 73 74 72 69 6E 67 76 61 72 EQUS A$ 3 0 OF B 4 EQUB 180 3010 D 4 30 EQUW 12500 3012 80 8D 5B 00 EQUD 6E6 The above printout shows that location &3000 contains &73, the hexadecimal ASCII code for ‘s’, &3001 contains the hexadecimal ASCII code for ‘t’, etc 441 Machine code entry points The BBC Microcomputer is unusual in a number of respects, not least because of the care taken to ensure that everything that can be done by programs written in the input/output processor (the BBC Microcomputer) can also be done in the second processor which is on the far side of the Tube . If a piece of machine code alters a particular memory location that controls the screen display directly, then that same piece of machine code will not work in the second processor because the screen will not be affected by any memory location in the second processor. It is vital that programmers avoid reading and writing to specific memory locations such as the screen memory, zero page locations used by BASIC, and memory mapped input/output devices. System calls are provided to enable you to access all these important locations and use of these system calls will ensure that your programs interact successfully with the machine. Don’t feel that we are trying to hide anything from you - on the contrary we are offering you access to all the I/O routines that BASIC uses! Cultivate the habit of using system calls and then you will not need to rewrite your code when you move it to the second processor. 442 45 The operating system calls Machine code user programs should communicate with the operating system by calling routines in the address range &FF00 to &FFFF. These routines then call a specific internal routine whose address may change in different operating systems. The address of the specific routine is held in RAM between locations &200 and &2FF. The user may change the address held in these RAM locations to intercept any operating system call he or she wishes. Thus the ‘output the character in A’ routine is entered at &FFEE in all environments. The routine indirects through location &20E in all machines. The contents of locations &20E and &20F will vary depending on the machine and the version of the operating system. In one particular machine the address in &20E and &20F is &E0A4 which is the local internal address of the normal ‘output the character in A’ routine. Parameters are passed to the routines in various ways using either the 6512 A, X and Y registers, zero page locations or a parameter block. All routines should be called with a JSR and with the decimal flag clear (ie in binary mode). In the detailed descriptions which follow A refers to the accumulator; X and Y refer to the registers; C, D, N, V and Z refer to the processor flags. The table on the next page gives a summary of operating system calls and indirect vectors. Files Files are treated as a sequence of eight bit bytes. They can be accessed in one operation (using OSFILE) or in blocks (using OSGBPB) or a byte at a time (using OSBGET and OSBPUT). The following attributes may be associated with each file. Load address is the address in memory to which the file should normally be loaded. This can be over-ridden when the file is loaded, if necessary. Execution address is meaningful only if the file contains executable machine code, in which case it is the address where execution should start. If the file contains a high level language program then the execution address is unimportant. Length is the total number of bytes in the file. It may be zero. Pointer is an index pointing to the next byte of data that is to be processed. The value of ‘pointer’ may be read or written (using OSARGS), and it does not indicate whether the appropriate byte has yet been transferred from file to memory or vice versa. Pointer is automatically updated by OSBGET and OSBPUT. 443 OSWRSC Writes a byte (contained in A) to the screen. The display memory location to be written should be set up in &D6 (LSB) and &D7 (MSB). Y (on entry) is used to contain an offset from this address. The effect of this call can be illustrated (somewhat crudely!) by the program shown below: 10 MODE 7 20 ?&D7=&7C 30 Y%=255 : A%=&45 40 FOR J=0 TO 127 STEP 2 50 ? & D 6 = J 60 CALL ( &FFB3 ) 70 NEXT J On exit, A, X and Y are preserved, C is undefined. OSRDSC Reads a byte from the screen, the display memory location concerned being contained in &F6 (LSB) and &F7 (MSB). The byte is returned in A. The following program illustrates the effect of this call: 10 MODE 7 20 FOR K=1 TO 255 30 PRINT "E"; 40 NEXT K 50 ?&F7=&7C 60 VDU14 70 FOR J=0 TO 127 80 ? &F 6 = J 90 PRINT ~USR ( &FFB 9 ) 100 NEXT J This routine can also be used to read bytes from paged ROM, with Y (on entry) set equal to the ROM socket number. In this context, this call has in the past been referred to as OSRDRM. OSFIND Opens a file for writing or reading and writing. The routine is entered at &FFCE and indirects via &21C. The value in A determines the type of operation. A=0 Causes a file or files to be closed. A=&40 Causes a file to be opened for input (reading). A=&80 Causes a file to be opened for output (writing). A=&C0 Causes a file to be opened for input and output (random access). 444 Routine Vector Summary of function Name Address Name Address UPTV 222 User print routine EVNTV 220 Event interrupt FSCV 2 IE File system control entry OSWRSC FFB3 - - Write byte to screen OSRDSC FFB9 - - Read byte from screen OSFIND FFCE FINDV 21C Open or close a file OSGBPB FFD1 GBPBV 21A Load or save a block of memory to file OSBPUT FFD4 BPUTV 218 Save a single byte to file from A OSBGET FFD7 BGETV 216 Load a single byte to A from file OSARGS FFDA ARGSV 214 Load or save data about a file OSFILE FFDD FILEV 212 Load or save a complete file OSRDCH FFEO RDCHV 210 Read character (from keyboard) to OSASCI FFE3 - - Write a character (to screen) from A plus LF if (A)=&0D OSNEWL FFE7 - - Write LF,CR (&0A,&0D) to screen OSWRCH FFEE WRCHV 20E Write character (to screen) from A OSWORD FFF1 WORDV 20C Perform miscellaneous OS operation using control block to pass parameters OSBYTE FFF4 BYTEV 20 A Perform miscellaneous OS operation using registers to pass parameters OSCLI FFF7 CLIV 208 Interpret the command line given /IRQ2V 206 Unrecognised IRQ vector /IRQ IV 204 All IRQ vector /BRKV 202 Break vector USERV 200 Reserved If A=&40, &80 or &C0 then Y (high byte) and X (low byte) must contain the address of a location in memory which contains the file name terminated with CR(&0D). On exit A will contain the channel number allocated to the file for all future operations. If A=0 then the operating system was unable to open the file. 445 If A=0 on entry then a file, or all files, will be closed depending on the value of Y. Y=0 will close all files, otherwise the file whose channel number is given in Y will be closed. On exit C, N, V and Z are undefined and D=0. The interrupt state is preserved, however interrupts may be enabled during the operation. OSGBPB The operating system call to get or put a block of bytes to or from a file which has been opened with OSFIND. The routine is entered at &FFD1 and vectors via &21A. This call is not available on the cassette filing system, and is fully documented in the appropriate disc filing system user guides. OSBPUT Writes (puts) the byte in A to the file previously opened using OSFIND. The routine is entered at &FFD4 which indirects through &218. On entry Y contains the channel number allocated by OSFIND. On exit A, X and Y are preserved, N, V and Z are undefined and D=0. The interrupt state is preserved but interrupts may be enabled during the operation. OSBGET Gets (reads) a byte from a file into A. The file must have been previously opened using OSFIND and the channel number allocated must be in Y. The routine is entered at &FFD7 which indirects via &216. On exit C=0 indicates a valid character in A. C=1 indicates an error and A indicates the type of error, A=&FE indicating an end of file condition. X and Y are preserved, N, V and Z are undefined and D=0. The interrupt state is preserved but interrupts may be enabled during the operation. OSARGS This routine enables a file’s attributes to be read from file or written to file. The routine is entered at &FFDA and indirects via &214. On entry X must point to four locations in zero page and Y contains the channel number. if Y is non-zero then A will determine the function to be carried out on the file whose channel number is in Y. A=0 Read sequential pointer. A=1 Write sequential pointer. A=2 Read length A=&FF Ensure that this file is up to date on the media. 446 If Y is zero then the contents of A will determine the function to be carried out. A=0 Return, in A, the type of filing system in use. The value of A on exit has the following significance: 0 No filing system. 1 1200 baud cassette filing system. 2 300 baud cassette filing system. 3 Sideways ROM filing system. 4 Disc filing system. 5 Econet filing system. 6 Teletext filing system 7 IEEE filing system 8 Advanced Disc Filing System A=1 Return address of the rest of the command line in the zero page locations. A=&FF Ensure that all open files are up to date on the media. On exit X and Y are preserved, C, N, V and Z are undefined and D=0. The interrupt state is preserved but interrupts may be enabled during the operation. OSFILE This routine, by itself, allows a whole file to be loaded or saved. The routine is entered at &FFDD and indirects via &212. On entry A indicates the function to be performed. X and Y point to an 18 byte control block anywhere in memory. X contains the low byte of the control block address and Y the high byte. The control block is structured as follows from the base address given by X and Y. 447 OSFILE control block 00 01 Address of file name, which must be terminated by &0D LSB MSB 02 Load address of file LSB 03 04 05 MSB 06 Execution address of file LSB 07 08 09 MSB 0A Start address of data for write operations, LSB 0B or length of file for write operations OC 0D MSB 0E End address of data, that is byte after last LSB OF byte to be written or file attributes 10 11 MSB The table below indicates the function performed by OSFILE for each value of A. A=0 Save a section of memory as a named file. The file’s catalogue information is also written. A=1 Write the catalogue information for the named file (disc only). A=2 Write the load address (only) for the named file (disc only). A=3 Write the execution address (only) for the named file (disc only). A=4 Write the attributes (only) for the named file (disc only). A=5 Read the named file’s catalogue information. Place the file type in A (disc only). A=6 Delete the named file (disc only). A=&FF Load the named file and read the named file’s catalogue information. When loading a file the byte at XY+6 (the LSB of the execution address) determines where the file will be loaded in memory. If it is zero then the file will he loaded to the address given in the control block. If non-zero then the file will be loaded to the address stored with the file when it was created. The file attributes are stored in four bytes. The least significant eight bits have the following meanings (for the Econet filing system): 448 Bit Meaning 0 Not readable by you 1 Not writable by you 2 Not executable by you 3 Not deletable by you 4 Not readable by others 5 Not writable by others 6 Not executable by others 7 Not deletable by others File types are as follows: 0 Nothing found 1 File found 2 Directory found A BRK will occur in the event of an error and this can be trapped if required. See ‘Faults, events and BRK handling’ towards the end of this chapter. On exit X and Y are preserved, C, N, V and Z are undefined and D=0. The interrupt state is preserved but interrupts may be enabled during the operation. OSRDCH This routine reads a character from the currently selected input stream into A. The routine is called at location &FFE0 and indirects via &210. The input stream can be selected by an OSBYTE call with A=2. See chapter 43. On exit C=0 indicates a successful read and the character will be in A. C=1 indicates an error and the error type is returned in A. If C=1 and A=&1B then an escape condition has been detected and the user must at least acknowledge this by performing an OSBYTE call with A=&7E; BASIC will normally do this for you. X and Y are preserved, N, V and Z are undefined and D=0. The interrupt state is preserved. OSASCI This routine writes the character in A to the currently selected output stream by using OSWRCH. However, if A contains &0D then OSNEWL is called instead. The actual code at location &FFE3 is FFE 3 C 9 0D OSASCI CMP #&0D FFE 5 DO 07 BNE OSWRCH FFE7 A 9 AO OSNEWL LDA # & 0A FFE 9 20 EEFF JSR OSWRCH FFEC A 9 0D LDA # & 0D FFEE 6 C 0 E 0 2 OSWRCH JMP ( WRCHV) 449 On exit A, X and Y are preserved, C, N, V and Z are undefined and D = 0. The interrupt state is preserved. OSNEWL This call issues an LF CR (line feed, carriage return) to the currently selected output stream. The routine is entered at &FFE7. On exit X and Y are preserved, C, N, V and Z are undefined and D = 0. The interrupt state is preserved. OSWRCH This call writes the character in A to the currently selected output stream. The output stream may be changed using an OSBYTE call with A=3. See chapter 43 for more details. OSWRCH is entered at location &FFEE and indirects via &20E. On exit A, X and Y are preserved, C, N, V and Z are undefined and D=0. The interrupt state is preserved but interrupts may be enabled during the operation. All character output from BASIC, the operating system and anything else uses this routine. It is, therefore, easy to pass all output to a user provided output routine by placing the address of the user routine at WRCHV (&20E). However, the user should note that all control characters have special significance. For example, &1C is followed by four bytes which define a text window. See chapter 34 on VDU codes for a complete listing of control characters. If the user wishes to intercept any control characters then his or her routine must check for all control characters. The routine must arrange to skip however many bytes follow a particular code since these bytes might, inadvertently, contain a control code. For example, the BASIC statement GCOL 1,3 is passed to the operating system as a string of bytes through OSWRCH. In fact, in this case the bytes would be &12,1,3. OSWORD The OSWORD routine invokes a number of miscellaneous operations all of which require more parameters or produce more results than can be passed in A, X and Y. As a result, all OSWORD calls use a parameter block somewhere in memory. The exact location of the parameter block is given in X (low byte) and Y (high byte). The contents of A determine the exact nature of the OSWORD call. All OSWORD calls are entered at location &FFF1 which indirects through &20C. The table below summarises the OSWORD calls. 450 OSWORD summary A= Summary of function 0 Read a line from the current input stream to memory 1 Read the elapsed time clock 2 Write the elapsed time clock 3 Read interval timer 4 Write interval timer 5 Read a byte in the input/output processor memory 6 Write a byte in the input/output processor memory 7 Generate a sound 8 Define an envelope for use with the SOUND statement 9 Read pixel colour at screen position X,Y A Read dot pattern of a specific displayable character B Read the palette value for a given logical colour OSWORD with A=0 This routine accepts characters from the current input stream and places them at a specified location in memory. During input the delete code (ASCII 127) deletes the last character entered, and CTRL U (ASCII 21) deletes the entire line. The routine ends if RETURN is entered (ASCII 13) or an ESCAPE condition occurs. The control block contains five bytes: 00 Address of buffer for input line 01 MSB 02 Maximum length of line 03 Minimum acceptable ASCII value 04 Maximum acceptable ASCII value Characters will only be entered if they are in the range specified by XY+3 and XY+4. On exit C=0 indicates that RETURN (CR; ASCII code 13 or &D) ended the line. C not equal to zero indicates that an escape condition terminated entry. Y is set to the length of the line, excluding the CR if C=0. 451 OSWORD call with A=1 Read clock This call is used to read the internal elapsed time clock into the five bytes pointed to by X and Y. This clock is the one used by BASIC for its TIME function. The elapsed time clock is reset to zero when the computer is switched on and if a hard reset is executed. Otherwise it is incremented every hundredth of a second. The only thing that will cause it to lose time is pressing the BREAK key and keeping it pressed. On entry X and Y should point to the memory locations where the result is to be stored. Y contains the high byte and X the low byte of the address. On exit X and Y are undefined and the time is given in location XY (LSB) to XY+4 (MSB). The time is stored in pure binary. OSWORD call with A=2 Write clock This call is used to set the internal elapsed time clock from the five bytes pointed to by XY. On entry X and Y should point to the memory locations where the new time is stored. Y contains the high byte and X the low byte of the address. The least significant byte of the time is stored at the address pointed to by XY and the most significant byte of the time is stored at address XY+4. A total of five bytes are required. OSWORD call with A=3 Read interval tinier In addition to the clock there is an interval timer which is incremented every hundredth of a second. The interval is stored in five bytes pointed to by X and Y. See OSWORD withA=l. OSWORD call with A=4 Write interval timer On entry X and Y point to five locations which contain the new value to which the clock is to be set. The interval timer increments and may cause an event when it reaches zero. Thus setting the timer to &FFFFFFFFFE would cause an event after two hundredths of a second. OSWORD call with A=5 Read I/O processor memory This call enables any program to read a byte in the I/O processor no matter in which processor the program is executing. On entry X and Y point to a block of memory as follows: XY LSB of address to be read XY+1 XY+2 XY+3 MSB of address to be read 452 On exit the eight bit byte will be stored in XY+4. A further feature is available on machines fitted with OS 2.00. The feature enables an additional 12K of memory to be accessed, which exists as sideways RAM at locations &8000-&AFFF. It is accessed by ROM IDs 128-255 (ie any value with the top bit set), and hence will not receive service calls. Furthermore, the MOS will not find it to contain a language. Bytes may be read from the RAM with the top two bytes of the memory block set to &FFFE. Note: references to this 12K of memory which are made in this manual are not necessarily applicable to other Acorn products. OSWORD call with A=6 Write to I/O processor memory As pointed out previously, programs that are to work through the Tube must not attempt to access memory locations in the I/O processor directly. This call provides easy access to locations in the BBC Microcomputer wherever the user’s program happens to be. On entry X and Y point to a block of memory initialised as follows: XY LSB of address to be changed XY+1 XY+2 XY+3 MSB of address to be changed XY+4 Byte to be entered at address given Bytes may be written to the sideways RAM described under OSWORD with A=5 by setting the top two bytes of the memory block to &FFFE. Note that shadow RAM locations &A000 to &AFFF should only be used for user supplied ‘VDU driver’ machine code programs. When shadow mode is on, the MOS VDU drivers will access shadow display RAM, not ‘normal’ display RAM (ie &3000 to &7FFF). Any access to &3000 - &7FFF by machine code running in &A000 - &AFFF will automatically be diverted to shadow display RAM. This facility gives faster VDU access when in shadow mode. OSWORD call with A=7 Make a sound This call can be used to generate a sound. The eight bytes pointed to by locations XY to XY+7 are treated as four two-byte values. These four values determine the sound effect. See the keyword SOUND for a detailed description. 453 XY Channel LSB 1 01 XY+1 MSB 00 XY+2 Amplitude LSB -15 FI XY+3 MSB FF XY+4 Pitch LSB 200 C8 XY+5 MSB 00 XY+6 Duration LSB 20 14 XY+7 MSB 00 The example figures on the right of the table show first the required decimal value and secondly the two hexadecimal values required. The figures are only illustrative. On exit X and Y are undefined. OSWORD call with A=8 Define an envelope This call is used to define an envelope which can be used by a SOUND statement or equivalent OSWORD call. On entry X and Y point to an address in memory where 14 bytes of data are stored. Y contains the high part of the address and X the low part. The envelope number is stored at XY and the following 13 locations contain data for that envelope. See the entry for the ENVELOPE keyword for more details. On exit X and Y are undefined. OSWORD call with A=9 Read a pixel This call enables the machine code programmer to read the status of a graphics point at any specified location. On entry X and Y point to a block of five bytes. Y contains the most significant byte of the address and X the least significant byte. On entry the first four bytes are set up thus: XY LSB of X coordinate XY+1 MSB of X coordinate XY+2 LSB of Y coordinate XY+3 MSB of Y coordinate On exit XY+4 contains the logical colour of the point or &FF if the point is off the screen. X and Y are undefined. 454 OSWORD call with A=&OA Read character definition Characters are displayed on the screen as an eight by eight matrix of dots. The pattern of dots for each character in MODES 0 to 6, including user defined characters, is stored as eight bytes (see chapter 34). This call enables the eight bytes to be read into a block of memory starting at an address given in X and Y. On entry the ASCII code of the character is the first entry on the block. On exit the block contains data as shown below. X and Y are undefined. XY Character required XY+1 Top row of displayed character XY+2 Second row XY+8 Bottom row of displayed character OSWORD call with A=&OB Read palette The reader will be aware that each logical colour (0 to 15) has an actual (or displayed) colour associated with it. The actual to logical association can be changed with VDU19. This OSWORD call enables one to determine the actual colour currently assigned to each logical colour. On entry the X and Y registers contain the address of the start of a block of five bytes. The first byte should contain a value representing the logical colour. On exit the following four bytes will contain the same four numbers used when VDU19 assigned an actual colour to the same logical colour. Suppose that logical colour 2 was in fact set to blue (4) by the statement VDU 19,2,4,0,0,0 then this call would produce the following result: XY 2 Logical colour XY+1 4 Actual colour (blue) XY+2 0 XY+3 0 } Padding zeros for future XY+4 0 } expansion 455 Command line interpreter (&FFF7) The machine operating system CLI is usually accessed from a high level language by starting a statement with an asterisk. For example: *MOTOR 0 , 1 The command line itself (excluding the asterisk) is then passed, without any further processing, to the CLI. Machine code programs can use all operating system commands by placing the address of a command line in the X (LSB) and Y (MSB) registers and calling &FFF7. This routine indirects through location &208. The command line should not start with an asterisk and must end with an &0D. In fact any leading asterisk or spaces will be stripped. The following BASIC program illustrates this: 10 DIM C 20 20 $ C = " MOTOR 1" 30 X%=C MOD 256 40 Y % = C DIV 256 50 CALL &FFF7 When RUN the cassette motor will turn on. The computer will have allocated a space for C - perhaps at location &1B0A in which case successive bytes would contain: Address Contents &1B0A 4D (M) &1B0B 4F (O) &1B0C 54 (T) &1B0D 4F (0) &1B0E 52 (R) &1B0F 20 (Space) &1B10 31 (1) &1B11 0D (Return) Of course, this particular example would have been easier as a *FX call or simply as *MOTOR 1 . However, complex commands may need this call. 456 Faults, events and BRK handling It is necessary to provide some means to enable programs to deal with faults such as Illegal command or Division by zero. BASIC uses the 6502 BRK instruction when dealing with faults like this and user written programs can also use the same facility. In BASIC (for example), a BRK instruction is followed by a sequence of bytes giving the following information: - BRK instruction, value &00. - Fault number. - Fault message (may contain any non-zero character). - &00 to terminate message. When the 6512 encounters a BRK instruction the operating system places the address following the BRK instruction in locations &FD and &FE. Thus these locations point to the ‘fault number’. The operating system then indirects via location &202. In other words control is transferred to a routine whose address is given in locations &202 (low byte) and &203 (high byte). The default routine, whose address is given at the location, prints the default message. The BRK handling outline above enables the user to intercept normal procedures and to generate his or her own special messages and error numbers in user written machine code routines. The CALL demonstration program towards the beginning of this chapter shows this in practice. See also IRQ at the end of this chapter. While faults are in general, ‘fatal’, there is another class of events, called ‘events’, which are informative rather than fatal. This class of events includes, for example, a key being pressed on the keyboard. The user may wish to detect such an operation or may be happy to ignore it. When the operating system detects an ‘event’ then, if that event is enabled (by using *FX1 4 ) it indirects via &220 with an event code in the accumulator. The contents of X and Y depend on the event. The event codes in A indicate the following: Accumulator description 0 Buffer empty 1 Buffer full 2 Keyboard interrupt 3 ADC conversion complete 4 Start of TV field pulse (vertical sync) 5 Interval timer crossing zero 6 ESCAPE condition detected X = buffer identity X = buffer identity Y = character that could not be stored 457 The user supplied event handling routine is entered with interrupts disabled and it should not enable interrupts. The routine should return (RTS) after a short period, say one millisecond maximum, and should preserve the processors P, A, X and Y registers. Interrupt handling The whole machine runs under continuous interrupts but nonetheless the user can easily add his or her own interrupts and handling routines. Because the machine runs under interrupts, software timing loops should not be used. Several hardware timers are available to the user and these should be used wherever possible. NMI - non-maskable interrupt In general these should be avoided. When a disc filing system ROM is fitted NMIs will be handled by the ROM. Again, it should be emphasised that NMI is reserved for the operating system. IRQ - interrupt request When an IRQ is detected the operating system immediately indirects through location &204 (IRQ1V) to an operating system routine which handles all anticipated internal IRQS. If the operating system is unable to deal with the IRQ (because it has come from an unexpected device such as the user 6522), then the system indirects through &206 (IRQ2V). Thus the user routine for handling IRQs should normally be indirected via IRQ2V but if top priority is required the user routine can be indirected via IRQ IV. In either case the user supplied routine must return control to the operating system routine to ensure clean handling. 458 The operating system handles BRK and IRQs with the following code. STA &FC \temporary for A PLA PHA \get processor status AND #&10 BNE BRK JMP ( & 0 2 0 4 ) \IRQ1V BRK TXA \BRK handling PHA \save X TSX LDA & 1 0 3 , X \get address low CLD SEC SBC #1 STA &FD LDA & 1 0 4 , X \get address high SBC #0 STA FE Note that A is stored in location &FC so that it can be accessed by user routines. When the computer indirects through &202 (BRKV), &204 (IRQ1V) and &206 (IRQ2V) X and Y will contain correct values. The user must not enable interrupts during his or her IRQ service routine. 46 Analogue input 459 The BBC Microcomputer is fitted with a socket at the back marked ‘analogue in’. Into this socket you can plug paddles and joysticks as well as voltages which the computer can measure. Paddles usually consist of a box with a knob like a record player volume control. The computer can tell the position of the paddle and so it can be used in games and more serious programs to move things around the screen. Joysticks, on the other hand, can be moved left and right as well as up and down. As a result you can move an object anywhere on the screen not just up and down a particular line. Both paddles and joysticks can be fitted with push buttons and the computer can detect when these buttons are pressed. The BBC Microcomputer can be connected to four paddles or two joysticks. The BASIC function ADVAL can be used to detect the position of each control and of the fire buttons. A second use for the analogue input is to measure voltages. Note that the analogue inputs have no built-in protection against excess voltages. You must therefore be careful not to apply a voltage greater than 1.8 volts or less than 0 volts to any of these inputs. In addition you should keep leads plugged into the analogue inputs away from devices which produce large static voltages such as televisions and some other mains equipment. Each of the four inputs can accept voltages in the range 0 to 1.8V and will produce a corresponding number in the range 0 to 65520. Since it is possible to use a transducer to produce a voltage proportional to temperature, light intensity, smoke density, water pressure, gas concentration etc, it is possible to use the computer to monitor all these things. If the unit is to be used to measure absolute voltages then it should be calibrated individually. In practice 1.0V input typically produces a reading of 35168. Although the unit is fitted with a 12 bit converter, the user should not rely on more than 10 bit accuracy unless great care is taken with screening and analogue ground connections. 460 Digital input/ouput using the eight bit user port The BBC Microcomputer contains an eight bit user port which can be connected to a wide range of devices such as bit pads and general interfacing boxes. The user port can be read from or written to in BASIC and in assembly language, but in either case the user will need to know how to use the 6522 versatile interface adapter integrated circuit. A 6522 data sheet will be essential and the user will discover that this extremely versatile chip is also quite difficult to master. What follows is essential information that you will need to work the chip rather than a course in using it. Once you have learned to use it you will realise that at least 20 pages would be needed to give a decent introduction to it! The 6522 lives in the memory map between locations &FE60 and &FE6F. The A side is used for the parallel printer port and the B side is used for the user port. The timers and shift register are also available for the user. When writing small programs the user can address the device directly either in BASIC or in Assembly Language. However programs that address the device directly will not work on the far side of the Tube. Machine code calls are provided to address the device whichever side of the Tube the program is on. Firstly, though, here are some programs in BASIC and assembly language to read and write to the port. 10 REM Read data in 20 REM Set Data Direction Register B 30 REM for all inputs 40 ? & FE 6 2 = 0 50 REM read a value in and PRINT it 60 X= ? & FE 6 0 70 PRINT X 80 GOTO 60 The next program sets up the 6522 to output to the user port and then transfers the bottom eight bits of X to the user port. Again the initialisation need only take place once. 10 REM All outputs 20 ? & FE 6 2 = & FF 30 REM now put X out 40 ? & FE 6 0 = X And here are those two programs in assembly language. First to read data into the accumulator: 100 LDA #0 110 STA &FE62 120 LDA &FE60 461 and secondly to write data out to the user port. This time the program is presented as two subroutines. The first, called INIT, sets up the 6522 and the second subroutine, WRITE, actually puts the data out from the accumulator onto the user port. 200 .INIT LDA #&FF 210 STA &FE62 220 RTS 230 .WRITE STA &FE60 240 RTS As has been made clear above, these programs will not work from the second processor. The 6522 is one of the memory input/output devices in the area of memory referred to as SHEILA. SHEILA controls the section of memory map in the range &FE00 to &FEFF, and the VIA (versatile interface adaptor) uses addresses between &FE60 and &FE6F which are therefore SHEILA+&60 to SHEILA+&6F. Two OSBYTE calls (see chapter 43) are provided to read and write to SHEILA. Here are the same two routines shown above but written so that they will work over the Tube. 100 110 120 130 140 150 160 LDA # & 9 7 \OSBYTE to write to SHEILA LDX #&62 \Offset to Data direction reg . LDY #0 \Value to be written JSR &FFF4 \ Cal 1 OSBYTE LDA # & 9 6 \OSBYTE to read from SHEILA LDX #&60 \Offset to data register JSR &FFF4 \ Cal 1 OSBYTE to get value And the next routine to I N I T andWRITE to the user port: 200 .INIT LDA #&97 \OSBYTE to write to SHEILA 210 LDX #&62 \Offset to Data direction register 220 LDY # & FF \A11 outputs 230 JSR &FFF4 \Call OSBYTE 240 RTS 250 .WRITE TAY \Move value to Y 260 LDA # & 9 7 \ Wr i t e-t o-S HE I LA code 270 LDX #&60 \Offset to data register 280 JSR &FFF4 \OSBYTE call 290 RTS In practice the user will often wish to use the handshake lines with data transfers. For information on this topic you are referred to other books. Space simply does not permit an adequate explanation here. 462 47 Error messages If the computer is unable to proceed for some reason then it will report the fact to you by printing an error message on the screen. The printing of the message can be suppressed by an ON ERROR statement - for example ON ERROR PROCerror ON ERROR may be followed by any statement or multiple statement. As well as the error message, the computer sets two variables each time an error occurs. ERR gives the error number. ERL gives the number of the line in the program where the error occurred. REPORT is a command to print the last error message. For example ON ERROR REPORT : PRINT" at line " ; ERL : END will give the same response that the computer gives without an ON ERROR statement. The error messages are listed below in alphabetical order together with their error numbers. Accuracy lost 23 If you try to calculate trigonometric functions with very large angles you are liable to lose a great deal of accuracy in reducing the angle to the range of plus or minus PI radians. In this case the computer will report Accuracy lost, eg PRINT SIN (10000000 ) Arguments 31 This error indicates that their are too many or too few arguments for a given function or procedure. Array 14 This indicates that the computer thinks that an array is to be accessed but does not know the array in question. Bad call 30 This indicates that the use of PROC or FN to call a defined procedure or function is incorrect. 463 Bad DIM 10 Arrays must be dimensioned with a positive number of elements. An error will be produced, for example, by: DIM A (-3 ) Bad HEX 28 Hex numbers can only include 0 to 9 and A to F. An attempt to form a hex number with other letters will result in this error, eg PRINT &y Bad key 251 An attempt has been made to provide a function key defenition with a key number greater than 15. Bad MODE 25 This indicates an attempt to change mode inside a procedure or function, or to select a mode for which their is insufficient memory. Bad program 0 There are a number of occasions on which the computer checks to see if the program that it contains starts and ends in memory. The untrappable and fatal error Bad program indicates that the computer could not follow a program through successfully to an end mark in memory. This is caused by a read error or by loading only part of a program or by overwriting part of a program in some way. Unless you are prepared to check the contents of memory a byte at a time there is little that can be done to recover a bad program. Bad string 253 A string more than ten characters long has been passed with an operating system command (cassette filing system only). Block? 218 This is an error generated by the cassette filing system. It indicates that an unexpected block number was encountered. Rewind the tape a short way and play it again Sam. Byte 2 An attempt was made, during an assembly language section, to load a register with a number requiring more than one byte, eg LDA #345 464 Can’t match FOR 33 There is no FOR statement corresponding to the NEXT statement. Channel 222 This error is generated by the cassette filing system if an attempt is made to use a channel that was not opened. Data? 216 This an error generated by the cassette filing system and it means that the computer has found a cyclic redundancy check (CRC) error. The CRC is stored on tape along with other information. Rewind the tape a short way and play it again. DIM space 11 An attempt was made to dimension an array for which there was insufficient room. Division by zero 18 Division cannot be done, eg PRINT 34/0 This error can also be caused by a division within a procedure or function using a LOCAL variable which has not been set to a new value. When a variable is declared as LOCAL it is set to zero. $ range 8 The user may put strings into any place in memory except zero page - that is locations with addresses lass than &100. Thus this is illegal: $40=" hello " Eof 223 This error is generated by the cassette filing system when the end of the file is reached. Escape The ESCAPE key has been pressed. 17 465 Exp range 24 The function EXP cannot deal with powers greater than 88. Thus the following is illegal: X=EXP ( 90 ) Failed at When renumbering a program the computer attempts to look after all references made by GOTO and GO SUB statements. Thus the program 133 GOTO 170 170 END would become 10 GOTO 20 20 END when renumbered. However, the computer will not be able to deal with 133 GOTO 140 200 END If the user attempts to renumber this program he or she will get the error message Failed at line 10 and the renumbered program will be 10 GOTO 140 20 END File? 219 This error indicates that an unexpected file name was encountered by the computer. FOR variable The variable in a FOR following is illegal: FOR 5=3 TO 10 Header? 217 This an error generated by the cassette filing system and it indicates that a header cyclic redundancy check error has occurred. Rewind the tape a short way and play it again. 34 . . .NEXT loop must be a numeric variable. Thus the 466 Index 3 This indicates an error in specifying an index mode when using the assembler, eg LDA Z,Z Key in use 250 An attempt has been made to define a function key while another function key is being expanded, eg *KE Y 0 *KE Y 1 RUN |M |M followed by pressing fO would produce this error message. LINE space The computer has no room left to insert the line in the program. Log range 22 An attempt was made to calculate the LOG of a negative number or of zero, eg PRINT LOG (—10) Missing , 5 This error indicates that the computer expected to find a comma in the line, and didn’t do so, eg D $ =M I D $ (A$ ) Missing ” 9 The computer expected to find a double quote, eg LOAD "FRED 27 Missing ) The computer expected to find a closing parenthesis, eg PRINT TAB ( 10 , 10 Missing # The computer expected to find a #, eg 467 45 A=BGET Mistake 4 This indicates that the computer could not make any sense of the input line. -ve root 21 An attempt was made to calculate the square root of a negative number, eg PRINT SQR(-IO) This may also occur with A S N and ACS. No FN 7 This indicates that the computer detected the end of a function but had not called a function defenition, eg =FN1 inda No FOR 32 A NEXT statement was found when no GOSUB statement had been encountered. No GOSUB 38 A RETURN statement was found when no GOSUB statement had been encountered. No PROC 13 This indicates that the word ENDPROC was found without there being a corresponding D E F PROC statement. No REPEAT 43 The interpreter found an UNTIL statement when no REPEAT statement had been encountered. 468 No room 0 This untrappable and fatal error indicates that while the computer was running a program it used up all available memory. No such FN/PROC 29 If the interpreter meets a name beginning with F N (eg FNf red) or PROC (eg PROCrob) it expects to find a corresponding function or procedure defenition somewhere. This error indicates that no matching defenition was found. No such line 41 The computer was told to GOTO orGOSUB a line number which does not exist. No such variable 26 All variables must be assigned to or made LOCAL, before they can be accessed in PRINT statements or before their values can be assigned to other variables. The initial assignment can simply be, for example, X= 0 . No TO 36 A FOR. . .NEXT loop has been set up with the TO part missing. A correctly formed line is shown below. FOR X= 10 TO 55 Not LOCAL 12 This indicates the appearance of LOCAL outside a procedure or function. ON range 40 The control variable was either less than 1 or greater than the number of entries in the ON list. For example, if X=3 then the following will fail: ON X GOTO 100,200 since there are only two destinations. 469 ON syntax 39 The ON . . . GOTO statement was incorrectly formed. For example, the following is illegal: ON X PRINT The word ON must be followed by a numeric which must in turn be followed by the word GOTO orGOSOB. Out of DATA 42 An attempt was made to read more items of DATA than there were in the DATA list. The word RESTORE can be used to reset the data pointer to the start of the DATA if required. Out of range 1 An attempt was made to branch out of range of the branch instruction in an assembly language program. Silly This message will be issued if you attempt to renumber a program or enter AUTO mode with a step size of 0 or more than 255, eg AUTO 100,0 Syntax 220 This error is generated by the cassette filing system and indicates that a syntax error, such as an illegal * OP T statement has occurred. String to long 19 The maximum length of a string is 255 characters. Subscript 15 This implies that an attempt was made to access an element of an array less than zero or greater than the size of the array. For example, these two lines together will produce this error: 100 DIM A (10) 120 A (15) =3 470 Syntax error 16 A command was terminated wrongly, for example LIST PRINT Too big 20 A number was entered or calculated which was too large for the computer to handle. Too many FORs 35 An attempt was made to nest too many FOR. . .NEXT loops. The maximum nesting allowed is ten. This can sometimes be caused by returning to a FOR statement without executing a NEXT statement, eg 10 FOR X=1 TO 6 20 GOTO 10 Too many GOSUBs 37 An attempt was made to nest too many GOSUB . . .RETURN loops. The maximum nesting allowed is 26. This can somtimes be caused by returning to a GOSUB statement without executing a RETURN statement, eg 10 PRINT "WRONG" 20 GOSUB 10 Too many REPEATS 44 An attempt was made to nest too many REPEAT. . .UNTIL loops. The maximum nesting allowed is 20. This can sometimes be caused by returning to a REPEAT statement without executing an UN T I L statement, eg 10 REPEAT 20 GOTO 10 Type mismatch 6 This error indicates that a number was expected and a string was offered or vice versa, eg 10 A$=X 471 Error number Error message 1 Out of range 2 Byte 3 Index 4 Mistake 5 Missing , 6 Type mismatch 7 No FN 8 $ range 9 Missing " 10 Bad DIM 11 Dim space 12 Not LOCAL 13 No PROC 14 Array 15 Subscript 16 Syntax error 17 Escape 18 Division by zero 19 String too long 20 Too big 21 -ve root 22 Log range 23 Accuracy lost 24 Exp range 25 Bad MODE 26 No such variable 27 Missing ) 28 Bad HEX 29 No such FN/PROC 30 Bad call 31 Arguments 32 No FOR 33 Can't match FOR 34 FOR variable 35 Too many FORs 36 No TO 37 Too many GOSUBs 38 No GOSUB 39 ON syntax 472 40 ON range 41 No such line 42 Out of DATA 43 No REPEAT 44 Too many REP 216 Data? 217 Header? 218 Block? 219 File? 220 Syntax 222 Channel 223 Eof 250 Key in use 251 Bad key 253 Bad string 254 Bad command Note: disc filing system errors are described in the appropriate disc filing system user guide . 48 Minimum abbreviations 473 This chapter lists the minimum abbreviations that can be used for BASIC keywords. The third column lists the hexadecimal number that is used to store the keyword in memory. This is often referred to as the ‘token’. Notice that the abbreviation never needs an opening parenthesis because the token includes the parenthesis ABS ABS 94 ENDPROC E . El ACS ACS 95 ENVELOPE ENV . E2 ADVAL AD . 96 EOR EOR 82 AND A . 80 EOF EOF C5 ASC ASC 97 ERL ERL 9E ASN ASN 98 ERR ERR 9F ATN ATN 99 ERROR ERR . 85 AUTO AU . C6 EVAL EV . AO BGET B . 9A EXP EXP A1 BPUT BP . D5 EXT EXT A2 CALL CA . D6 FALSE FA . A3 CHAIN CH . D7 FN FN A4 CHR$ CHR . BD FOR F . E3 CLEAR CL . D8 GCOL GC . E6 CLG CLG DA GET GET A5 CLOSE CLO . D9 GET$ GE . BE CLS CLS DB GOSUB GOS . E4 COLOUR C . FB GOTO G . E5 COS COS 9B HIMEM H . 93 COUNT COU . 9C (right) DATA D . DC HIMEM H . D3 DEF DEF DD (left) DEG DEG 9D IF IF E7 DELETE DEL . C7 INKEY INKEY A6 DIM DIM DE INKEY $ INK . BF DIV DIV 81 INPUT I . E8 DRAW DR . DF INSTR ( INS . A7 ELSE EL . 8B INT INT A8 END END E0 LEFT $ ( LE . CO LEN LEN A9 PTR PT . CF LET LET E9 (left) LINE LIN . 86 RAD RAD B2 LIST L . C9 READ REA . F3 474 LN LN AA REM REM F4 LOAD LO . C8 RENUMBER REN . CC LOCAL LOC . EA REPEAT REP . F5 LOG LOG AB REPORT REPO . F6 LOMEM LOM . 92 RESTORE RES . F7 (right) RETURN R . F8 LOMEM LOM . D2 RIGHT$ ( RI . C2 (left) RND RND B3 MI D $ ( M . Cl RUN RUN F9 MOD MOD 83 SAVE SA . CD MODE MO . EB SGN SGN B4 MOVE MOV . EC SIN SIN B5 NEW NEW CA SOUND SO . D4 NEXT N . ED SPC SPC 89 NOT NOT AC SQR SQR B6 OFF OFF 87 STEP S . 88 OLD 0 . CB STOP STO . FA ON ON EE STR$ STR . C3 OPENIN OP . 8E STRING$ ( STRI . C4 OPENOUT OPENO . AE TAB ( TAB ( 8A OPENUP OPENUP AD TAN T . B7 OPT OPT THEN TH . 8C OR OR 84 TIME T I . 91 OS CL I OSC . FF (right) PAGE PA . 90 TIME T I . D1 (right) (left) PAGE PA . DO TO TO B8 (left) TRACE TR . FC PI PI AF TRUE TRUE B9 PLOT PL . F0 UNTIL U . FD POINT ( PO . BO US R USR BA POS POS B1 VAL VAL BB PRINT P . FI VDU V . EF PROC PRO . F2 VPOS VP . BC PTR PT . 8F WIDTH W . FE (right) 49 BASIC H 475 This chapter details differences between BASIC II and the original version of BBC BASIC, and as such is mainly intended for those who are already familiar with the latter language. Two new keywords are introduced by BASIC II: OPENUP and OSCLI. These are described in chapter 33. BASIC II makes available the following alterations and extensions to existing BASIC keywords. ABS The unary minus operator may be used, for example PRINT -ABS (-1 ) will give the value -1. In BASIC this gave a Type mismatch error. COUNT This has been altered so that COUNT is reset to zero after a change of MODE , as shown by the following example program: 10 PRINT "Hello"; 20 MODE 3 30 PRINT "Goodbye"; 40 PRINT COUNT In BASIC this would leave the screen showing: Goodbye 12 With BASIC II the following is obtained: Goodbye 7 ELSE In BASIC, ON ... GOTO ... ELSE or ON . . . GOS UB . . . E L S E could not be used inside procedures or functions - only the ON. ..GOTO (or ON . . . GOSUB) part was available. This limitation is not present in BASIC II. EVAL In addition to its BASIC functions, in BASIC II EVAL can be used to evaluate the pseudo-variables COUNT, ERL, ERR, HIMEM, LOMEM, PAGE, TIME and TOP. 476 INPUT If more than one string or value is to be input at a time then the variable identifiers have to be separated from each other. In BASIC this was done by using commas, eg INPUT NAME $ , AGE, HEIGHT In BASIC II either commas or semicolons may be used, eg INPUT NAME $ , AGE; HEIGHT When entering numerics however, these should be separated by commas as before, not semicolons. INSTR In BASIC II, INSTR has been extended such that an instruction such as PRINT INSTR ( "1" , "Hello" ) will return the value 0. This is useful in that a statement such as X=INSTR ( A$ , B$ ) can be included inside a procedure or function without having to check that A$ is longer than B $ . ON ERROR In BASIC II, the comand ON ERROR GOTO . . . can be used with any line number. (In BASIC, ON ERROR GOTO 9 9 9 9 , for example, could not be used). OPENIN and OPENUP In both BASIC and BASIC II an existing file can be opened to allow data to be read or altered, or to allow more data to be added to the end. In BASIC, this function was performed by the instruction OPENIN. In BASIC II it is done by OPENUP. Since these keywords give exactly the same result, the token for them both is &AD. Hence, if a program containing the instruction OPENUP is written on a BBC Microcomputer containing BASIC II then the instruction will be tokenised to &AD. If this program is then saved and loaded into a machine containing BASIC, the program will work in exactly the same way, but when listed it will display the instruction as OPENIN. This will apply the other way around as well so existing programs do not need to be altered to run in BASIC II. 477 The keyword OP ENIN does exist in BASIC II, but has a different meaning. BASIC II uses the keyword OPENIN to open a file for read-only operations; this was not possible in BASIC. Since this is a new facility it has a new token, &8E. Note that programs written in BASIC II which contain the instruction OPENIN will not run in BASIC. The following new features are available to assembly language programmers: ASC In BASIC II ASC " : " may be used in the assembler. In the original BASIC this lead to confusion. EQUB, EQUD, EQUS, EQUW These new features are detailed in chapter 44. OPT In BASIC II, bit 2 of the OPT statement’s operand is used to determine whether assembled machine code is placed in memory at the address given by 0% (the code origin) or P% (the program counter). See chapter 44 for full details. 478 Appendix A Teletext (MODE 7) displayed alphanumeric characters ^ s # 479 N* N* ,4 V & -V* Alpha green Normal * height Graphic cyan 0 1 3 fi ii 0 P C E 00 Alpha yellow . Double height Graphic white 0 3 5 G I 0 H 00 Alpha blue Nothing — Conceal display □ 3 h E : 0 1 00 Alpha magenta Nothing Contiguous graphics * 0 3 m r' E ! E 0 00 Nothing Separated graphics £1 3 E E E E 0 00 Graphic red Nothing 3 3 5 E I E E 0 00 Flash Graphic green Black * background Til 3 ; E F l 0 01 Graphic yellow New background 3 3 E 7 El I 0 01 Nothing Graphic blue Hold graphics 0 P 0 0 01 Nothing Graphic magenta Release * graphics = 0 75 0 0 0 d * every line starts with these options 480 Appendix B Teletext (MODE 7) displayed graphics characters \ * <& si* # ojfc s* 0 Nothing Down Nothing Move cursor to 00 E s no 31 1 0 a H 1 Next to printer Up Disable VDU Move cursor 9 9 an 3! E E □ 0 2 Start printer Select mode □ 0 □ 3H Hi E B B 3 Stop printer Start of line Reprogram characters □ El Hi ■ ■ ]i E 0 0 0 4 Nothing Paged mode Nothing □ S El 31 E u B n 5 Nothing Scroll mode Nothing D 0 0E 0 E § □ 6 Enable VDU Nothing Nothing H Q E 1 E B □ 7 Beep Nothing Nothing n U 9 3! 1 u 0 a Back space and delete 8 Back Nothing Nothing D □ 13 0* 30 0 El H Nothing 9 Forward Nothing Nothing a I S IB r z q Alpha red Each character has a code. Thus H is code 72 since it is in column 70 row 2. 481 .-A V s* s* Alpha green Normal * height Graphic cyan ■ E □ □ q E E B E Alpha yellow Double * height Graphic white □ u □ El 1 E Q B 0 Alpha blue Nothing Conceal display □ s E g □ E i H Alpha magenta Nothing Contiguous graphics * a a a E b E 0 □ Nothing Separated graphics E Q H E 0 0 C H □ Alpha * white Graphic red Nothing E □ 9 E EE 0 G a Back space and delete Black * background 0 ID ID E 0 0 G 0 H ■ Steady * Graphic yellow New background a a V E 0 H e □ 0 I Nothing Graphic blue Hold graphics G □ a b q a 0 B B I Nothing Graphic magenta Release graphics * B a a 0 a b 0 D 0 1 * every line starts with these options 482 Appendix C ASCII (MODES 0 to 6) displayed character set and control codes \ I Nothin. Next to printer i'p Start printer Clear text Stop printer Start of line ^ ^ m Reprogram characters B9 rrwninpnil K BT7 UJL1UJ [9 m 484 Appendix D Hexadecimal codes Appendix E 485 Text planning sheet 486 Graphics planning sheet 1 (grid related to character positions) 1120 1280 1024 487 Graphics planning sheet 2 (decimal) 1000 1100 1200 1280 488 User defined character planning sheet MODES 0 to 6 Appendix F 489 Keyboard codes Z is &5A z is &7A All numbers are in hexadecimal. The editing keys only produce codes if enabled with * F X4. 490 Appendix G Printed circuit board layout for the BBC Microcomputer 491 Appendix H External connections at the rear of the BBC Microcomputer 492 Appendix I External connections underneath the BBC Microcomputer. Appendix J 493 Memory map and memory map assignments Hex Decimal Operating system ROM Memory mapped input/output Operating system ROM Four paged ROMs eg BASIC RAM used for high resolution graphics Dynamic variable storage User’s BASIC program area Reserved for operating system use -&FFFF 65535 -&FF00 65280 -&FC00 64512 &C000 49152 &AFFF 45055 &8000 32768 - - Movable boundary &4000 16384 &3000 12288 Movable boundary &2000 8192 &0E 00 3 5 84 &MM 0 494 Memory map (detail) 3584 3328 3072 2816 2560 2304 2048 1024 768 512 256 0 495 Memory map assignments FFOO to FFFF FEOO to FEFF FDOO to FDFF FCOO to FCFF C000 to FBFF 8000 to BFFF 0000 to 7FFF 1900 to 1AFF E00 E00 to 18FF E00 to 1CFF D9F to DFF D00 to D9E COO to CFF BOO to BFF A00 to AFF 900 to 9FF 800 to 8FF 400 to 7FF 300 to 3FF 200 to 2FF 100 to IFF 000 to OFF Operating System ROM. Internal memory mapped input/output (SHEILA). External memory mapped input/output (JIM). External memory mapped input/output (FRED). Operating System ROM. One or more sideways ROMs (eg BASIC, VIEW, BCPL, PASCAL). Read/write RAM. Econet filing system workspace (if fitted). Default setting of PAGE . Disc filing system workspace (if fitted). Advanced disc filing system workspace (if fitted). ROM workspace. Used by NMI routine (eg by disc or Econet filing system). User defined character definitions. User defined function key (soft key) definitions. RS423 receive. RS423 transmit, sound and speech workspace. Sound and buffer workspace. Sideways ROM workspace. VDU, cassette and keyboard workspace. Operating system workspace and indirection vectors. 6512 stack. Zero page. Zero page FF FD to FE FC DO to FB B0 to CF A8 to AF A0 to A7 90 to 9F 70 to 8F 0 to 6F The top bit is set during an ESCAPE condition. Address following detected BRK instruction. User IRQ routine save slot for A register. Used by machine operating system. Allocated to current filing system. Used by machine operating system. Allocated to disc or Econet filing system. Allocated to Econet filing system. Free for user routines (in BASIC only). BASIC language (or currently selected ROM). Sideways (shadow) RAM 8000 to AFFF Paged RAM 3000 to 7FFF Shadow screen RAM 496 Appendix K Circuit layouts Printer, User I/O, 1 MHz Bus and Tube circuits 497 Video outputs RS 423 CONNECTOR 500 Cassette interface Appendix L 501 VDU code summary Decimal Hex CTRL ASCII abbreviation Bytes extra Meaning 0 0 @ NUL 0 Does nothing 1 1 A SOH 1 Send next character to printer only 2 2 B STX 0 Enable printer 3 3 C ETX 0 Disable printer 4 4 D EOT 0 Write text at text cursor 5 5 E ENQ 0 Write text at graphics cursor 6 6 F ACK 0 Enable VDU drivers 7 7 G BEL 0 Make a short bleep 8 8 H BS 0 Backspace cursor one character 9 9 I HT 0 Forwardspace cursor one character 10 A J LF 0 Move cursor down one line 11 B K VT 0 Move cursor up one line 12 C L FF 0 Clear text area 13 D M CR 0 Move cursor to start of current line 14 E N SO 0 Page mode on 15 F O SI 0 Page mode off 16 10 P DLE 0 Clear graphics area 17 11 Q DC1 1 Define text colour 18 12 R DC2 2 Define graphics colour 19 13 S DC3 3 Define logical colour 20 14 T DC4 4 Restore default logical colours 21 15 U NAK 0 Disable VDU drivers or delete current line 22 16 V SYN 1 Select screen mode 23 17 w ETB 9 Reprogram display character 24 18 X CAN 8 Define graphics window 25 19 Y EM 5 PLOT K,x,y 26 1A Z SUB 0 Restore default windows 27 IB [ ESC 0 Does nothing 28 1C \ FS 4 Define text window 29 ID ] GS 4 Define graphics origin 30 IE A RS 0 Home text cursor to top left 31 IF US 2 Move text cursor to x, y 127 7F DEL 0 Backspace and delete 504 Appendix N FX and OSBYTE call summary Decimal Hex Function 0 0 Prints operating system version number. 1 1 Reserved for application programs. 2 2 Selects input device. 3 3 Selects output devices. 4 4 Enable/disable cursor edit keys. 5 5 Select printer type. 6 6 Set printer ignore character. 7 7 Set RS423 receive baud rate. 8 8 Set RS423 transmit baud rate. 9 9 Set flash period of first colour. 10 A Set flash period of second colour. 11 B Set auto-repeat delay. 12 C Set auto-repeat period. 13 D Disable various events. 14 E Enable various events. 15 F Clear all or just input buffer. 16 10 Select number of ADC channels. 17 11 Force start of conversion on ADC channel. 18 12 Reset user defined function keys. 19 13 Wait for field synchronisation. 20 14 Explode soft character RAM allocation. 21 15 Clear selected buffer. 114 72 Control shadow/main memory selection 117 75 Read VDU status byte. 118 76 Read CTRL/SHIFT key status. 119 77 Close * SP OOL and *EXECfiles. 123 7B End of user print routine. 124 7C Reset ESCAPE flag. 125 7D Set ESCAPE flag. 126 7E Acknowledge detection of escape condition. 127 7F Check end of file status. 128 80 Read ADC channel/fire buttons/last conversion. 129 81 Read key within time limit. 130 82 Read machine high order address. 131 83 Read top of operating system RAM address. 132 84 Read bottom of display RAM address. 505 133 85 Read lowest address for particular mode. 134 86 Read text cursor position. 135 87 Read character at text cursor position. 137 89 Turn cassette motor on/off. 138 8A Insert character into specified buffer. 139 8B Set file options. 140 8C Select cassette file system and set speed. 142 8E Select sideways ROM. 144 90 Alter TV display position/interlace. 145 91 Remove character from buffer. 146 92 Read from I/O area FRED. 147 93 Write to I/O area FRED. 148 94 Read from I/O area JIM. 149 95 Write to I/O area JIM. 150 96 Read from I/O area SHEILA. 151 97 Write to I/O area SHEILA. 152 98 Examine specified buffer. 158 9E Read from speech processor. 159 9F Write to speech processor. 209 D1 Speech on/off. 210 D2 Sound on/off. 218 DA Read/write size ofVDU queue 239 EF Read/write shadow display mode state 224 E0 Cancel VDU queue. 225 El Set base number for function key codes. 226 E2 Set base number for SHIFT function key codes. 227 E3 Set base number for CTRL function key codes. 228 E4 Set base number for SHIFT CTRL function key codes. 229 E5 ESCAPE=&1B. 230 E6 Enable/disable normal ESCAPE key action. 231 E7 Enable/disable user 6522 IRQ. 232 E8 Enable/disable 6850 ACIA IRQ. 233 E9 Enable/disable system 6522 IRQ. 235 EB Return presence of speech processor. 253 FD Last reset type. 255 FF Write start-up option byte. 506 Appendix O Operating system calls Routine Vector Summary of function Name Address Name Address UPTV 222 User print routine EVNTV 220 Event interrupt FSCV 21E File system control entry OSWRSC FFB3 - - Write byte to screen OSRDSC FFB9 - - Read byte from screen OSFIND FFCE FINDV 21C Open or close a file OSGBPB FFD1 GBPBV 21A Load or save a block of memory to file OSBPUT FFD4 BPUTV 2 Save a single byte to file from A OSBGET FFD7 BGETV 216 Load a single byte to A from file OSARGS FFDA ARGSV 214 Load or save data about a file OSFILE FFDD FILEV 212 Load or save a complete file OSRDCH FFEO RDCHV 210 Read character (from keyboard) to A OSASCI FFE3 - - Write a character (to screen) from A plus LF if (A)=&0D OSNEWL FFE7 - - Write LF,CR (&0A,&0D) to screen OSWRCH FFEE WRCHV 20E Write character (to screen) from A OSWORD FFF1 WORDV 20C Perform miscellaneous OS operation control block to pass parameters OSBYTE FFF4 BYTEV 20A Perform miscellaneous OS operation using registers to pass parameters OSCLI FFF7 CLIV 208 Interpret the command line given IRQ2V 206 Unrecognised IRQ vector IRQ IV 204 All IRQ vector BRKV 202 Break vector USERV 200 Reserved Index 507 Abbreviations for keywords 473 ABS 173 Acknowledge escape conditions 407 ACS 174 Accuracy of calculations 55 Actual colour numbers 141 Addressing modes 432 ADSR envelope 216 ADVAL 175,405,414,459 Aligning columns when printing 57-66 Amplitude envelope 159,216 Analogue input connections 459 Analogue to digital converter 201,403,414,477 AND 123,178 Animation 145 Appending programs 371 Application note 421 Arc-cosine 172 Arc-sine 181 Arc- tangent 182 Arrays 102,208 ASC 54,180 ASCII 54, 180, Appendixes A-D ASN 181 Assembly language CALL 186,392 Assembly language DIM 208,436 Assembly language examples 429,435,436,438 Assembly language introduction 428 Assembly language OPT 283,438 Assembly language USR 340,392 ATN 182 Attack Phase 157,217 AUTO 43,183 Automatic line numbers 43,183 Autopaging 30,350 Auto repeat of keys ,403 Background colours 45,50,139 Bad program 463 Base value of function keys 424,425 BASIC II 475 Baud rate selection of cassette 419 Baud rate selection on RS-423 402 BGET# 184 Bitwise AND 178 Boolean types 178 BPUT# 185 Branch instructions 84 BREAK key 13,120 BRIAN 39 BRK 456 Buffer flushing all 406 input 404 keyboard 406 sound 406 Buffer get character 415,420 Buffer insert character 418 Buffer status 413,414,456 Calendar program 111 CALL 186,392 CAPS LOCK key 11 Cassette file tape format 369 filing system 167,360 leads 7 loading 261 motor control 360 motor relay on/off 386,417 recordings 366 Catalogue 28,385 Catalogue of cassette tape 28,361 Centronics printer 373 CHAIN 28,188,261 Channels when using files 165 Character counting 198 Character set Appendix A Character - user defined 146,354 CHR$ 54,189 Circuit board layout Appendix G Circuit diagrams Appendix K CLEAR 190 Clear graphics window 49,191 Clearing the screen 193,350 Clearing text window 49,193 CLG 49,191 CLI 455 Clock 6,333,451 Clock program 111 CLOSE# 192 CLS 16,49,193,350 COLOUR 45,49,194,233 Commands 15 Command line interpreter 455 Command mode 21 Comments in assembly language 431 Comments in BASIC programs 43,304 Concatenation of strings 53 Connectors Appendixes H and I 508 Contents of memory 378 Control codes 348 Co-ordinates on screen 46 COPY key 13,71,397 Correcting errors 22 COS 197 Cosine 197 COUNT 198 CTRC register access 355,420 CTRL key 14 CTRL U 1 Cursor control codes 65,66,350 Cursor editing 22 Cursor off 66 Cursor position 294,345,421 Data 107,199 Data files on cassette 365 Data logging 366 Date 111 Decimal places 60,296 Decimal point 15 DEF 202 Defining characters 146,354,405 DEG 206 Degrees from radians 206 DELETE 24,53,207 Delete current entry 389 DELETE key 13 Delete whole line 24,43,234 Demonstration programs Age 67 BL and Lotus 108 Brian 39 Call 455 Div and Mod 110 Double height Teletext 41 Draw 71 Drinks 164 Fourpnt 35 Geography quiz 200 GOTO 24 Hand mouth ear 237 Hangman 117 Hanoi 300 Hours, mins, sec 111 Hypno 89 H2 64 Leap Years 113 Lunar lander 151 Man 146 Month 107 Monthly 32 People and arrays 103 Persian 37 Polygon 31 Quadrat 33 React 87 Read screen character 417 Reverse string 115 Rocket 148 Role 64 Sine 40 Sine in Teletext 135 Sqr root 37 Stars and stripes 78 Sums in 15 secs 74 Tartan 35 Temperature 97 Too late 70 Windows 51 DIM 121,208,436 Display position, changing 17,443 DIV 122 DRAW 46,49,137,138,211 Econet filing system 370 Editing a line 22 Editing keys 22,71,401 Editing key produced codes 71,401 ELSE 213 Enable screen output 350 END 214,365,413 ENDPROC 215 Entry point in assembly language 441 ENVELOPE 156,216,452 EOF# 220 EOR 123,221 EQUB 439,477 EQUD 439,477 EQUS 439,477 EQUW 439,477 Erasing the screen 50,193,356 ERL 126,222 ERR 126,223 Error codes 126,223 Error handling 125,277,308,367 Error handling in assembly language 438 Error line 126,222 Error messages 462 Error numbers 368,471 Errors, correcting 22 Escape acknowledge 407 Escape detected (assembly language) 457 ESCAPE key 13,407,426 Escape reset 407 EVAL 224 Evaluate a string 224 Event enable 403,404 Event disable 403 Event handling 403 Exclusive OR in BASIC 221 EXP 226 Expansion bus 421 Exponent 15 EXT# 227 509 FALSE 76,85,228 Fault handling 456 Fields 57 Field sync 404,456 Field width 58,71 Filenames 366 Files 190,442 File pointer 301 Filing an area with colour 138 Fire button on game paddles 176,414,459 Flashing colours 141,403 Flash rate selection 403 Flush keyboard buffer 404,406 Flush input buffer 404,406 Flush VDU queue 424 FN 259 FOR.. .NEXT 77,231,273,325 Foreground colours 45,61,162 FOURPNT 35 Free space left 383 FRED 422 Functions 94,203,230 Function keys 16,119,131,405,424 FX call summary 396,398 Games paddles 176,414,459 GCOL 233,139,57 Geography quiz 200 GET 234 Get character from buffer 70,243,415,420 GET$ 235 Global variables 91 GOSUB 96,236 GOTO 100,238 Graphics 45,137 Graphics origin 358 Graphics planning sheet Appendix E Graphics windows 47,355 Hard reset 120 Hexadecimal 61 High order address 415 HIMEM 240,383 HYPNO 89 IF THEN ELSE 4,213,242,332 Indirection operators 378 INKEY 243,415,420 INKEY$ 246 INPUT 67,247 INPUT# 249 INPUT LINE 248 INPUT line 469 Input/Output devices 399,459 Input stream selection 399 INSTR 115,250 Instruction set for 6502 Appendix M INT 251 Integer arithmetic 110,210,267 Integer variables 55,379 Internal file format 299,369 Internal format in memory of BASIC 473 ofVariables 55,56 Interrupts 426,427,457,458 Interval timer 451,457 Inverse colour 143 IRQ handling 426,457,458 JIM 444 Joysticks 175,460 Keyboard 11 Keyboard auto repeat 5,403 Keyboard testing for BASIC 243 Key depressions, detecting 70 Keyword definitions 170 Keywords — details 170 Keywords — summary 473 Leads for cassette 7 Leap year calculation 128 LEFT$ 114,251 LEN 115,254 Length of a file 227 Length of a program 383 Length of a string 115,254 LET 18,256 Line Feed 402 Line numbers 20 LIST 257 LISTO 80,259 List options 80,259 LN 260 LOAD 261 Loading machine code 363 Loading programs 27,363 LOCAL 90,263 LOG 264 Logarithm 264 Logical colour 140 LOMEM 265,383 Loops 74,77 Lunar lander game 151 Machine code 428 Machine operating system 444 Man shaped character 146 Mantissa 55,56 Memory maps Appendix J Memory pointers 240,265,288,335,383,416 Memory - saving 168 Merging programs 371 MID$ 114,266 Mistake 467 MOD 122 MODE 45,137,194,269 MODE7 128 510 Monitor lead 7 MONTHLY 32 Motor on/off 417,455 MOVE 46,271 Multiple statement lines 44,84 Music 7 Musical notes 156 Natural logarithm 260 NEW 272,275 NEXT 77-83,231,273 NMI 457 Noise generator 317 NOT 274 Note synchronisation 189 Number to string conversion 116,363 Number accuracy 55 Numeric range 55 Numeric variables 56 Old 279 ON ERROR 280 ONGOSUB 100,280 OPENIN 283 OPENOUT 285 OPENUP 286 Opening file for input 283 Opening file for output 285 Opening file for random access 286 Operating system call summary 444 Operating system statements 384 Operator precedence 122 OPT 287,457 OR 123,289 Origin move 357 OSARGS 445 OSASCI 448 OSBGET 445 OSBPUT 445 OSBYTE calls 395,397 OSCLI 290,455 OSFILE 446 OSFIND 443 OSGBPB 445 OSNEWL 450 OSRDCH 448 OSRDSC 443 OSWORD 450 OSWRCH 377,450 OSWRSC 443 Output stream select 400 PAGE 292,383 Page mode 39,350 Panic button 21 Parameters 89 Parameter block in CALL 186 PEEK 378 PERSIAN 37 PI 293 Pitch envelope 182,216 Pling indirection operator 378 PLOT statement 290 PLOT a point 144 POINT 293 Pointers to memory 240,265,288,335,383,415 POKE 378 POLYGON 31 POS 294 Precedence of operators 122 PRINT 295 PRINT# 299 Printer Choosing 481 Connections 373 Drivers 377 On/Off 350,377,401 Parallel 373 Serial 375 Print formatting 57 PROC 300 Procedures 87,300 Program deletion 272 Program line renumbering 43,305 Program listing 0,257,259 Program recovery 275 PTR 301 Quadrat 33 Query indirection operator 378 Qume printer 376 RAD 302 Radians from degrees 302 RAM 383 Random numbers 73,308 Range, numeric 55 REACT 87 READ 107,303 Read key 70,234,243,415 Read screen character 416 Read screen point 293,443,472 Real variables 55 Recording programs 26 Red keys 16,119,405,424 Relay on/off 417,455 REM 43,304 Remarks in assembly language 459 Remarks in programs 43,304 Remote control tape recorder 455 RENUMBER 24,54,305 REPEAT ... UNTIL 74,307 REPORT 127,307 Report error 127,307 Reserved words 473 Reset 120 Resident integer variables 55 511 RESTORE 108,310 RETURN key 5,13 RETURN 97,310 RIGHT$ 114,311 RND 73,312 Rocket graphics shapes 148 ROM filing system 370 RS232C printers 375 RS423 as input 400,420 RS423 connections 375 RUN 313 SAVE 314 Saving A section of memory 362 BASIC programs 26,314,360 Data 303 Machine code 363 Memory space 168 Single characters 186 Save format 299 Screen editor 22 Scroll mode 30,350 Sequential access file 163,442 Serial port 375,399,427 Serial printer connections 375 Serial ULA bit meanings 422 Shadow screen mode 387 SHEILA 421 SHIFT key 11 SHIFT LOCK key 11 Sign of a number 315 Significant figures 55 SIN 316 SINE program 40,135 SGN 315 Sockets on computer Appendixes H and I Soft reset 119 SOUND 155,317,471 Spaces - printing on screen 323 SPC 323 Speeding up programs 168 SQR ROOT 48,324 SQR 324 Squares in graphics 138 Statements 15 Star commands 370,385 Stars and Stripes 78 STEP 325 STOP 325 STR$ 327 STRING$ 135,328 String concatenation 53,328 String functions 94,114 String indirection operator 378 String — length of 115,254 String — multiple copies of 116,328 String — searching for one in another 115,250 String-to-number conversion 116,342 String variables 53,115,327 Structures in BASIC 382 Subroutines in BASIC 96,236 Syntax explanation 170 TAB 62,330 Tabulation 62 TAN 331 Tangent 331 Tape filing system 163,360,370 TARTAN 35 Telephone book program 167 Teletext 128 Teletext character set 14 Teletext control codes 128,425 Teletext filing system 370 Text colours 45 Temperature conversion program 97 Text planning sheets 485 Text windows 48 THEN 332 TIME 73,333 Tokens 473 TO 334 TOP 335,383 TRACE 336 TRUE 74,85,338 Tuning a TV 3 Types of variables 52 Unplot a point 144 UNTIL 74,339 User defined characters 146 User defined function keys 16,119,405,42 User input/output port addresses 421 User supplied printer driver 377 USR 340,392 VAL 342 Variables 18,52,102 VDU 343,347 VDU summary 348 Version number of operating system 399 VIA user port address 422 Volume settings 360 VPOS 345 V24 port 375 Wait until next frame for animation 405 Welcome cassette 7 Whole number arithmetic 114,210,269 WIDTH 346 Windows 57,350,355-357 XY cursor addressing 62 1 MHz expansion bus 421 6522 address 422 512 *Star commands 370,385 *ADFS 370,385 *CAT 385 *DISC 370,385 *EXEC 386 *FX commands 385 *IEEE 370,386 *KEY 385 *LOAD 385 *MOTOR 386 *NET 370,385 *OPT 385 *ROM 370,386 *RUN 385 *SAVE 385 *SHADOW 386,387 * SPOOL 371,386 *TAPE 370,385 *TAPE3 370,385 *TAPE12 370 *TELESOFT 370,386 *TV 386 B B m m 22.72.120.401 22.71.120.401 22,71,120,401 22,71,120,401 II 14 i 14 % 14 Vi 14 % 14 + addition 15,123 - subtraction 15,123 * multiplication 15,122 / division 15 ? Indirection operator 378 ! Indirection operator 378 $ Indirection operator 378 < 123 <= 123 = 123 > 123 >= 123 + concatenation of strings 53,328 : multiple statement 44,84 ; in PRINT 21 ; in VDU 379 $ for string 53 & Hex number 61 @% Print format 60 # immediate 451,458 \ comment in assembly language 431 A exponentiation 15 () brackets 122,208 [] square brackets 452 ” quotation marks 53,199,295 ’ apostrophe 295,296,298 \ 14 { 14 } 14