Atari XL Handbook

THE

ATARIXL

HANDBOOK

THE

ATARIXL

HANDBOOK

PETER LUPTON

&

FRAZER ROBINSON

CENTURY COMMUNICATIONS

LONDON

Copyright e Peter Lupton and Frazer Robinson 1984

All rights reserved

First published in Great Britain in 1984 by Century Communications Ltd,

12 - 13 Greek Street,

London WI V 5LE

ISBN 0 7126 02259

Reproduced, printed and bound in Great Britain by

Hazell Watson & Viney Limited,

. Member of the BPCC Group,

Aylesbury, Bucks

Typeset by Spokesman, Bracknell

CONTENTS

Acknowledgements

Introduction

1 Computers and Programs

2 Setting up your XL

3 First Steps

4 Programming

5 Program Control

6 Data and Programs

7 Pieces of Strings

8 Functions

9 Logical Thinking

10 Memory Managemen

t

11 Sound and Music

12 Characters

13 Graphics

14 Advanced Graphics

15 Player-Missile Graphics

16 Permanent Storage

17 Advanced Techniques

APPENDICES

1 BASIC Commands

77

105

136

153

172

188

196

31

45

52

63

70

74

1

3

8

18

204

2 BASIC Error Codes

3 Speeding Up Programs

4 N umbering Systems

5 Graphics Modes

6 Colour Codes

7 Character Codes

8 Display List Instructions

9 Useful Memory Locations

10 Music Frequencies

11 Musical Notation

12 System Memory Map

Index

215

223

225

230

231

232

234

236

238

239

243

244

ACKNOWLEDGEMENTS

Our thanks to all those who in some way made the creation of this book possible, especially friends and colleagues whose enthusiasm and interest was invaluable as our stocks of midnight oil burnt low.

Special thanks to Herbert for all his contributions, and to Richard Gollner for his continuing optimism and sense of humour.

Once again our families have given us much encouragement, for which we are grateful, and sound nutritional advice, which we have ignored.

INTRODUCTION

This book is an introduction to the ATARI XL range of computers and their facilities. It takes you from the first steps in BASIC programming, pointing out the pitfalls and explaining how to make the most of the features of ATARI BASIC, through to a detailed understanding of how to write good, structured programs, with many illustrative examples along the way.

A chapter is devoted to the extensive sound capabilities of the XL, including details of the sound generating chip itself which allow you to create a wider range of effects than is possible using the standard commands.

Four chapters cover the incredible graphics facilities of the machines, including many spectacular examples and useful programs.

An advanced chapter shows how a detailed knowledge of the way graphics are created leads to even more exciting effects.

A comprehensive set of appendices ensure that the book will remain a valuable reference work long after you have grasped the principles of the machine.

We hope that you will find this book both enjoyable and instructive and that it enables you to exploit the potential of your XL to the full.

PETER LUPTON and FRAZER ROBINSON 1984

CHAPTER 1

COMPUTERS AND PROGRAMS

Despite everything you may have heard about computers being hyperintelligent machines which are on the point of taking over the world, a computer is not really intelligent at all. A computer is at heart little more than a high-speed adding machine, similar to an electronic calculator, but with more sophisticated display and memory facili ties.

The feature that gives computers their immense flexibility and potential is this: they can store lists of instructions which they can read through and obey at very high speed. These lists of instructions are called programs, and most of this book is concerned with how these programs are written.

The computer instructions which form programs must follow certain rules, or the computer will not be able to understand them. The rules for writing programs resemble the rules of a spoken language, and so the set of instructions is often said to form a programming language. There are many different computer programming languages; the one that the Atari XL understands (in common with most other personal computers) is called BASIC. (The name BASIC is an acronym for Beginners' AIIpurpose Symbolic Instruction Code.)

A programming language is much simpler than a human language because computers, for all their power, are not as good at understanding languages as people are. The BASIC language used by the

1

2 The Atari XL Handbook

XL has only about 80 words.

The rules for combining the words - the (grammar' of the language - are much more strict than for a language like English, again because it is difficult to make computers that can use languages in the relaxed sort of way in which we speak English.

These may seem like limitations, but in fact as you will discover BASIC is still a powerful language, and it is possible to write programs to perform very complex tasks.

Finally, remember this. Your computer will not do anything unless you tell it to, so whatever happens, you're the boss. The XL won't take over the world unless you make it!

CHAPTER 2

SETTING UP YOUR Xl

Before you can use your Atari XL, you must connect it to a power supply and to a television. To

Peripheral port

Parallel

Bus

TV socket

Monitor Power aNI socket socket OFF

The Atari XL Connectors

-

Rear View

0 0

PORT 1 PORT 2

The Atari XL Connectors

-

Side View

load and save programs you will also need to connect a cassette unit to the XL.

Before connecting anything, make sure you know what should plug in where. The diagrams above show all the connector sockets of the XL.

3

4 The Afar; XL Handbook

There are several sockets through which the XL passes and receives information, and one through which it gets the electrical power it needs to operate. The sockets are clearly labelled, but refer to the diagram before plugging anything in.

POWER

The XL needs a low voltage DC supply, and this is obtained from the power supply unit supplied with your computer. Plug this power supply into the mains, and plug the output lead into the computer at the socket labelled PWR. IN on the back of the computer. Do not switch on yet!

DISPLAY

The Atari XL uses a standard domestic TV to communicate with you, and for this almost any TV will do. To get the best results, use a modern, good quality colour TV. If you use a black and white set, the colour displays produced by the computer will appear as shades of grey.

To connect the XL to the TV, plug the supplied aerial lead into the aerial socket of the TV, and plug the other end into the socket labelled TV at the back of the computer (check the diagram). The lead has a different type of plug at each end, so take care that you don't try to force in the wrong one.

CASSETTE RECORDER

The Atari XL, like most other small computers, uses cassette tapes to save programs or information, so that you don't have to type them in every time you need them.

You cannot use an ordinary cassette recorder wi th the XL: you must use the special Atari cassette unit, which is available at extra cost. The power for

Setting up your XL 5 the cassette unit is taken from the adaptor supplied with it. Plug this adaptor into the mains, and plug the lead into the socket labelled POWER IN on the back of the cassette unit. To connect the cassette unit to the computer, plug one end of the lead supplied into the socket marked PERIPHERAL on the back of the computer, and the other end into either of the sockets marked I/O CONNECTORS on the back of the cassette unit.

DISK DRIVE

Programs can also be stored on floppy disks.

A floppy disk drive is faster than a cassette unit and is more flexible in use.

(It is also much more expensive). The disk drive connects to the socket labelled PERIPHERAL at the rear of the computer.

The disk unit requires its own power supply and so must also be plugged into the mains.

PRINTER

If you have a printer, its lead plugs into one of the

I/O CONNECTORS socket at the rear of the cassette unit. If you are using a disk drive you should plug the printer lead into the spare socket at the rear of the disk drive. The printer also has a separate mains lead.

SWITCHING ON

When you have connected everything together, you are ready to switch on. The equipment must be switched on in the right order, or there is a risk of damaging the computer.

First switch on the TV.

6 The Atari XL Handbook

Second, switch on the disk drive and then the printer. Remember that you should never switch a disk unit on or off with disks inside it.

Last, switch on the computer itself, using the switch on the back next to the PWR. IN socket.

Switching off should be carried out in the reverse order: first the computer, then the disk drive and printer, and last of all the TV.

'fUNING

To get a display to appear on the TV screen, tune to channel 36, or, on a pushbutton set, use a spare channel and keep tuning until you see this appear on the screen:

READY.

If you are unable to tune the television, perform the following checks:

Setting up your XL 7

1 Check that the aerial lead is connected.

2

3

Make sure the computer is connected to the mains and switched on.

The red power indicator should be on.

Try tuning the TV again, and - if possible try a different TV.

With a little time and careful tuning, it is possible to get a clear and stable display on nearly all types of TV. If you are unsuccessful, consult your dealer.

If you are thinking of buying a TV especially for use with your XL, it's worthwhile taking the computer to the shop, as certain types of TV seem to give better results than others.

For the best quality display, you could buy a monitor. This is a display specifically designed for use with a computer, and contains no circuitry for

TV reception. However, a good colour monitor can cost twice the price of the computer!

The XL provides a standard output for a monitor, through the socket next to the TV socket. Your dealer can advise you on the connecting leads required.

CHAPTER 3

FIRST STEPS

Before you can make use of your Atari, you must find out how to communicate with it.

Communication is a two-way process: you must give the Atari instructions, and you must be able to find out how it responds to them.

You give instructions using the keyboard, and the computer displays its response on the TV screen.

Type the following phrase on the keyboard:

PRINT "ATARI XL"

You will see the letters appear on the screen as you press the keys. The light blue square - called the cursor - will move to indicate where the next letter you type will appear.

Nothing else happens though: the computer has not yet obeyed your instruction.

Now press the RETURN key. The words

XL' appear on the screen, and the word

ARI is printed to tell you that the computer is waiting for your next command. If instead of printing

XL' the computer prints 0, or prints and reprints your instruction, it means you have made a typing mistake. Try again!.

So, to give the Atari a direct command you type it at the keyboard and press RETURN. Try another command:

8

First Steps

9

PRINT "HELLO!"

Again, the letters between the quotation marks are printed.

You can tell the Atari to print any sequence of letters, but you must remember to put them between quotation marks.

Try making it print your name.

You don't have to type

(1

RINT in full every time the command can be abbreviated to PRe (the full stop must be included), or to a question mark. Try:

PRe "HELLO" and:

?

"ATARI"

(remembering to press RETURN after typing each command, of course).

MANAGING THE DISPLAY

As well as typing on to the screen, there are a number of different ways to alter the display.

The Cursor

The cursor can be moved around the screen using the key labelled CONTROL at the left of the keyboard and the four keys at the right with arrows marked on them. To move the cursor, hold down the CONTROL key and press one of the arrowed keys.

This method of assigning two functions to a key is similar to the use of a

SHIFT

key on a typewriter, except that on the Atari computer it is taken a stage further, with the CONTROL key as well as

10 The A tar; XL Handbook the SHIFT keys, so any of the other keys may have three different uses.

Try moving the cursor around the screen (it will continue to move for as long as you press the key).

Notice that when the cursor reaches one edge of the screen it reappears at the opposite edge.

The TAB key is used in the same way as that on a typewriter. Pressing TAB moves the cursor right to the next preset posi tion. You can set and clear the tab stops by using SHIFT and TAB together to add a new stop at the current horizontal position of the cursor, and CONTROL and TAB together to clear a stop. When you first switch on the Atari there are five TAB stops set automatically, at eight column intervals across the screen.

You will notice after a little experimenting that the

TAB key may move the cursor to different positions on successive horizontal lines. This is because, as you will discover later, it is possible to type an instruction which extends over two or even three lines of the screen, and the TAB key is moving to preset points along this 114 character line.

Screen Scrolling

When you move the cursor down to the bottom of the screen with the cursor keys, it will reappear at the top and continue to move downwards.

However, if you use the RETURN key to move downwards, when the cursor reaches the bottom the display will begin to move upwards.

This movement is called scrolling.

Anything which disappears from the top of the screen can not be recovered, as the screen will only scroll one way.

First Steps 11

Corrections

If you notice - before you press RETURN - that you have made a typing mistake, you can correct the error by using the cursor keys. For example, if you type:

PRONT "ATARI XL" and then realise your mistake, position the cursor over the offending 0 using the cursor keys, and type an I. You can now press RETURN there's no need to move the cursor to the end of the line.

This is a general rule; pressing RETURN will tell the computer to consider the line upon which the cursor rests as a command. This means you must be careful to delete any rubbish from the line before pressing RETURN, or the Atari will not understand the instruction.

To remove large amounts of text, use the DELETE

BACK SPACE key which will delete characters to the left of the cursor for as long as it is held down.

If you hold down CONTROL and press DELETE

BACK SPACE characters to the right of the cursor will be deleted. To insert text in a line, hold down the CONTROL key and press the INSERT key this will create a space for your additions.

You can delete whole lines from the screen by holding down SHIFT and pressing DELETE BACK

SPACE; using SHIFT and INSERT will insert blank lines.

Clearing the screen

This can be done in three ways. You can hold down the SHIFT key or the CONTROL key and press the

CLEAR key to simply clear the screen. Pressing the RESET key (in the row at the right hand edge of the keyboard) will clear the screen, return the

12 The Atari XL Handbook screen colour to blue and display the READY message.

NUMBERS

As well as printing words, the Atari XL can also handle numbers. Try:

PRINT 5

(RETURN)

You can do arithmetic:

Addition:

PRINT 5+3

Subtraction:

PRINT 7-4

Multiplication:

PRINT 3*5

Division:

PRINT 15/6

Powers (exponentiation):

PRINT 31\2

You can ask the computer to calculate longer expressions, such as:

PRINT 3*5 + (11+5)/(9-7) * 3/4

(the answer should be 21).

First Steps 13

In working out expressions like this, the computer follows strict rules about the order in which the' various arithmetical operations are performed.

First The expressions inside brackets are evaluated.

Second Any powers (squares, cubes, etc.) indicated by /\ are worked out.

Third The computer performs multiplications and divisions.

the

Fourth The additions and subtractions are performed.

If you are unsure, put brackets round the bit you .

want calculated first. Try the following examples, and see if you can work out the answers yourself to check that you know what the computer is doing.

PRINT 2 * 2

PRINT 2 + 2

+

*

2

2

*

+

PRINT 6 + 3 / 5 - 4

2

2

PRINT 3 A 2 + 4 /\ 3 / 2

PRINT (3 + 4)

*

2

Both text and numbers can be printed by a single

PRINT command if they are separated by a semicolon. For example:

PRINT "3+5 = "; 3+5

DISPLAYING CHARACTERS

There are several ways in which the characters displayed may be altered.

14 The Atari XL Handbook

Upper and Lower Case Letters.

When you first switch on and type, all the letters on the screen are displayed as capitalsCupper case).

The Atari can also display letters in lower case.

PRINT "THIS IS A TEST" will print the message (THIS IS A TEST'. Now, press the CAPS key and type the line again: print "this is a test"

The Atari will tell you you have made an error because it can not understand commands typed in small letters. You can however print both sizes:

PRINT "LARGE AND small letters" is OK.

You can use the SHIFT key to get the upper case letters, or press CAPS again to return normal upper case only mode.

Reverse Mode

Up to now, all our characters have been in light blue on a dark blue background. You can invert this by pressing the key at the bottom right of the main part of the keyboard, with the black and white triangle marking - the reverse key. From now on, anything you type appears reversed, with dark characters on a light background. To turn this off press the reverse key again. The Atari will not understand instructions typed in reverse video, but reversed characters can be included in commands.

PRINT

Graphics Characters

The Atari has a set of graphical symbols which can be displayed by holding down the CONTROL key

First Steps 15 and typing letters. To switch permanently to this mode, press CONTROL and CAPS together. Any letter key pressed will now display a graphics character. Pressing SHIFT and CAPS restores the normal upper case mode.

The command:

POKE 756, 204 will replace these graphics characters with the

International character set a collection of accented letters. The command:

POKE 756, 224 restores the graphics characters, as does pressing the RESET key. The POKE command is described in Chapter 10.

CONTROL CHARACTERS

When you use the cursor control keys, or the TAB key, or any other of the controlling keys, the effect is usually immediate.

It is possible however to write

PRINT

commands which perform the same functions as the control keys.

Try the key sequence:

PRINT "[ESC] [ SHIFT

&

CLEAR]"

(The symbol [ESC] means press the key marked·

ESC.

Don't type the brackets).

Instead of the screen clearing when the CLEAR key is pressed, a control character (looking like '\.) representing the screen clear key is added to the print command.

When you press RETURN, and this control character is printed, it has the same effect as pressing CLEAR usually does.

16 The A tar; XL Handbook

You can use the ESC key in a similar way to insert cursor controls, INSERT, DELETE BACK SPACE and TAB actions into

PRINT

commands. The ESC key affects only the next key to be pressed, so to enter a sequence of commands, you must type, for example:

PRINT "[ESC] [CLEAR] [ESC] [CONTROL/UP]

In the rest of this book we will, where possible, use symbols such as l' to represent the typing of ESC and a control character.

If you want to display the control symbols on the screen, pressing ESC twice will produce an ESC control character to precede the control character symbol to be displayed.

MULTIPLE COMMANDS

You can put more than one instruction in one command if you separate the instructions with colons (:). Try this:

PRINT "3+4=": PRINT 3+4

Or this:

PRINT ",,"THIS": PRINT "IS": PRINT

"AN": PRINT "ATARI XL": PRINT

"COMPUTER"

It doesn't matter if you run over the end of the line on the screen, as long as there are no more -than three screen lines of characters (including the spaces) in the command. A beep will sound to warn you as you approach the maximum number of characters.

Entering a number of instructions together like this can get very cumbersome. In the next chapter

First Steps 17 we will discover a way of giving the computer a large number of instructions all at once - in the form of a program.

SUMMARY

PRINT instructs the XL to print something on the screen.

PRINT "ABCDEFG" prints the characters between the quotation marks.

PRINT 3+4 prints the result of the sum.

These can be combined:

PRINT "3+4*5+7 = ".

, 3+4*5+7

Upper and lower case characters, reversed characters and graphics characters are available.

Control characters to clear the screen and move the cursor can be included in a PRINT command.

Multiple Commands

A number of separate instructions may be included on one line but must be separated by colons (:).

CHAPTER 4

PROGRAMMING

At the end of Chapter 3, we saw that it is possible to give the computer a number of instructions at one time by writing them one after another on one line.

We will now look at a much more powerful way of doing this - using a program.

A computer program is a numbered list of computer instructions which are entered together and stored by the computer to be obeyed later. Let's look at the last example of Chapter 3 and see how we can turn it into a program. In Chapter 3 we had:

PRINT PRINT "IS": PRINT

"AN": PRINT "ATARI XL": PRINT

"COMPUTER"

Rewritten as a program it would look like this:

10

20

30

40

50

PRINT" "-THIS"

PRINT "IS"

PRINT "AN"

PRINT "ATARI XL"

PRINT "COMPUTER"

To enter the program into the computer, type .each

line in turn, remembering to press RETURN after each. You will see that the Atari does not obey the instructions as they are entered, and that the tREADY' message does not appear after you press

RETURN. The computer recognises that each line

18

Programming 19 is part of a program because each begins with a number, and the program is stored for future use.

When you have typed in all the lines, you can inspect the stored program by typing LIST (and pressing RETURN, of course). The instructions are listed in numerical order on the screen. You can list parts of the program by specifying line numbers. For example:

LIST 30, 50

LIST 20 lists all lines from 30 to 50 lists only line 20

If a program is very long, it will occupy more lines than there is space for on one screen, so the screen scrolls upwards to display the program. The listing can be suspended by holding down CONTROL and pressing 1. Repeating the CONTROL 1 sequence will restart the listing.

To order the computer to act on the program you use another

BASIC

command:

RUN.

Type this in

(followed by RETURN). This tells the computer to read through the stored program and obey each instruction in order. The program gives exactly the same results as the multiple instructions in

Chapter 3.

A program may be R UN as many times as you like.

It will be stored in the computer's memory until you switch it off. Programs can be altered or added to at will. To alter an instruction, retype the Iine.:

If you enter:

40 PRINT "ATARI 600XL" the original line 40 is replaced by the new one.

Extra lines are added by typing them as before:

20 The Ateri XL Handbook

35 PRINT "AN EXTRA LINE"

Notice that the computer inserts the extra line in the appropriate place: it is not put at the end.

NOTE It is a good idea always to number program lines in steps of 10 to allow extra or forgotten lines to be inserted without having to renumber the whole program.

To delete a line, type the line number and press

RETURN. The line is deleted from the list in the computer's memory.

Now see if you can alter the example program to

PRINT different words on the screen.

When you have exhausted the possibilities of this program, you can use the command NE W to delete the whole program. You will then be ready for the next program.

This program will demonstrate some of the graphics facilities of the Atari. Don't worry if you don't understand all the BASIC commands, they are explained later in this book.

10

20

30

40

50

60

70

80

90

GRAPHICS 9

SETCOLOR 4,1,0

FOR Y=50 TO 0 STEP -10

FOR X=O TO 24

LUM = X+3: IF X>ll THEN LUM. =

27-X

Y2 = X+Y

DrAM = SQR(144-(X-12) *

(X-12»/2

COLOR 15-LUM

PLOT Y2, Y+7-DIAM

100 DRAWTO Y2, Y+7+DIAM

110 COLOR LUM

Programming

120 DRAWTO Y2, 180+DIAM-Y

130 NEXT X

140 NEXT Y

200 FOR T=l TO 1000: NEXT T

210 FOR C=O TO 15

220 SETCOLOR 4,C,O

230 FOR T=l TO 500: NEXT T

240 NEXT C

250 GOTO 210

ARITHMETIC IN

PROGRAMS

VARIABLES

21

We discovered in Chapter 3 that the Atari will print the answers to arithmetical problems in response to commands such as:

PRINT 3+5

This sort of instruction can be included In a program like this:

10 PRINT" ","

20 PRINT "3+5 = "; 3+5 but it would be difficult to make much use of this for working out your personal finances.

What gives a computer the power to perform complex data processing (or (number crunching') tasks is its ability to do algebra: to use names as symbols for numbers. This means that programs can be written with names to symbolise numbers, .

and then RUN with different numbers attached to the names.

These number-names are called

VARIABLES

because they represent varying numbers. Let's try an example:

The area of a rectangle is equal to the width of the rectangle multiplied by the height.

By using

The Atari XL Handbook

22

HEIGHT

I

AREA = WIDTH

X

HEIGHT

1

..

WIDTH

•

names instead of numbers we can write a program to work out the area of any rectangle:

10 WIDTH = 8

20 HEIGHT

30

= 12

AREA

=

WIDTH

40 PRINT "AREA

*

HEIGHT

= "; AREA

This program, when RUN, will print the area of a rectangle 12 inches by 8 inches. By changing the numbers in lines 10 and 20 we can obtain the area of a rectangle of any size.

VARIABLE NAMES

A variable name can have as many as 114 characters. All variable names must begin wi th a letter, but the other characters may be numbers, for example Al, EGG7, LAST1. There must not be any spaces in a variable name as this will confuse the computer.

NOTE You cannot use as a variable name any of the BASIC command words, or any name which begins wi th the letters of a

BASIC command word. If you do, the

Atari will complain of errors as soon as you type the line.

For example, the name LENGTH begins with LEN, and

Programming

23

COST begins with COS. (See Appendix

1 for a full list of all the BASIC reserved words.)

TYPES OF VARIABLE

There are two different types of variable, one represents numbers, the other represents words.

The type which represents numbers is called the numeric variable; the type representing words is the

string

variable, so called because it contains a sequence or (string' of characters. The variables in the rectangle program were all numerical variables.

Numeric Variables

These are used to represent numbers and can have any value, whole numbers or fractions.

Real variables should be used in all arithmetical programs. Real variables can have any value, for example:

REAL

SIZE

= 3.72

= 87.3

*

2.5

String Variables

A string variable is used to store words, letters or numbers. The variable name for a string variable must be followed by a dollar sign ($) to distinguish it from the a numeric variable. Examples are:

NAME$ = "WINSTON CHURCHILL"

STRING$ = "ABCDEFG"

Note that the letters defining the variable contents are enclosed by quotation marks.

A string variable can hold almost any number of characters, limited only by the amount of memory

24 The Ateri XL Handbook available, but you must set the maximum number of characters allowed for each string variable before using the variable.

The DIM command

(short for dimension) is used for this:

DIM BONZO$(30) sets the string variable BONZO$ to have a maximum length of 30 characters. If you then try to store more than 30 characters in BONZO$ an error message will be displayed.

NOTE It is possible to have variables of different types with the same name, such as:

NAME

NAME$

= 87.76

= "GEORGE WASHINGTON"

The computer will not be confused, but you might, so be careful!

ARRAYS

Numeric variables (but not strings) may be used in a special form - an array.

In normal use a variable name represents one stored number. In an array, the variable name represents a collection of stored information, with one or more reference numbers identifying the individual items. An array can be pictured as a collection of boxes. The table on the next page is a diagram of an array which uses one reference number.

The array has 6 elements numbered from 0 to 5.

The array would be used like this:

10

20

30

40

DIM BOXES(5)

BOXES{O)

BOXES{l)

BOXES(2)

=

=

11

92.73

6

Programming 25

REFERENCE

NUMBER

0

1

2

3

4

5

The Array BOXES

CONTENTS

11

92.73

6

0.4971

-3

125.6

50

60

70

BOXES(3)

BOXES(4)

BOXES ( 5 )

100 FOR N

= 0.4971

= -3

= 125. 6

= 0 TO 5

110 PRINT N, BOXES{N)

120 NEXT N

As mentioned before, an array may have more than one reference number.

The number of reference numbers per item is called the number of dimensions of the array. Here is a program using an array with two reference numbers per item:

10

20

30

40

50

60

DIM TABLE{2,1)

TABLE{O,O) = 34

TABLE{O,l)

TABLE{1,O)

TABLE{1,1)

TABLE{2,O)

= 3.7

7

6.85

= 302

70 TABLE{2,1)

100 FOR A

110 FOR B

=

=

0

0.35

TO 2

0 TO 1

120 PRINT TABLE(A,B),

130 NEXT B

26 The A tar; XL Handbook

140

150

PRINT

NEXT A

Here again the array can be thought of as a collection of storage boxes, this time laid out in a grid as shown in the table below. The two columns could represent for example the number of iterns in stock, and the price of each item.

A=O

A = 1

A=2

B

=

0

34

7

302

B = 1

3.7

6.85

0.35

The Array TABLE

You can use arrays with one or two dimensions, but no more.

The maximum allowed value of each reference number may be anything up to several thousand, but this depends on how much memory is occupied by the program itself, and by the other variables of the program.

A DIM command must be included at the beginning of any program which will use arrays.

The command tells the computer to reserve memory space for the array. If you try to use an array without setting its dimensions the Atari will give the error message 9'.

GETTING VALUES INTO PROGRAMS

It would be very inconvenient to have to alter a program to make it handle different numbers, so there are a number of instructions which allow

Programming 27 numbers or letters to be given to a program while it is running. The first of these is INPUrr.

When the computer finds an INIlUl' statement in a program, a question mark is displayed on the screen. The computer waits for you to type in a number or letter string, which it will then store as a variable before continuing with the rest of the program.

Type NE W (RETURN) to delete any program currently in the computer's memory and then try this example:

10 INPUT NUMBER

20 PRINT NUMBER

When you

RUN

this program you will see a question mark on the screen. Type a number and press RETURN: the number is printed.

The

INPUT

you specify a string variable in the command: instruction can handle strings too, if

INPUT

10 DIM NAME$(20)

20 INPUT NAME$

30 PRINT NAME$

This program will accept both numbers and letters and store them as a string variable. Pressing

RETURN enters a null string but this time nothing is displayed.

It is possible to use one

INPUT

two or more numbers or strings.

command to input

10 DIM NAME$(20)

20 INPUT NAME$, AGE

30 PRINT NAME$

40 PRINT AGE

28 The A tar; XL Handbook

The variables must have commas separating them in the INPUT command. If you press RETURN before entering all the items the XL will print another question mark and wait for the remaining items. When you type in the information you must use a comma to separate numeric entries, or press

RETURN to mark the end of each string entry (a comma would be counted as part of the string). If you enter too many items the surplus ones will be ignored.

Let's use what we know about variables and

INPUT to improve the area program. Clear the computer's memory by typing NEW (RETURN), and then type in this program.

10

20

30

40

50

60

70

80

REM IMPROVED AREA PROGRAM

PRINT

"

"

"

PRINT "ENTER WIDTH" ;

INPUT WIDTH

PRINT "ENTER HEIGHT";

INPUT HEIGHT

AREA = WIDTH * HEIGHT

PRINT: PRINT "AREA =

"

,

.

AREA

This program will read the two numbers you type in for width and height and print the area of the rectangle. Using INPUT has made the program much more flexible: we don't have to alter the program to use different numbers.

REMARKS

Line 10 of the last program is a

remark

or

comment

statement. These are used to hold comments, notes and titles to make the purpose of a program and the way in which it works clear to someone reading the listing. Remarks are identified by the word REM before the remark and have no effect when the program is RUN.

Programming 29

You should always put plenty of remarks in programs, because although you may understand how a program works when you write it, three months later you will have forgotten. If you need to modify a program at some time after writing it, remarks will make it much easier to remember how the program works.

SUMMARY

PROGRAMS

A program is a numbered sequence of computer instructions.

LIST

RUN

NEW displays the lines of a program.

starts the execution of a program.

deletes a program from the computer's memory.

VARIABLES

Variables represent numbers and words In programs.

There are two types of variable:

Numeric

String representing numbers.

representing words or numbers, and distinguished by

$

after the name.

INPUT

is used to enter numbers or words into variables while a program is running.

30 The Afar; XL Handbook

Numeric variables may take the form of arrays, with one variable name representing a number of different values, with reference numbers to distinguish between the items.

REMARKS

REM

is used to put remarks into programs for the programmer's benefit.

CHAPTER 5

PROGRAM CONTROL

In the last chapter we defined a program as anumbered list of instructions to the computer, which are obeyed in order from beginning to end.

In this chapter we will find out how to write programs in which some instructions are executed more than once.

This makes programs more efficient, as instructions which must be repeated need to be written only once. We will also discover that the computer can make decisions.

REPETITION

A section of program can be repeated many times using the instructions FOR and NEXT. Try this example:

10 FOR COUNT = 1 TO 10

20 PRINT COUNT

30 NEXT COUNT which prints the numbers from 1 to 10 on the screen. The section of program between the FO R and the NEXT commands is repeated for each value of COUNT, and COUNT is automatically increased by one every time the NEXT command is met in a loop.

.

The variable in a FO R ... NE XT loop need not be increased by 1; any increase or decrease can be

31

32

The Ateri XL Handbook specified using the command STE P. Try changing line 10 to:

10 FOR COUNT = 0 TO 30 STEP 3

Then RUN the program again.

COUNT is now increased by 3 each time the loop is repeated. The

STEP

can also be negative, so that the numbers get smaller:

10 FOR COUNT = 50 TO 0 STEP -2.5

In summary, the instructions are used like this:

FOR V = X TO Y STEP Z is used to begin a loop (V is a variable and X, Y and

Z may be variables or numbers), and

NEXT V marks the end of the loop. Note that, unlike some other microcomputer BASICs, in Atari BASIC the variable must be specified after

NEXT.

The variables used in

FOR ... NEXT

elements of arrays may not be used.

loops must be simple numeric variables; string variables, and

Nested Loops

FOR ...

NEXT loops can be nested - that is, one loop can be contain one or more other loops. Here is an example:

10

20

30

40

FOR A

FOR B

PRINT

NEXT B

=

=

1

1 TO 4

A, B

TO 3

Program Control 33

50 NEXT A

For any nested loops, it is very important that the enclosed loop is completely within the outer loop, otherwise the program will not work.

To clarify this, let's examine the order in which instructions are obeyed in the program.

START r+ 10 FOR A

20 FOR B

= 1 TO 5

= 1 TO 5

L

30 PRINT A, B

40 NEXT B

- - - - - - 50

END

NEXT A

The two loops are arranged so that one is completely within the other.

If lines 40 and 50 were swapped over, the result would be as shown in the next example. You can see that the two loops

START

10 FOR A

20 FOR B

30

40

50

END

PRINT

NEXT A

NEXT B

= 1 TO 5

= 1 TO 5

A, B overlap and the order in which the instructions.

should be obeyed is not clear and the Atari will give

This rule always applies to FOR ...

NEXT loops the loops must always be one within the next.

34 The Ateri XL Handbook

JUMPS

The computer can be instructed to jump from one program line to another using the command

GOTO.

Enter this program:

10

20

30

GOTO 30

PRINT "LINE 20"

PRINT "LINE 30"

When it is RUN the message (LINE 20' is not seen because the

GOTO

instruction in line 10 sends the

XL straight to line 30.

The jump can be to a lower numbered line. The following program will go on for ever, unless you stop it by pressing the BREAK key:

10

20

PRINT "ON AND ";

GOTO 10

This is called an endless loop, and is to be avoided!

DECISIONS

The commands IF and THEN are used in programs to make decisions. A variable is tested, and one of two alternative actions is taken, depending on the result of the test.

10

20

30

40

50

FOR X

PRINT X

IF X

=

FOR P

NEXT X

=

=

1 TO 10

5 THEN PRINT "X =

0 TO 200 : NEXT P

5"

The format of IF ... T-HEN is:

IF

(condition) THE N (instructions)

Program Control 35

The condition after IF may be one of many possible alternatives. Examples are:

IF COUNT = 10

Continue when variable (COUNT) equals a number, in this case 10.

IF COUNT

<

100

Continue when variable less than a number.

IF COUNT

>

100

Continue when variable greater than a number.

IF NUMBER

< >

VALUE

One variable not equal to another (greater than or less than).

IF X >= Y

Greater than or equal to.

IF X

< = .y

Less than or equal to.

Two conditions may be combined, as in:

IF X = 1 OR X = 2

IF A = 3 AND NAME$ = "ATARI XL"

In all of these, the items being compared may both be variables, or one may be a variable and the other a number. (There's not much point in comparing two numbers!) For further details see Chapter 9,

Logical Thinking. The instructions after THEN

36

The Ateri XL Handbook are carried out if the condition is met, otherwise the program continues at the next line.

IF' ... THEN can also be used to control jumps:

10

20

30

40

PRINT "WHAT NUMBER AM I

THINKING OF";

INPUT N

IF N

< >

3 THEN PRINT

"WRONG" : GOTO 10

PRINT "CORRECT!"

If the jump is the only instruction after THEN, the word GOTO can be omitted:

10

20

30

40

50

60

70

80

DIM A$(20)

PRINT "WHAT AM I " ;

INPUT A$

IF A$ = "ATARI XL" THEN 80

PRINT: PRINT A$;"?

NO, TRY

AGAIN"

PRINT

GOTO 20

PRINT: PRINT "GOOD GUESS!"

Variables can be used to direct a GOTO command:

10

20

30

40

JUMP = 40

GOTO JUMP

PRINT "THIS LINE IS MISSED"

PRINT "JUMPS TO HERE"

EXAMPLE PROGRAM - SORTING NUMBERS

As an example of what can be done using loops and decisions, here is a program which sorts ten numbers into ascending order.

10 REM SORTING PROGRAM

Program Control 37

20

30

40

50

60

70

80

DIM NUM(10),SORT(10)

REM INPUT 10 NUMBERS

PRINT" '\."

FOR N=l TO 10

PRINT "ENTER A NUMBER";: INPUT

NUMBER

NUM(N)=NUMBER

NEXT N

100 REM COpy NUMBERS TO ARRAY SORT

110 FOR C=1 TO 10

120 SORT(C)

130 NEXT C

250 SORT(N+l)

= NUM(C)

200 REM SORT NUMBERS

210 COUNT = 0

220 FOR N=l TO 9

230 IF SORT(N+l)

>=

SORT(N) THEN

280

240 TEMP = SORT(N+l)

= SORT(N)

260 SORT(N) = TEMP

270 COUNT = COUNT + 1

280 NEXT N

300 IF COUNT

>

0 THEN 210

310 FOR Z=1 TO 10

320 PRINT NUM(Z),SORT(Z)

330 NEXT Z

The REMarks indicate the functions of the different sections of the program, which works like this:

Line 20 dimensions the two arrays used in the program.

Lines 40 to 80 input ten numbers from the keyboard and store them in the array

NUM( ),

using a FOR ... NEXT loop.

Lines 110 to 130 copy the numbers from

NUM( )

to a second array SORT( ) which will be sorted, while NUM retains the numbers in the order in

38 The Atari XL Handbook which they were typed in.

Again, a loop is used.

R ...

The lines from 210 to 280 sort the numbers in the array SORT(IO) into ascending order.

A

FOR ... NEXT loop compares each element of the array SORT with the next, and swaps them over if they are in the wrong order. A variable,

COUNT, keeps track of the number of these swaps and, if at the end of the loop this isn't zero, the loop is repeated.

Lines 310 to 330 complete the program by

PRINTing the two sets of numbers.

PAUSES

To make a pause during the running of a program, we can use a FOR ... NEXT loop which does nothing at all except count its way through the steps. The following program is an example:

10

20

30

FOR T

NEXT T

= 0 TO 5000

PRINT "ABOUT 10 SECONDS"

Another way to make a pause is to use the Atari's built in timer: The three storage locations 18, 19 and 20 are incremented about 5 times a second, so for a 10 second wait, try:

10

20

30

POKE 19,0:POKE 20,0

IF PEEK(19)

<

2 THEN 20

PRINT" [BELL] "

(The BELL character is obtained by pressing ESC then holding down CONTROL and pressing 2.)

Program Control 39

SUBROUTINES

A subroutine is a section of program which is executed at a number of different times in the course of a program, but is written only once. The computer is diverted from the main program to the subroutine, and returns to the main program at the point from which it left.

The command GOSUB followed by a line number or a variable diverts the computer to the subroutine in much the same way as the GO TO command.

The difference is that the end of a subroutine is marked by a RETURN command which sends the computer back to the instruction after the GOSUB command. As an example of two very simple subroutines, type in the following program:

10

20

30

40

GOSUB 110

C = A+B

GOSUB 210

END

100 REM SUBROUTINE TO INPUT A & B

110 INPUT A

120 INPUT B

130 RETURN

200 REM SUBROUTINE TO DISPLAY

RESULT

210 PRINT" " "

220 PRINT

230 PRINT Ai" + niBin

240 RETURN

= niC

Lines 10 to 40 are the main program. Line 10 calls the subroutine at line 110, line 20 adds A and B and stores the result as C, and line 30 calls the subroutine at line 210. Line 40 marks the end of the program - as the last line to be executed is not the last line of the program we have to tell the

40 The Atari XL Handbook

Atari not to go on to the following lines, which contain the subroutines.

The subroutine at line 110 inputs two numbers and stores them as A and B. The subroutine at line 210 clears the screen and prints the two numbers and their sum.

When the program is RUN, the computer begins, as always, at the lowest numbered line, line 10.

This line calls the first subroutine, and the computer is diverted to line 110. Line 130 returns the computer to the instruction after the GOSUB, which is in line 20.

The program proceeds as normal to line 30, which causes another diversion to line 210. Line 240 returns the computer to line

40, and the program ends.

So, a GOSUB command causes a diversion from the sequence of a program to a subroutine, and the

RETURN command ends the diversion.

The use of subroutines saves a lot of effort in writing programs, as the program lines in the subroutine need to be written only once, instead of being retyped at every point in the program where they are needed. There is no limit to the number of times a subroutine may be called.

Another advantage of subroutines is that they can make the design of a program simpler. If you use subroutines for the repetitive and less important parts of a program the main program becomes much easier for you, the programmer, to follow, and is therefore much easier to write.

Subroutines, like loops, may be nested - that is, the subroutines may call other subroutines, which may in turn call others. However, a subroutine may not be called by another subroutine which it has itself called, or an endless loop occurs. The program will

Program Control 41 crash (come to an undignified halt) when the Atari runs out of the memory it uses to store all the line numbers for the RETURNs, and the message

2 AT LINE xx' will be displayed.

STOPPING AND STARTING

In the last example program the new command

END was used to indicate the last line of the program to be executed. The command tells the computer to stop running the program. There is a second command, STOP, which also halts programs, but the two have slightly different effects. END stops the program running, and the message is displayed, but when STOP is used the program halts and the message

AT LINE xxx' is printed (xxx is the line number of the STOP instruction). Unless you need to know where the program halted, END is the one to use.

If a program has been halted by one of these instructions, or by pressing the BREAK key, it may be restarted with the instruction CONT.

The program will continue from the instruction after the last one to have been executed. CONT may only be used immediately after the program has been stopped.

You may not use CONT within programs.

ON ... GOSUB (GOTO)

Both GOSUB and GOTO may be used in a second type of decision command which selects one of a .

number of destinations depending on the value of a chosen variable.

ON N GOTO 100, 200, 300 results in:

42 The Ateri XL Handbook

GOTO 100

GOTO 200

GOTO 300 ifN ifN ifN

=

1

=

2

=

3

Similarly:

ON P GOSUB 500,200,500,400,100 selects the P'th destination in the list and calls it as a subroutine. If the value of the variable is greater than the number of destinations in the list, or if it is zero, no destination is selected and the program continues at the next instruction. If the value is less then zero, the program stops with the error message (ERROR - 3'. To avoid this, make sure you only use positive values by testing them first.

For example, instead of:

250 ON NUM GOTO 1000, 2000, 3000 write:

250 TEST=NUM

260 IF TEST<O THEN TEST=O

270 ON TEST GO TO 1000, 2000, 3000

SUMMARY

LOOPS

The loop structure has the form:

FOR

V = A TO B STEP C ...

NEXT

in which the instructions between the FO R and NEXT commands are repeated once for each value of V indicated by the FOR ...

STEP

command. A, Band

C may be variables or numbers; V must be a variable. Loops may be nested one within another.

Program Control 43

DECISIONS

IF (condition) THEN (instruction) decides between two actions. The condition after If' is usually a test of a variable.

There may be more than one instruction after THEN, in which case colons (:) must be used to separate them. The complete IF ...

THEN

command must be in one line.

GOTO

need not be included if it would be the only command after THEN, but it must be if there is another command before it:

100 IF ACE 1 THEN 20 but 100 IF ACE 2 THEN KING

7:GOTO 20

SUBROUTINES

Diversions from a program, using the commands

GOSUB

and

RETURN.

GOSUB

calls the subroutine, and RETURN at the end of the subroutine returns the computer to the instruction after the

GOSUB.

ON ... GOSUB

and ON ... GOTO

Select a destination from a list:

ON P GOSUB X, Y, Z causes a

GOSUB

to the P'th item in the list.

ON N GOTO A, B, C

This causes a jump to the N'th destination - if there is an N'th item in the list.

END halts the running of a program.

44

The Afar; XL Handbook

STOP halts a program and prints

LINE xxx'.

CONT will restart a program.

AT

CHAPTER 6

DATA AND PROGRAMS

So far we have not examined all of the ways in which information can be given to programs, nor those by which programs can print out information on to the screen. There are several ways of getting information into programs which we will examine in this chapter, and we will also examine in more detail the use of PRIN'1' and other commands to display data.

READ AND DATA

To enter large amounts of data which will be the same each time the program is R UN we use the data entry command READ.

Look at this program:

10

20

30

40

50

DIM STOCKS(10),PRICE{10)

FOR ITEM=1 TO 10

READ DTAl,DTA2

STOCKS{ITEM)=DTAI

PRICE(ITEM)=DTA2

60

70

80

90

NEXT ITEM

PRINT" '\."

FOR X=l TO 10

PRINT X,STOCKS(X),PRICE(X)

100 NEXT X

110 END

120 DATA 5,1.41,1,5.75,10,2.63,

100,9.95,0,2.23,5,5.6,123,

1.49,8,9.12,20,5.55,1,6.59

45

46 The Afar; XL Handbook

This method of entering information is much easier to use than simply writing it into the program by setting variables. Imagine the typing involved in entering lots of lines like:

PRICE(2)=5.75

With DATA lines you can layout the information in a clear tabular form and check it much more easily. You can also alter it if you need to without touching the main program.

When a program is running, the computer uses a

'pointer' stored in its memory to keep track of how many DATA items it has read.

Every time a

READ command is met, the DATA item indicated by the pointer is copied into the variable, and the pointer moves on to the next item. This means you must have a DATA entry corresponding to each occurrence of READ. The program will stop if there are too many READ commands and the

Atari will print an error message: ERROR-6.

The command RE STO RE can be used to reset the data pointer to the first DATA item in the program. RESTORE followed by a line number, such as:

50 RESTORE 2000 will reset the data pointer to the first DA which occurs after that line.

l' A item

The variables used in the READ command must match the type of data in the corresponding DATA entry. A READ with a string variable will read anything as a string, but READ with a number variable must find a number in the DATA entry or the error message ERROR-6 will result.

Data and Programs 47

GET

The GET instruction provides a second way of feeding information to programs from the keyboard. G Err is used in handling many different peripheral devices, but here we will consider it only as a keyboard command. (See Chapter 15 for more details on other uses ofGErl').

GET halts the program and waits for a key to be pressed. The ASCII code for the key character is stored in a variable.

Before GET may be used, a channel must be opened to the keyboard, by the command

OPEN#1,4,0,"K:"

The GET instruction then takes the form:

GET#I,KEY

Try this short program to see GET in action.

10

20

30

40

50

REM READING KEYS WITH GET

OPEN#1,4,0,"K:"

GET#I,KEY

PRINT KEY,CHR$(KEY)

GOTO 30

GErr is not the ideal command for taking large amounts of information from the keyboard. It is very useful to be able to halt a program until a key is pressed, before, for example displaying a new screen of information.

GET also alows one-key entries to be made when selecting items from menus - lists of options. Try this program:

10

20

REM MENU

OPEN#I,4,0,"K:"

48 The Atari XL Handbook

40

50

60

70

80

30 PRINT " " + + + SELECT OPTION":REM

REVERSED TEXT

PRINT " .J,RED"

PRINT

PRINT

" + YELLOW"

" + BLUE"

GET#l,KEY

IF KEY=ASC("R") THEN SETCOLOR

2,3,6

90

100

IF KEY=ASC("Y") THEN SETCOLOR

2,2,8

IF KEY=ASC( "B") THEN SETCOLOR

2,9,4

110 GOTO 30

This kind of menu is useful in many types of program when asking the program user to make simple decisions and selections. The program is made extremely easy to use as keystrokes are reduced to a minimum.

MORE ABOUT PRINTING

So far, we have described the use of PRINT (or ?) simply to display single iterns of data on the screen.

The Atari has a number of extra facilities which allow you to control the screen layout and produce clear and orderly output from programs.

Punctuating PRINT Statements

You can PRINT more than one item on a single line by including 'punctuation' in the PRINT command. You can use commas to separate the items, in a PRINT statement, and the effect is rather like the 'tab' function of a typewriter.

Output on the screen is formatted at every tenth column. For example:

PRINT "A","B","C","D"

Data and Programs 49 will give a display:

A B C D

The first item in the PRINT statement appears in the first column. The next item - the first after a comma - appears in column 10. The other items appear in columns 20 and 30.

\

The semicolon (;) can be used in a similar fashion.

This leaves no space between successive items in the PRINT statement.

PRINT "A"j"B"j"C"j"D"j"E"j"F" will display:

ABCDEF

The same rules-apply when printing numbers, and numerical variables.

The effect of commas and semicolons is not confined to the PRINT you end one statement in which they appear. If

PRINT statement with a comma or semicolon then the data printed by the next

PRINT command will appear on the same line, with spaces as appropriate.

POSITION

The command POSITION can be used to make the·

Atari begin printing at any point on the screen. To place the cursor at column C, row R, use:

POSITION C,R

On the standard text display, C must be between 0 and 39, and R must be between 0 and 23.

50

Cursor Control

The Ateri XL Handbook

Just as we can use the cursor keys to move the cursor round the screen, so can we use a program to move it.

As described in Chapter 3, the XL interprets four characters as control codes governing the cursor movement.

The characters can be used within strings.

For example:

50

60

70

80

90

10

20

30

40

DIM CD$(20): DIM CU$(20)

PRINT" "

PRINT "TOP LINE"

CD

s "

-It -It -It -It -It -It -It -It -It -It " cus = "i i i i i i i iii"

FOR T

FOR T

CDS

=

PRINT CDS; "MIDDLE LINE"

=

0 TO 500:NEXT T

0 TO 500:NEXT T

PRINT CU$;"BACK TO THE TOP!"; prints the three messages in various parts of the screen using the strings CD$ and CU$ to move the cursor.

You can use the other control characters in a similar way - to insert or lines and characters, to set and clear tabs, and so on.

The way in which data is presented on the screen is an important part of the program and can make the difference between a program being easy to use, or frustrating and confusing. Make good use of the commands available!

Data and Programs 51

SUMMARY

READ and DATA are used to copy large amounts of information into variables without writing lots of program lines.

POSITION C,R places the cursor at column C, rowR.

There are a number of special commands which allow control of the format of data PRINTed on the screen.

CHAPTER 7

PIECES OF STRINGS

String variables are used to store sequences -

(strings' - of characters. There are a number of

BASIC commands which are used to manipulate strings, and these are described in this chapter.

We saw in Chapter 4 that a string variable can store a sequence of characters. For example:

NAME$

GAME$

= "WI LLIAM SHAKESPEARE"

= "SPACE INVADERS"

REFERENCE$ =" ABC123D"

A string can almost any number of characters, limited only by the amount of memory space available. The maximum length of any string must be specified in a DIM statement before the string is used. The characters in a string may be letters, figures, punctuation marks, spaces - in fact any of the characters the Atari can print. The number of characters in a string can be counted using the function LEN. Try this:

10

20

30

40

DIM NAME$(25)

NAME$

=

"JOHN SMITH"

PRINT NAME$

PRINT LEN(NAME$)

This program prints the length as 10, which is the number of characters in NAME$ (including the space).

52

Pieces of Strings 53

NUMBERS AND LETTERS

Computers cannot handle characters (numbers, symbols and letters) directly - they use numbers to represent the characters. Each letter, and each of the other characters the XL can print, is represented by a different code number. There are several systems used in different computers for deciding which code number represents which letter. The Atari uses a system called ASCII - the

American Standard Code for Information

Interchange - which is the most common system for micros. The table in Appendix 4 shows how the

ASCII code relates to letters and numbers. (In fact the Atari code differs from standard ASCII in certain respects, but this is not too important.)

From the table we see that the XL uses a code number 65 to represent the letter A, code number

66 for B and so on. Even numerals have codes. For example the character has the code number 57.

There are two functions in BASIC which allow us to convert characters to numbers and numbers to characters.

The function CHR$ converts a number to a string containing the corresponding character:

10

20

30

40

DIM A$(l)

C

A$

=

=

65

CHR$(C)

PRINT A$

This displays the letter tA'.

You can use a variable with CHR$, as in the example, or a number:

PRINT CHR$(42)

This displays an asterisk (*).

54 The Atari XL Handbook

CHR$ can only give one character at a time. There are 255 possible characters, and if the number or variable in the brackets is increased beyond 255 the character set is repeated. The value used must be less then 65536, and must not be negative, or an

ERROR-3 message will result.

The function ASe complements CHR$. ASC finds the ASCII codes of characters.

10

20

B = ASC ( "B" )

PRINT B displays which is the ASCII code for the letter

B. You can also use string variables with ASe:

10

20

30

DIM B$(l)

B$ = "B"

PRINT ASC(B$) also displays If the variable contains more than one character, ASe gives the code for the first character only:

PRINT ASC( "ABCDE" ) displays the code for

FIGURES IN STRINGS

A string can contain any characters - including numerals. There are two functions which can be used to make strings of number characters from actual numerals and to find the numeric value of the number characters in a string.

The function STR$ creates from a number a string containing the characters of that number:

10

20

DIM A$(5)

A$ = STR$(12345)

Pieces of Strings 55

30 PRINT A$

This converts the single number 12345 into a string by assigning the characters 12345 to A$ and displays the string.

The complementary function to STR$ is V A L.

V AL evaluates the numerical characters in a string:

10

20

30

40

DIM A$ (10)

A$ =

A

"123456"

=

VAL(A$)

PRINT A

This displays the number (123456'.

If the string evaluated contains letters as well as numbers then only the numbers which appear to the left of all the letters are converted by VA L.

This means that '

PRINT VAL(n123ABC45") displays (123'.

The signs

+ and - may appear before the number characters. They will be treated by VAL as the sign of the number. If the first characters of the string are not numeric the message ERROR-18 is displayed by the Atari.

TESTING STRINGS

Strings can be compared with each other,in the same way as numbers, using IF ... THEN. A routine like this one can be used to check that an entry is suitable for the program:

10 DIM PW$(50)

56

20

30

40

The Ater! XL Handbook

PRINT "WHAT'S THE PASSWORD";:

INPUT PW$

IF PW$

<>

"BEANS" THEN PRINT

"WRONG !!!":GOTO 20

PRINT

As well as testing to see if two strings are the same, you can use the (greater than' (» and (less than'

( <) tests to compare strings.

10 IF "B"

>

"A" THEN PRINT "OK"

This works because the letters are stored as numbers. The computer is in fact comparing the

ASCII codes of the letters in the strings. This is very useful because it allows strings to be sorted into alphabetical order, as we will show later in this chapter.

STRING MANIPULATION

Atari BASIC differs from most other dialects of

BASIC in the way in which strings are handled.

Characters can be copied from strings, and can be inserted into strings to replace some of the characters already there. To illustrate the various commands we will use a string A$ which contains the first 10 letters of the alphabet. The string is defined by:

Position 1

Character A

2 3 4 5 6 7 8 9 10

B C D E F G H I J

The string

A$

DIM A$(30)

Pieces of Strings 57

A$="ABCDEFGHIJ"

Portions of strings can be copied by specifying the posi tions of the beginning and end of the substring to be removed, like this:

B$=A$(FIRST, LAST)

So to print the fifth to eighth characters of A$, use the command:

PRINT A$(5,8)

New characters may be inserted into a string by specifying the position of the first character:

A$(4)="XYZ"

PRINT A$

Notice that end of the string is reset to the end of the new section, despite the fact that the original string was longer. The end of the first string can be recovered by adding a character to the end:

A$(ll)="Q"

PRINT A$

You can add material to the end of a string by this method as long as the total length does not exceed the maximum set by the DIM command. A useful formula to add one string to the end of another is:

A$(LEN(A$)+l)=B$

It is quite legitimate to add a new string at a position beyond the end of the first string. Try adding more letters to A$ like this:

A$(20)="LMNOP"

PRINT A$

58 The Afar; XL Handbook

The characters in the middle of the string, which have not been defined by any of your commands, turn out to be hearts. The character code of a heart is zero, and the memory is filled with zeros when the Atari is switched on.

The facility to manipulate strings in this way and the fact that a string may hold a very large number of characters allows a single string variable to be used in a similar fashion to a numeric array variable. An "array" or ten strings, each with a length of20 characters of less, can be fitted into one string with a length of 200 characters by allowing

20 character positions for each of the substrings.

The substings can then be read and replaced using the commands outlined above.

There are two problems in using strings like this.

The first is that the string is initially filled with hearts. This can be overcome by filling the string with spaces before adding any data to it:

10

20

30

40

DIM ARRAY$(200)

FOR Z=1 TO 200

ARRAY$(Z)=""

NEXT Z

The second problem is that every time new material is inserted into a string, the position of the end of the main string is reset to the position of the end of the inserted portion. The answer to this is to write a space in the last character space of the main string after every modification:

150 ARRAY$(140)="NEW STUFF"

160 ARRAY$(200)=""

There are one or two other point to watch out for.

Garbage characters maybe left behind if you replace an element with a shorter string.

This again can be solved by writing spaces into the

Pieces of Strings 59 appropriate portion of the string before inserting the new material.

The program below uses these string handling techniques to sort 20 words into alphabetical order.

The string ALL$ with a maximum length of 440 characters is used as the array.

10

20

30

40

50

60

70

80

90

REM STRING SORTING PROGRAM

PRINT" <, "

DIM WORD$(20),ALL$(440)

FOR L=l TO 440: ALL$(L)=" ".

NEXT L

REM INPUT 20 WORDS

FOR C=l TO 20

PRINT "ENTER A WORD";

INPUT WORDS

ALL$(C*20)=WORD$

100 NEXT C

110 ALL$(440)=""

120 REM PRINT UNSORTED LIST

130 GOSUB 1000

190 REM SORT THE WORDS

200 SWAPS=O

210 FOR Z=l TO 19

220 ZO=Z*20

230 Zl=ZO+20

240 IF ALL$(Zl,Zl+19) >=

ALL$(ZO,ZO+19) THEN 300

250 WORD$=ALL$(Zl,Zl+19)

260 ALL$(Zl)=ALL$(ZO,ZO+19)

270 ALL$(ZO)=WORD$

280 ALL$(440)=""

290 SWAPS=SWAPS+l

300 NEXT Z

310 IF SWAPS>O THEN 200: REM

REPEAT UNTIL NO SWAPS

320 REM PRINT SORTED SEQUENCE

330 GOSUB 1000

60 The Ateri XL Handbook

340

990

1000

1010

1020

1030

1040

END

REM DISPLAY STRINGS

PRINT'" "

FOR P=l TO 20

PRINT ALL$(P*20, P*20+19)

NEXT P

RETURN

This program works in the same way as the sorting program at the end of Chapter 5. You can input twenty strings, which may contain more than one word.

The "array" string ALL$ is not used very efficiently. The first 19 characters are not used at all. There is no reason why, with a bit of extra calculation, these characters should not be used.

Remember that there is no character at ALL$(O) if you try to use this position the message ERROR-

9 will be displayed and the program will stop.

If you run this program a few times you will discover how effective string sorting can be. Notice that any characters will be sorted into ASCII order, so strings containing characters such as */@£?% and /@$&)# will be sorted. You will find that lower case letters are treated as completely different from upper case letters, because of course they are represented by different codes, and the Atari is interested only in the codes. The lower case letters have higher codes than the capitals, so they appear at the end of the sorted list. (You can switch in and out of lower case by pressing the CAPS key).

SUMMARY

A string is a sequence of characters. These can be stored in string variables, which are distinguished by $ after the variable name.

Pieces of Strings

61

LEN(string) gives the number of characters in a string.

ASCII code is used by the Atari to symbolise letters. There is a table of the characters in Appendix 10.

ASCII

codes for the

CHR$ converts a code number to the equivalent character:

PRINT CHR$(65) displays

Ase gives the

ASCII

code for a character:

P RI NT ASC (

II

A

II ) displays

STR$ turns a number into a string of number characters:

10 A$ STR$(123)

20 PRINT A$

V AL evaluates the numerical characters In a string:

PRINT VAL("123ABC789") displays (123'.

COMPARISONS.

Strings can be compared using relational operators = <

IF ...

and >.

THEN and the for testing inputs and sorting strings.

This is useful

62 The Aten XL Handbook

Strings can be modified by inserting characters at any position. Substrings can be read by specifying the beginning and end of the portion required.

CHAPTER 8

FUNCTIONS

A 'function' in computing is an instruction which performs a calculation on a number. There are a number of functions available on the Atari XL. For example, in Chapter 7 functions were described which operate on numbers to give strings, or on strings to give numbers. In this chapter we will look at some more of the functions available on the

Atari.

SQUARE ROOTS

The square root of a number is calculated by the function SQR(N). The square root of a number or variable N is the number which when multiplied by itself, or squared, gives N. Try:

PRINT SQR(4)

The Atari displays 2, because 2 squared (2*2) is 4.

10 FOR N = 1 TO 10

20 PRINT N, SQR(N)

30 NEXT N prints the numbers 1 to 10 and their square roots.

63

64 The Ater! XL Handbook

ABSOLUTE VALUES

The function ABS finds the absolute value of a number: the value of the numeric part, disregarding the sign.

The function changes negative numbers to positive numbers, but has no effect on positive numbers.

PRINT ABS(123.456) displays e

123.456' - no change. But

PRINT ABS(-543.345) displays e

543.345 ' - the minus sIgn has been removed.

INTEGER CONVERSION

The function INT removes the fractional part of a number and returns the next lower whole number.

Try:

PRINT INT(123.4567)

Only the whole number part - 123 - is displayed.

Be careful with numbers less than zero. The INT function finds the first whole number lower then the number you give it.

This means that for negative numbers the answer is not the whole part of the initial number, but one less (or minus one morel), Therefore:

PRINT INT(-2.875) displays t_3'.

Functions 65

SIGNS

SGN(N) returns a value which indicates the sign of a number or variable N. If the number is positive, the result is 1; if the number is zero, the result is zero; and if the number is less than zero, SGN returns -1.

PRINT SGN (5)

PRINT SGN CO)

PRINT SGN (-7) displays (1' displays (0', and displays (-1'.

TRIGONOMETRY

The trigonometric functions of sine, cosine and arctangent are available in Atari BASIC. They are written:

SIN(N)

COS(N)

ATN(N) where N is a number or a variable.

The angles may be expressed either in radians or degrees. When first switched on, and after a RUN command, the Atari treats all angles as being in radians. To switch to degrees use the command

DE G. The corresponding command RAD returns .

the Atari to radian mode.

A radian is the ratio of the length of an arc of a circle to the radius. In the diagram overleaf, the angle A in radians is L/R.

The circumference of a circle is 2n times its radius (21TR), so it follows that

360 degrees is equivalent to 21T radians.

66 The Ateri XL Handbook

L /

I

,

I

I

, I

\

/

/

/

, /

/.

R

The angle A is L/R radians

LOGARITHMS

Natural logarithms are provided by the function

LOG(number or variable).

PRINT LOG ( 10) displays t2.30258509'

PRINT LOG ( 5000) displays t8.5171932'

The antilogs of natural logarithms are calculated by EXP(N). Try:

PRINT EXP(LOG(100»

The answer is 100.

The Atari actually prints

99.9999976 - the calculation is not quite accurate.

Logs can be used to calculate roots. This program finds the cube root of 8.

10 A = 8

20 R = LOG(A)/3

30 PRINT EXP(R)

Functions

67

The answer is 2, because 2 cubed (2*2*2) is 8.

(Again the Atari is not quite accurate, printing

1.9999999.)

This program calculates fifth roots.

10 N

20 R

30

= 243

= LOG(N)/5

PRINT EXP(R}

The program will display (3' (actually 2.99999993), because 3 to the power 5 (3/\5 or 3*3*3*3*3) is 243.

Base ten logarithms are catered for by the function

CLOG.

The antilogs can be calculated by using the formula:

Antilog

zz:

1 o log

So,

if:

TENLOG

=

CLOG (A)

Then:

A

=

lOI\TENLOG

RANDOM NUMBERS

The Atari XL has a function which provides random numbers. The function is RND(X), which prints a number between 0 and 1. The 'seed' value

- X - can be any number or letter, but does directly influence the value of the random number that results. Try PRINTing RND(I) a few times. You will get a different number each time.

68 The Atari XL Handbook

Dice Throwing

We can use random numbers in programs to imitate the throwing of dice. Try this:

10 OPEN#I,4,0,"K:"

20 PRINT "'PRESS ANY KEY TO THROW

THE DICE"

30

40

50

60

GET#l,KEY

DICE

PRINT

= INT(RND(0)*6)

PRINT "YOUR NUMBER IS "iDICE

70 FOR DELAY

80 GOTO 20

+ 1

= 0 TO 500:NEXT DELAY

This program will throw the die every time you press a key. Pressing the program.

BREAK

key will stop the

SUMMARY

There are several built-in functions in Atari

BASIC

which operate on numbers or variables:

SQR(N) calculates the square root of a number N.

ABS(N) returns the absolute value of a number changing negative numbers into positive ones, and leaving positive numbers unchanged.

INT(N) returns the integer value of N, removing any fractional part.

SGN(N) gives 1 ifN is positive, 0 ifN is zero and-1 ifN is negative.

The trigonometric functions:

SIN(N)

Functions 69

COS(N)

return the values for the angle N (which may be in radians or degrees).

ATN(N) returns the value of the angle whose Tangent is N.

DEG

switches to degrees mode.

RAD

switches back to radians.

LOG(N)

returns the natural logarithm of a number, N.

CLOG(N)

returns the base ten logarithmofN.

RND(N) returns a pseudo random number between o and 1.

CHAPTER

9

LOGICAL THINKING

As well as doing arithmetic, the Atari XL can perform tests to compare numbers and strings. We have already seen this when using IF ...

way in which the XL makes comparisons.

THEN. In this chapter we will examine in more detail the

Consider the program line:

220 IF A = 3 THEN 500

The line instructs the Atari to branch to line 500 if the variable A holds the value 3. What does the

Atari do when it encounters this program line?

The first thing the computer must do is decide whether A is equal to 3; or to put it another way, whether t.A

=

3' is true. The XL tests this, and if the expression is true, it returns the answer 1; if the expression is false, the answer is O. If the result is true (1), the commands after THEN are obeyed.

If the answer is false (0) the program will continue with the next line.

Why are we telling you all this?

Because the computer can compare numbers and return true or false values without IF. Try this short program:

10 A = 3

20 PRINT A

=

3

The program displays t.1'.

If you change line 10 to

70

Logical Thinking 71

10 A

= 5 (or any other number) the program will print O.

This applies to all the other comparisons listed in

Chapter 5 as conditions for IJ.":

Equal

< Less than

> Greater than

<> Not equal

>= Greater than or equal

<= Less than or equal

Try this:

10 A = 5

20 PRINT A

<=

7

If you tryout the other tests, you will find they all behave in the same way.

LOGIC

If two tests are combined, the same true and false answers are still obtained:

10 A = 5

20 PRINT ( A

<

7 ) AND ( A

>

3 )

Now, if the computer evaluates simple relational expressions such as A<7, A>3 as 1 or 0, what happens when two are combined, and what does the

AND do?

There are three BASIC commands which can be used with relational expressions: AND, OR and

72 The Atari XL Handbook

NOT.

Forgetting about numerical representations of true and false for the moment, let's look at what these commands do.

Using AND to relate two expressions, as in the example above, it seems fairly obvious that the final result will be true only if both smaller expressions are true. This is in fact what happens.

Here is a table of the possible combinations, with the two simple expressions represented by X and Y.

This type of table is called a 'truth table'. -.

x

TRUE

TRUE

FALSE

FALSE

Y

TRUE

FALSE

TRUE

FALSE

XANDY

TRUE

FALSE

FALSE

FALSE

Truth Table for AND

The command 0 R also does the obvious thing, returning 1 if X or Y or both are true.

x y

XORY

TRUE

TRUE

FALSE

FALSE

TRUE

FALSE

TRUE

FALSE

TRUE

TRUE -

TRUE

FALSE

Truth Table for OR

NOT changes from true to false, and vice versa, so:

Logical Thinking

10 A = 3

20 PRINT NOT A>lO prints 1 (true). Here is the truth table for NOT:

73

X

TRUE

FALSE

NOTX

FALSE

TRUE

Truth Table for NOT

SUMMARY

The Atari XL can not only handle numerical calculations: it can solve logical problems too.

Relational tests such as those performed after IF are represented numerically:

True

False is represented by the number 1.

is represented by the number o.

There are three logical operators:

AND A AND B is true if both A and Bare true.

OR A OR B is true if either A is true or B is true or both are true.

NOT NOT A is true if A is false.

CHAPTER 10

MEMORY MANAGEME'NT

The memory of a computer is built up from a large number of storage units, each of which can hold one number.

Each storage unit is called a memory location, and each location has a unique reference number called its address.

In a microcomputer like the Atari XL, each memory location can hold an eight-bit binary number, which can have a value between 0 and 255. Larger numbers need more than one location. (If you are not sure about binary numbers, they are explained in Appendix 4.) Eight bits of storage are called a byte in computer jargon, so the memory locations of the XL hold one byte of data each.

The microprocessor used in the XL, the 6502, is able to make use of 65535 memory locations (65535 is 2 16 , or 64k). Not all of this can be used for your

BASIC programs (the amount available depends on which model XL you have), some of it is reserved for use by the computer.

The arrangement of memory on the XL is shown in Appendix 13.

There are two types of memory used in a computer.

Random access memory, or RAM, is used to store data, and its contents can be changed as the data changes. Read only memory, or ROM, cannot be altered and is used to store the programs the computer needs to work at all; the BASIC interpreter for example.

There are two BASIC commands, PEEK and

POKE, which are used to look at individual memory locations.

PEEK finds out the contents of a location, and POKE writes a new number into

74

Memory Management 75 the memory. You can tryout PEEK and POKE by using them to control the display.

Type in these instructions:

GRAPHICS O:POKE 82,20

The cursor will jump to the middle of the screen, and from now on you will only be able to use the right-hand side of the display. This is because the

XL uses location 82 to control the width of the screen - this location normally contains the value

2, resulting in a 2 column left-hand margin. The

POKE command placed a 20 in this location, causing the left-hand margin to move to column 20.

If you now type:

PRINT PEEK(82) the XL will print the answer 20, telling you that the number in location 82 is 20. (It should be, as you've just put it there!) Before you continue, type:

POKE 82,2 which will reset the margins to their normal positions.

What makes the POKE and PEEK commands so important is that not all the memory addresses of the XL are used for memory. Some of the addresses.

are used to point to the control registers of the special chips which control the sound and graphics facilities.

This means that the chips can be controlled simply by writing a number into these control registers using the POKE command. These special chips will be described in the following chapters.

76

The Ateri XL Handbook

EMPTY SPACE

As you write longer programs, you will find that the amount of memory space provided by the XL is not unlimited. If a program uses large arrays for data storage, the memory can be eaten up at an alarming rate. The function FRE(X) tells you how much memory is left unused.

Check the memory immediately after switching on your XL, then enter a program and try it again - a smaller number will be printed. If you RUN the program and use FRE(X) a third time the number printed will be smaller still, as the XL has taken memory space to store the variables.

THE CLR COMMAND

The CLR command may occasionally be useful.

The command clears all the variables stored by a program, but leaves the program itself intact. You can then start a program with the maximum amount of memory free.

CHAPTER 11

SOUND AND MUSIC

The Atari XL has exceptional sound generating facilities. The built-in sound generator (POKEY) is a synthesiser on a chip, with four voices, a wide range of octaves, and control of the type of tone produced by each voice.

The XL has no built-in loudspeaker, but uses the loudspeaker of the television or monitor to which it is connected.

WHAT IS SOUND?

Sound is the effect on our ears of vibrations in the air caused by a vibrating object such as a guitar string or a loudspeaker.

............

························1·······················

.

............................•......« ..

.

.

Amplitude

Vibrating Guitar String

If we plot a graph of the way a vibrating object moves over time we get an undulating shape which is called a (wave'. What you hear depends on the properties of the wave: the rate at which the object vibrates (the frequency of the vibration)

77

78 The Ater! XL Handbook

...

Period

Amplitude

Simple Wave determines the pi tch of the sound, and the amplitude determines the volume.

High and Low Frequencies

Large and S mall Amplitudes (Loud and Soft)

Not all sounds have the same shaped waves, and this wave shape, or waveform, has a great effect on the quality of the sound. The shape of the sound waves produced by the XL looks like this: -

A Square Wave as produced by the XL

Sound and Music

79

- this is known as a square wave for obvious reasons.

USING

THE

SOUND GENERATOR

The sound generator chip (known as POKEY) provides four tone channels, each of which can play tones spanning three octaves. The four channels can be set to play independently of each other so that chords and harmonies can be created. The volume of each channel is independently controllable, as is the amount of distortion with which the tones are played.

The chip can be described as 'intel'ligent', meaning that once data about the sound you wish to hear has been passed to it, the computer can continue running your program while the sound is being generated. For this reason, using the POKEY chip will not slow the running of BASIC programs.

To provide control over the functions of the POKEY chip, there is a BASIC command -

SOUND

SOUND

The SOUND command must be followed by four numbers, separated by commas, which determine:

1

2

3

The tone channel to be used.

The period of the note to be played. The number is not the frequency of the note, but is inversely proportional to its frequency.

The distortion to be applied to the sound.

80 The Atari XL Handbook

4 The volume of the note on that channel.

A typical

SO UND

command is :

SOUND 0,126,10,15 where a note of period 126 is played at volume 15 on channel o.

The distortion parameter of 10 specifies no distortion - a pure tone is played.

Try the example, and vary the parameters to get a feel for the noises you can make. You can stop a note from sounding by pressing the RESET key.

The

SOUND

command has the format:

SOUND Channel, Period, Distortion, Volume

and the four parameters work as shown below. The numbers in brackets in the following paragraphs are the allowable range of values for these parameters - if you exceed them an error message is displayed.

CHANNEL (0-3)

This determines which of the four channels is to be used. The remaining parameters in the command pertain to that channel only, and have no effect on the other three channels.

PERIOD

(0-65535)

This controls the frequency of the tone, a low number means a high pitched tone; a high number means a low tone. Any number up to 65535 is allowed, but there are only 256 different frequencies which means that the the numbers 0 to

255 represent all possible tones, with these being repeated at higher values so that:

Sound and Music

SOUND 0,100,10,14

SOUND 0,356,10,14

SOUND 0,61540,10,14 all produce the same tone.

81

VOLUME (0-15)

This parameter controls the volume of the tone, with 15 being the loudest possible and 0 being silence. If you use more than one channel at a time, it is advisable to ensure that the total of the volume parameters isn't greater than 32 as exceeding this value may result in distorted sounds which are accompanied by buzzing of the TV speaker. The best way to ensure this is to use a volume parameter of 8 for each channel, which is quite loud enough for most purposes. If not, you can increase the volume using the volume control on the TV set.

DISTORTION (0-65535)

This controls the amount by which the pure tone is distorted. Numbers between 0 and 65535 are allowed but only even numbers have any effect, and there are only eight possible settings - for numbers higher than 16 the effect repeats. There are only seven different degrees of distortion, selected by the numbers 0, 2, 4, 6, 8 and 12; the' numbers 10 and 14 both specify a pure tone.

The term

distortion

requires further explanation.

As we have seen, the sound waves generated by the

POKEY chip are square waves:

82 The Ater! XL Handbook

A Square Wave

In order to create distorted tones, this regular pattern is modified by another square wave, this one having a random pattern like this:

A Random Square Wave

When you specify a distortion parameter other than 10 or 14, the square wave generated according to the three other SO UND command parameters is modified by the random square wave, such that the only pulses of the original wave you hear are those which coincide with a pulse of the random wave.

The diagram opposite shows how this works:

The result of the combination of the two waves would look like the lowest wave in the diagram. As well as distorting the quality of the sound, the pitch of the sound you hear is lower, as some of the cycles of the wave are removed.The different values of the distortion parameter select different random waves, producing different degrees of distortion.

The following program plays a tone while stepping through the different distortion values.

10

20

30

40

50

FOR DISTORT=O TO 14 STEP 2

SOUND 0,lOO,DISTORT,14

FOR DELAY=O TO 1000:NEXT DELAY

NEXT DISTORT

SOUND 0,0,0,0

Sound and Music 83

SQUARE WAVE

RANDOM WAVE

RESULTANT SOUND WAVE

The Distortion process

Notice how some of the values of the distortion parameter produce more regular sounding noises than others. This is because, despite the name, the pattern of random pulses which comprise the random waves is repetitive. The difference between the various random waves is the time over which the pattern repeats. If this time is short, the noises produced appear repetitive and are useful for engine noises, for example. The most random noises are good for explosions and gun shots.

SOUND CHECKER PROGRAM

The following program will allow you to experiment with the various SOUND parameters on each channel.

5

10

20

30

40

REM SOUND CHECKER

GOSUB 10000

GOSUB FUNKEY

GOSUB KEYBOARD

GOTO 20

84 The Aten XL Handbook

1993

1994

1995

1996

1997

1998

1999

2000

2010

993

994

995

996

997

998

999

1000

1010

1020

1030

1040

2020

2030

2100

2993

2994

2995

2996

2997

2998

2999

3000

3005

3010

REM

REM *********************

REM * *

REM * SUBROUTINE FUNKEY *

REM * *

REM *********************

REM

K=PEEK(53279)

IF K=3 THEN GOSUB OPTION

IF K=5 THEN GOSUB SELECT

IF K=6 THEN GOSUB START

RETURN

REM

REM ***********************

REM * *

REM * SUBROUTINE KEYBOARD *

REM * *

REM ***********************

REM

K=PEEK(764)

IF K=54 THEN POKE 764,255:

GOSUB DECREASE:GOSUB PLAY:GOTO

2100

IF K=55 THEN POKE 764,255:

GOSUB INCREASE:GOSUB PLAY:GOTO

2100

IF K=28 THEN GOSUB START:PRINT

" " " : END

RETURN

REM

REM *********************

REM * *

REM * SUBROUTINE OPTION *

REM

REM

*

REM *********************

*

POSITION 2,4:PRINT"CHANNEL

FREQUENCY DISTORTION VOLUME"

P=P+2:IF P=6 THEN P=O

X={{P=0)*11)+«P=2)*22)+

«P=4)*34)

Sound and Music 85

3020

3030

3040

3050

3060

4040

4050

4060

4993

4994

4995

4996

4997

4998

4999

5000

3070

3993

3994

3995

3996

3997

3998

3999

4000

4010

4020

4030

5010

5020

5993

5994

5995

5996

5997

5998

5999

IF P=O THEN M$="FREQUENCY":REM

THIS TEXT IN REVERSE

IF P=2 THEN M$="DISTORTION":

REM THIS TEXT IN REVERSE

IF P=4 THEN M$="VOLUME":REM

THIS TEXT IN REVERSE

POSITION X,4:PRINT M$

GOSUB KEYBOARD

RETURN

REM

REM *********************

REM * *

REM * SUBROUTINE SELECT *

REM * *

REM *********************

REM

POSITION 3,7+2*CHAN

PRINT " "

CHAN = CHAN+l

IF CHAN=4 THEN CHAN=O

POSITION 3,7+2*CHAN

PRINT ">"

RETURN

REM

REM ********************

REM * *

REM * SUBROUTINE START *

REM

REM

* *

REM ********************

FOR Z=O TO 3:S0UND Z,O,O,O:

NEXT Z

GOSUB 12010

RETURN

REM

REM ***********************

REM * *

REM * SUBROUTINE DECREASE *

REM * *

REM ***********************

REM

86 The Ateri XL Handbook

6020

6493

6494

6495

6496

6497

6498

6499

6500

6510

6520

7993

7994

7995

7996

7997

7998

7999

8000

6000

6010

8005

8010

8020

8030

8040

8050

9993

9994

9995

9996

9997

9998

9999

10000

10010

S{CHAN,P)=S{CHAN,P)-1-{P=2)

IF S(CHAN,P) <0 THEN S(CHAN,P)

=S(CHAN,P+l)

RETURN

REM

REM ***********************

REM * *

REM * SUBROUTINE INCREASE *

REM *

REM ***********************

REM

S(CHAN,P)=S{CHAN,P)+1+{P=2)

*

IF S(CHAN,P»S{CHAN,P+l) THEN

S{CHAN,P)=O

RETURN

REM

REM *******************

REM * *

REM * SUBROUTINE PLAY *

REM * *

REM *******************

REM

SOUND CHAN,S{CHAN,0),S{CHAN,2)

,S(CHAN,4)

XP=X+«P=0)*3)+{(P=2)*4)+

«P=4)*2)

POSITION XP,7+2*CHAN

PRINT" n:REM 3 SPACES

POSITION XP,7+2*CHAN

PRINT S(CHAN,P)

RE"URN

REM

REM ******************

REM

*

REM * SET UP DISPLAY *

REM *

REM ******************

REM

PRINT

POSITION 13,1

*

*

Sound and Music 87

10020 PRINT "XL SOUND CHECK":REM IN

REVERSE

10030 POSITION 2,4

10040 PRINT "CHANNEL FREQUENCY

DISTORTION VOLUME"

10050 POSITION 12,5:PRINT "(0-255)

(0-14) (0-15)"

10060 FOR CHAN=O TO 3

10070 POSITION 5,7+2*CHAN

10080 PRINT CHAN:NEXT CHAN

10090 CHAN=O:POSITION 3,7+2*CHAN

10095 PRINT n>"

10993 REM

10994 REM ************************

10995 REM * *

10996 REM * SUBROUTINE ADDRESSES *

10997 REM * *

10998 REM ************************

10999 REM

11000 FUNKEY = 1000

11010 KEYBOARD = 2000

11020 OPTION = 3000

11030 SELECT = 4000

11040 START = 5000

11050 DECREASE = 6000

11060 INCREASE = 6500

11070 PLAY = 8000

11080 POKE 752,I:REM TURN OFF CURSOR

11993 REM

11994 REM **************

11995 REM * *

11996 REM * INITIALISE *

11997 REM * *

11998 REM **************

11999 REM

12000 DIM S(3,5),M$(20)

12010 FOR CHAN=O TO 3

12020 FOR P=O TO 4 STEP 2

12030 S(CHAN,P)=O

12035 XP=«P=0)*14)+({P=2)*26)+

{{P=4)*36):GOSUB 8010

88

The Atari XL Handbook

12040 READ D:S(CHAN,P+l)=D

12050 NEXT P:RESTORE

12060 NEXT CHAN:P=-2:CHAN=O

12070 DATA 255,14,15:REM MAX DATA

12080 GOSUB OPTION

20000 RETURN

The program creates a display of the current

Frequency, Distortion and Volume parameters for each of the four tone channels, and allows you to change them to experiment with the SOUNI) command.

To select one of the four channels, press the

SELECT key on the function keypad - a indicates which channel is selected.

> symbol

To alter one of the parameters on that channel, use the OPTION key to make your choice - the heading of the currently selected column will be highlighted. Use the < and > keys to increase or decrease the value displayed. The sound you hear is changed with the changing parameters, and in this way you can tailor the four channels to create the noise you require and make a note of the parameters. If you make a mess of things, pressing the START key sets all the parameters back to zero.

HOW THE PROGRAM WORKS

The program consists of a number of modules, defined in lines 11000 to 11070.

The display is created by lines 10000 to 10080 and lines 12000 to 12080 create an array which contains the parameters for each channel (initially zero) and the maximum value for each parameter.

The program selects Channel 0 and Frequency options as a starting point by setting the variables

Sound and Music 89

P and CHAN in line 12060, then calling the routine normally called when the OPTION key is pressed.

When running, the program constantly cycles through the two routines FUNKEY and

KEYBOARD which read the two keyboards, until a keypress is detected. The appropriate subroutine is called and SOUND parameters for the selected channel changed by ei ther the DECREASE or

INCREASE subroutines.

After each keypress is processed, the PLAY routine at 8000 is called to update the noise, and the program awaits the next keypress.

PLAYING TUNES

To play tunes, we must (feed' a SOUND command with a series of frequency parameters in turn, and one way of doing this is shown in this example:

10 SOUND 0,INT(100*RND(I»,10,8:

GOTO 10

As mentioned earlier, all four channels are independant - allowing us to play four different tones at once on each channel.

5

10

20

30

40

50

REM CACOPHONY!

SOUND 0,INT(10*RND(1»,10,8

SOUND I,INT(50*RND(1»,10,8

SOUND 2,INT(lOO*RND{1»,10,8

SOUND 3,INT{250*RND{l»,10,a

GOTO 10

Nei ther of these examples could be described as music, for that we need a more controlled way of calculating frequencies. The table in Appendix 10 lists the frequency values corresponding to musical notes (if you are unfamiliar with musical notation, refer to Appendix 12). Using this table we can build

90

The Atari XL Handbook up a series of numbers corresponding to notes but we need a way of storing them. One way is to store them in a string, reading them one at a time as parameters for a

SOUND

command.

This technique is illustrated in the following program.

10

20

30

40

50

60

70

80

DIM TUNE$(ll)

TUNE$ = "QQQQDHHQQUQ"

FOR Z=1 TO LEN(TUNE$)

N = ASC(TUNE$(Z,Z»

FOR D =

NEXT Z

°

TO 100:NEXT D

SOUND 0,0,0,0

The main limitation with this method is that the notes are all the same length, which renders it impractical for all but the simplest music.

There are several ways of overcoming this: for example, a second string could be used to hold the data for the length of notes. Add these lines to the last program to hear this technique in action.

15

25

60

DIM L$(ll)

L$="32132121214"

FOR D=O TO 100*VAL(L$(Z,Z»:

NEXT D

This method is adequate for simple tunes but a better way is to hold information concerning notes and note lengths in DATA statements like this:

10

20

30

40

50

60

70

FOR Z=1 TO 13

READ NO,LE

SOUND 0,NO,10,8

FOR DUR=O TO 100*LE:NEXT DUR

SOUND 0,0,0,0

NEXT Z

SOUND O,O,O,O:END

Sound and Music 91

100 DATA 81,2,81,2,88,1,81,1,72,

2,96,4,108,4,122,2,122,2,128,1

,122,1,108,2,144,8

The next program uses DATA statements to playa tune in four part harmony. There are five D AT A items for each group of notes - the periods of the four voices, and a value for the note length. The program will play a tune you should recognise if you have run the Audio-Visual self checking routine on the XL.

It will take you quite a time to type in all the

DATA, but the end result is well worth all the effort.

You will notice that the values used for note periods are not the same as those given in the Atari

BASIC Reference Manual supplied with your XL.

We found that the values given by Atari were not well tuned, and that the note values given in

Appendix 11 sounded much better! Even so, the tuning is not perfect, as the 255 available periods are not sufficient to give a perfect sound. Later in the chapter we will describe how to increase the range and tuneability of the XL at the expense of reducing the number of voices from four to two.

10

20

D=10

VQ=0:Vl=1:V2=2:V3=3

30 VOL=4:VOLO=lO:VOL3=6

100 READ PO,Pl,P2,P3,T

110 IF T=255 THEN END

120 SOUND VO,PO,D,VOLO

130 SOUND Vl,Pl,D,VOL

140 SOUND V2,P2,D,VOL

150 SOUND V3,P3,D,VOL3

155 FOR DEL=O TO T*llO:NEXT DEL

160 GOTO 10

92 The Ateri XL Handbook

1000

1010

1020

1025

1026

1030

1040

1050

1060

1062

1070

1072

1080

DATA 85,0,0,0,2,95,0,0,0,2,

71,0,0,0,2,63,0,0,0,1,47,0,0,0

,1,56,0,0,0,2

DATA 63,0,0,0,1,47,0,0,0,1,

56,0,0,0,2,71,0,0,0,2,63,0,0,0

,2,85,0,0,0,2,95,0,0,0,2

DATA 85,113,143,171,2,

95,127,151,191,2,71,113,143,17

1,2,63,75,127,191,1,47,75,127,

191,1,56,75,95,227,2

DATA 63,75,127,191,1,47,75,

127,191,1,56,71,95,143,2,71,85

,113,171,2,63,85,101,255,2,85,

127,171,203,2

DATA 95,127,151,191,2

DATA 95,0,0,0,2,85,0,0,0,2,

113,0,0,0,2,95,0,0,0,1,85,0,0,

0,1,127,0,0,0,2

DATA 85,0,0,0,1,75,0,0,0,1,95,

0,0,0,2,47,95,191,0,2,56,95,14

3,0,2,63,95,171,0,1,71,95,171,

0,1,95,191,0,0,2

DATA 95,0,0,0,2,85,0,0,0,2,

113,0,0,0,2,95,0,0,0,1,85,0,0,

0,1,105,0,0,0,2

DATA 71,0,0,0,1,63,0,0,0,1,79,

0,0,0,2,39,79,159,0,2,47,79,11

9,0,2,52,79,143,0,1,59,79,143,

0,1,79,0,159,0,2

DATA 0,0,0,0,0

DATA 79,105,159,0,2,71,105,

143,0,2,79,105,159,0,2,71,105,

159,0,1,63,105,127,0,1,52,105,

0,0,1

DATA 71,105,143,0,1,79,105,

159,179,2

DATA 59,79,95,191,1,52,63,79,

211,1,47,59,79,239,1,39,0,0,23

9,1,44,52,71,211,1,47,59,79,19

1,1

Sound and Music 93

1082 DATA 52,63,79,159,1,44,0,85,

159,1,47,59,71,143,1,59,0,113,

143,1,52,63,79,159,2

1090 DATA 79,105,159,0,2,71,105,

143,0,2,79,105,159,0,2,71,105,

143,0,1,63,105,127,0,1,52,105,

0,179,1,71,0,143,0,1

1100 DATA 63,85,127,143,2,56,85,

113,143,2,63,85,127,151,2,56,8

5,113,143,1,47,85,95,143,1,42,

85,0,143,1

1102 DATA 56,85,113,143,1,63,85,

127,143,2

1110 DATA 47,63,95,151,1,42,50,63,

171,1,37,47,63,191,1,31,0,63,1

91,1,35,42,56,171,1,37,47,63,1

51,1,42,50,63,127,1

1112 DATA 35,0,56,127,1,37,47,75,

113,1,47,0,95,113,1,42,50,85,1

27,2

1120 DATA 37,63,75,151,1,50,63,74,

151,1,47,56,95,143,2,37,63,75,

191,2,56,71,101,171,2,37,63,75

,191,2,56,71,101,171,2

1130 DATA 47,75,95,227,1,63,75,95,

227,1,56,71,95,171,2,47,75,95,

227,2,56,71,95,171,2

1132 DATA 47,75,95,227,1,63,75,95,

227,1,56,71,95,171,2

1140 DATA 63,85,105,255,1,63,85,

101,255,1,75,95,127,191,2,71,9

5,113,171,2,63,85,105,255,1

1142 DATA 63,8q,101,255,1,75,95,

127,191,2,71,95,113,171,1,56,9 .

5,113,171,1

1150 DATA 63,85,101,255,2,75,95,

113,191,2,63,85,101,255,2,47,9

5,0,191,2,52,71,75,171,1,56,0,

113,171,1

1152 DATA 63,75,127,151,1,71,95,

113,143,1

94 The Afar; XL Handbook

1160 DATA 63,95,127,151,2,56,95,

113,143,2,47,63,75,151,2,42,50

,71,171,1,35,0,71,171,1,47,0,9

5,191,2,42,0,85,171,2

1170 DATA 47,0,95,191,2,52,71,85,

171,1,56,0,113,171,1,63,75,95,

151,1,71,95,113,143,1,63,95,12

7,151,2

1172 DATA 56,95,113,143,2,47,63,

95,151,2

1180 DATA 42,50,71,171,1,35,0,71,

171,1,47,95,0,191,2,42,85,0,17

1,2,47,95,0,191,2,0,0,0,0,0

1182 DATA 85,85,171,0,2,95,95,191,

0,2

1190 DATA 42,50,71,255,1,35,71,0,

255,1,47,95,0,191,2,42,85,0,17

1,2,47,95,0,191,2,0,0,0,0,0

1192 DATA 85,105,143,211,2,95,127,

151,191,2

1200 DATA 71,95,143,227,2,63,75,

127,255,1,47,0,95,255,1,56,71,

95,143,2,63,75,95,151,1,47,0,9

5,151,1

1220 DATA 56,71,95,171,2,71,95,113,

171,2

1230 DATA 63,85,101,255,2,85,101,

127,203,2,95,127,151,191,2,0,0

,0,0,0,85,113,143,171,2

1232 DATA 95,127,151,151,2,71,95,

143,171,2

1240 DATA 63,75,95,191,1,47,0,95,

191,1,56,71,95,143,2,71,85,113

,191,2,71,95,113,143,3

10000 DATA 0,0,0,0,255

Sound and Music 95

ADVANCED TECHNIQUES

The effects you can acheive using the SOUND command can be quite spectacular, but it is possible to do more, since this single BASIC command doesn't offer control over all of the functions of the POKEY chip.

With more understanding of how the chip operates you can have a greater influence on the noises it creates

SOUND REGISTERS

Like other chips in the XL, the POKEY chip is controlled by a number of registers, whose addresses and functions are shown here.

REGISTER FUNCTION

53760

53761

53762

53763

53764

53765

53766

53767

53768

ChannelO Frequency

Channel 0 Control

Channel 1 Frequency

Channell Control

Channel 2 Frequency

Channel 2 Control

Channel 3 Frequency

Channel 3 Control

POKEY Control

POKEY Chip Registers

96

The Atari XL Handbook

The SOUND command has a direct influence on the first six of these registers, and loads those pertinent to the specified channel with the appropriate data, for example:

SOUND 0,100,10,15 can be duplicated by the commands

POKE 53760,100

POKE 53761,175

The first PO KE command sets the frequency register for channel 0 to 100, which is fairly obvious, but the contents of the channel 0 control register (53761) require some explanation.

The eight bits of the control register control different aspects of the sound.

Bits 0, 1, 2 and 3 control the volume - four bits can represent the numbers 0 to 15, hence the restriction of 0 to 15 for the volume parameter in

SOUND

commands.

Bits 5, 6 and 7 control distortion. Three bits means eight different combinations or eight different degrees of distortion.

We POKEd a value of 175 into this register, and obtained a pure tone at volume 15. The volume is obtained by setting each of the four volume bits to

1. A pure tone involves setting bits 5 and 7 to 1, the value of bit 6 being irrelevant (that's why the numbers 10 and 14 give the same pure tone in a

SOUND

command).

Converting these bits to a decimal number gives:

Sound and Music

97

128

+

32

+

8

+

4

+

2 + 1 = 175

You can achieve the same effects as those obtainable with the SOUND command in this way, but this knowledge isn't much use. But what about bit 4 of the control register? This is the volume only bit and when set to 1, the value of the volume bits (0 to 3) is used to directly control the speaker of the TV.

Bits 0 to 3 select a voltage which is applied to the

TV speaker if bit 4 is set. The higher the value of bits 0 to 3, the higher the voltage and the greater the deflection of the TV speaker.

For example type:

POKE 53763,31

When you press the from the speaker.

RETURN

key, you will hear the beep as you press the key, followed by a click

POKE 53763,16 will select a zero voltage and the speaker will return to its rest position, with another click. This is how the XL creates noises with square waves, each pulse of which moves the speaker out then back to rest again.

Type in the following program, which steadily increases the voltage applied to the speaker.

10

20

30

40

50

60

FOR VOLTS=l TO 15

FOR Z=l TO 100

POKE 53763,16+VOLTS

POKE 53763,16:REM 0 VOLTS

NEXT Z

NEXT VOLTS

98

The Ateri XL Handbook

Each increase in voltage is alternately applied then switched off 100 times, causing a buzzing sound, which increases in volume with each increase in voltage. For each voltage the distance travelled by the speaker cone increases, displacing an increased amount of air, and so producing a louder sound.

We can change the frequency of the noise by inserting a delay loop:

35 FOR D=O TO 30:NEXT D

With this delay, you can hear the individual clicks as the speaker moves in and out.

Unfortunately the speed limitations imposed by

BASIC mean that this technique cannot be exploited fully without resorting to machine code.

THE POKEY CONTROL REGISTER

This register provides additional control of the

POKEY chip, and opens up many more sound generating possibilities.

The frequency of the square waves generated by the POKEY chip is governed by another square wave generated by the computer. The frequency of this stream of pulses (known as clock pulses) has a bearing on the range of frequencies you can generate. You can control the frequency of these clock pulses and thereby greatly increase therange of frequencies generated by the POKEY chip. Try this example:

SOUND 0,255,10,8 will generate the lowest frequency tone possible with a SOUND command. If you now type:

Sound and Music 99

POKE 53768,1 the frequency of the note produced will drop dramatically. This is because bit 0 of the POKEY control register (peR from now on) controls the frequency of the clock pulses applied to the POKEY chip. Bit 0 is usually set to 0, but when set to 1, the frequency of the clock pulses is reduced by a factor of about four - resulting in a much lower output frequency.

This effect applies to all four tone channels.

The other seven bits in peR are used to control other functions, as shown in this table.

BIT FUNCTION

0

1

2

Switch clock from 64kHz to 15 kHz

High Pass Filter on Channel 1

(controlled by Channel 3)

High Pass Filter on Channel

(controlled by Channel 2) a

3

4

5

6

Adds Channel 2 to Channel 3

Adds Channel a to Channel 1

Channel 2 clock

Channel 0 clock

=

1.79 MHz

=

1.79 Mhz

7 Increases repeat rate of random waves

POKEY Control Register Functions

100 The Ateri XL Handbook

Bi ts 5 and 6 provide a more flexible way of changing the clock frequency applied to channels 2 and 0, without affecting other channels. The clock frequency is increased by a factor of nearly 28 - try this example.

SOUND 2,200,10,15

POKE 53768,32

Note that the frequency of the output tone is much higher after setting bit 5 of peR.

10 FOR Z=l TO 12

20

30

40

50

60

70

READ N

SOUND O,N,10,8

SOUND

NEXT Z

2,N,10,8

FOR P=O TO 200:NEXT

SOUND O,O,O,O:END

P

100 DATA 251,230,217,204,193,182,

173,162,153,144,136,128

This program plays ascending notes on two channels at once. If you add the line:

45 POKE 53768,32 bi t 5 of the peR is set, increasing the clock frequency applied to channell - try it and see!

This method of increasing the frequency of the output tone is rather crude, and the peR provides facilities for more accurate control of frequencies.

In normal use we are restricted to 256 different frequencies per channel, since the frequency register for each channel consists of only eight bits.

This restriction means that many frequencies cannot be acheived. However, peR hits 3 and 4

Sound and Music 101 allow you to 'joiri' two channels together, forming one channel with a 16 bit frequency register.

Bit 3 joins channel 2 to channel 3, and bit 4 joins channel 0 to channel 1. So if you are prepared to reduce the number of independent tone channels, you will be able to generate a much wider range of freq uencies.

The following program demonstrates the increased frequency range by joining channels 0 an 1.

10

20

SOUND 0,0,0,0

POKE 53768,64+16

30

40

50

60

70

80

POKE 53761,160

POKE 53763,175

FOR Z=O TO 65535

HI = INT(Z/256)

LO

= Z-(HI*256)

POKE 53760,LO

90 POKE 53762,HI

100 NEXT Z

110 SOUND 0,0,0,0

The program will take a long time to run, so you might like to try adding a

NEXT

STEP

loop to speed things up a bit.

to the

FOR ...

The range of frequencies can be altered by setting other bits in peR which control the clock frequency applied to the to:

POKEY

chip. Try changing line

20

20 POKE 53768,16

This will select the normal clock frequency instead of 1.79 MHz (1 MHz is one million cycles per second) making the lowest frequency about 1 Hz (or one click per second). The lowest frequency can be reduced still further, by setting bit 1 of peR to

102 The Ateri XL Handbook reduce the clock frequency to 15 kHz (1 kHz is one thousand cycles per second):

20 POKE 53768,17

FILTERING

Bits 1 and 2 of peR control what are known as pass filters.

high

These are filters which cut out frequencies lower than a certain value. In the

POKEY

chip, the filtering frequency (that below which frequencies are cut off) is set on one of the tone channels and acts on another channel.

As shown in the table of

POKEY

control registers at the beginning of this section, if bit 1 of peR is set, any frequency on channell which is lower than that on channel 3 is filtered out. If bit 2 is set, channel 2 acts as a filter on channel O.

The principle is as follows:

At any instant, if the frequency on the tone channel is greater than that of the filter, the tone will be heard, otherwise it will not. This gives rise to an effect similar to, but much more regular than, distortion as illustrated in the diagram opposite.

Try this example to hear it in action.

10

20

30

40

50

60

70

SOUND 0,0,0,0

POKE 53768,4

POKE 53760,240

POKE 53761,168

POKE 53764,120

POKE 53765,160

GOTO 70

If the volume bits of the filter channel are set, that tone channel will be heard as well - change line 60 to:

Sound and Music

SQUARE WAVE

F

R

E

Q u

E

N

C y

-----.In

TIME

-

FILTERING WAVE n r

RESULTANT WAVE

•

60 POKE 53765,168

If you delete line 20, the two tone channels alone will be heard.

Finally, bit 7 of the peR controls the rate at which the pattern of the square wave controlling the distortion repeats, and when set to 1, makes any distorted sound much more regular, as the following example shows. You will hear the sound change after about two seconds, as line 50 sets bit 7 in the peR.

10

20

30

40

SOUND 0,10,8,8

SOUND 1,100,8,8

SOUND 2,255,8,8

FOR D=O TO 1000:NEXT D

103

104 The Ateri XL Handbook

50

60

POKE 53768,128

GOTO 60

As you can imagine, the number of possible combinations of the techniques mentioned in this chapter is virtually infinite and a good deal of experimentation is required to get the best out of the POKEY chip.

CHAPTER 12

CHARACTERS

Up to now all the displays we have created have comprised light blue characters on a dark blue background. You will probably be aware that the

XL is capable of more impressive displays than this, and the following four chapters will explain how to make the most of the graphics potential of your computer.

CHANGING

COLOURS

The XL is able to display 16 primary colours, and you can choose anyone of these to be the screen or border colour using a BASIC command

SETCOLOR

The syntax of the

SETCOLOR R,C,L command is:

The three parameters specify the register or store location into which we want to place colour information, the code specifying the colour we require, and the luminance, or brightness, of the colour.

COLOUR REGISTERS

The XL reserves five memory locations for storing colour information and these colour registers are numbered 0 to 4.

The colour of the screen is controlled by the contents of register 2, so to change

105

106 The Atari XL Handbook the screen colour we use the

SETCOLOR

command to alter the contents of register 2.

The sixteen colours from which we make our choice are represented by code numbers 0 to 15, as shown in this table:

CODE COLOUR

5

6

7

8

9

2

3

0

1

4

10

11

12

13

14

15

GREY

GOLD

ORANGE

RED

PINK

VIOLET

PURPLE

LIGHT BLUE

DARK BLUE

BLUE GREEN

BLUE

DARK BLUE

GREEN

DARK GREEN

OLIVE GREEN

ORANGE

XL Colour Codes

The actual colour displayed depends on the third parameter in the

SETCOLOR

command luminance. This controls the brightness of the colour and can have a value between 0 and 14. The

Characters 107 higher the luminance number the brighter the colour; so grey at luminance 0 will be black, but at luminance 14 will appear white. Odd values for luminance have the same effect as the lower even number, so there is a choice of eight.

To see

SETCOLOR

in action, type in the following program which cycles through the possible screen and border colour combinations.

10

20

30

40

50

60

FOR COLR=O TO 15

FOR LUM=O TO 14 STEP 2

SETCOLOR 2,COLR,LUM

FOR DELAY=O TO 200:NEXT DELAY

NEXT LUM

NEXT COLR

The program changes the contents of Register 2 which controls screen colour, if you change line 30 to:

30 SETCOLOR 4,COLR,LUM the border colour will be affected. It is also possible to alter the luminance of the characters, by altering register 1 as in this example, where characters are made to flash by controlling their luminance.

10

20

30

40

50

60

SETCOLOR 2,3,0

LUM = 14

SETCOLOR l,3,LUM

FOR PAUSE=O TO 100:NEXT PAUSE

LUM = 14-LUM

GOTO 30

38 OR 40

COLUMN

SCREEN?

All our programs so far have used a screen display of 38 columns by 24 rows, leaving a margin of two characters at the left of the display. This margin is

108 The Ateri XL Handbook preset to cater for the variety of TVs which might be connected to the computer.

It is possible to regain the two columns and allow a full 40 column display by altering the contents of one of the two memory locations that the XL uses to control the width of the display. Location 82 sets the position of the left hand margin, and location 83 the right hand margin. When the XL is switched on, a value of 2 is placed in location 82, setting the lefthand edge of the display at column 2, so to regain the two

(lost' columns, simply:

POKE 82,0

You will then have access to all 40 columns until you switch off the computer or press RESET.

DIFFERENT DISPLAYS

As we have said, the XL is capable of a variety of display types, and the different displays are referred to by a number - the MODE number. The

40 by 25 display is known as mode 0, and is the default mode which is selected when you switch on the XL. There is a BASIC command which is used to change modes -

GRAPHICS.

The syntax of the

GRAPHICS

command is:

GRAPHICS M where M is the number of the mode required. So typing:

GRAPHICS 0 would select the standard display - but there isn't much point in doing that since that's the mode we are using! There are two other modes which can be

Characters 109 used to display characters similar those in mode 0; modes 1 and 2.

GRAPHICS MODES 1 AND 2

The characters in mode 1 are twice as wide as those in mode 0, so the number displayable on one line is reduced to 20 (the preset margins do not apply to

Graphics 1 and 2 displays). Mode 2 characters are twice as high and twice as wide, bringing the

'number of characters per screen or resolution down to 20 by 12.

As well as the different sized characters the screen is split into two parts - a small section at the bottom is reserved for a four line mode 0 display while the top portion is reserved for mode 1 or 2 characters. The mode 0 window means that the number of rows of characters that can be displayed in modes 1 and 2 is reduced. To see these modes type in the following program:

5

10

20

30

40

50

60

REM GRAPHICS lAND 2 DEMO

MODE=l

GRAPHICS MODE

PRINT:PRINT"GRAPHICS 0 WINDOW"

POSITION 1,5

PRINT·#6; "GRAPHICS ";MODE;"

SCREEN"

GOTO 60

As you can see from the listing, PRINT cause characters to appear in the mode

°

window, but another technique is required to put characterson the mode 1 section of the screen.

To cause characters to appear on the mode 1 and 2 screens, the PRINT command is modified to

PRINT 16. This instructs the computer to send the data following the PRINT command to the destination specified after the I symbol. In this

110 The Ateri XL Handbook case the destination is 6, which represents the screen. This technique is commonly used to send data to peripheral devices such as the cassette unit

(see Chapter 16) and here we are treating the model screen as ifit were a peripheral device.

The colour and luminance of the message displayed by the last program is controlled by colour register

0, and can be changed with the SETCOLOR command. However it is possible to display characters in up to four different colours on a mode

1 or 2 screen, demonstrates.

as the following program

10

20

30

40

50

60

70

80

GRAPHICS 1

PRINT#6;"REGISTER ZERO"

PRINT#6

PRINT#6;"register one"

PRINT#6

PRINT#6;"REGISTER TWO":REM

THIS TEXT IN REVERSE

PRINT#6

PRINT#6;"register three":REM

THIS TEXT IN REVERSE

Notice that regardless of the case specified in the program, the messages on the screen are all in upper case and those which would appear in inverse video in mode 0 do not do so in mode 1. This is because the additional information which would affect case and inverse video in mode 0, is used to select one of four colour registers to determine the colour of the characters. The following table shows which registers are selected by which type of text.

Characters can be positioned on the mode 1 and 2 screens using the POSITION command as in mode

0, but remember that because the characters are larger than those in mode 0, the limits of screen coordinates are correspondingly reduced.

Characters 111

TEXT REGISTER

UPPER CASE lower case

0

1

INVERSE UPPER CASE inverse lower case

2

3

Colour Register use in Modes 1 and 2

There is another way to put characters on the mode

1 and 2 screens using the commands

COLO

Rand

PLOT.

Wi th this method, characters are placed on the mode 1 or 2 screens with a syntax of which is:

PLO'r

command, the

PLOT X, Y where X and Yare screen coordinates, and X

Y display.

=

0,

=

0 specifies the top left-hand corner of the

The character to be plotted is selected using the

COLOR

command. The syntax ef'-the

COLOR

command depends upon the graphics mode in use, and in modes 1 and 2, the syntax is:

COLOR C where C is a code which specifies not only a character but also the register in which colour information for that character is stored. For each.

character there are four possible code numbers, each specifying the same character, but a different colour register.

For example, the code 42 represents the asterisk, and specifies register 0 for colour information. The number 10 also represents an asterisk, but its colour is determined by the

112 The Aten XL Handbook contents of register 1. A full table of all these codes is given in Appendix 7.

The following program demonstrates how multicoloured messages can be displayed on the mode 1 and 2 screens using the COLOR and PLOT commands.

5

10

20

30

40

50

60

REM MODE 1

&

2 COLOR/PLOT DEMO

MODE=2

GRAPHICS MODE

FOR CHAR=O TO 18

READ COLR

COLOR COLR:PLOT CHAR,8/MODE

NEXT CHAR

70 GOTO 70

500 DATA 52,32,100,201,230,70,101

510 DATA 210,229,78,116,32,195,239

520 DATA 76,111,213,242,83

The DATA statements in lines 100 to 120 contain codes for the characters and their associated colour registers taken from the table in Appendix 7, and these are selected one at a time and plotted by the loop between line 30 and 60. The program will work equally well in mode 1.

The colours in which the characters are displayed are the default values for each of the colour registers, as shown in the table opposite.

The program can be made more interesting by changing these registers to give different colours and luminances. If you do change the value .in a register with the SETCOLOR command, all characters whose colour is determined by that register will instantly take on the new colour. You can see this by deleting line 70 and adding the following lines to the last program:

100 FOR COLR=O TO 15

Characters 113

LOC.

REG.

COLR.

LUM.

708

709

710

711

712

0

1

2

2

12

9

8

10

4

COLOUR

ORANGE

GREEN

BLUE

3 4 6 PURPLE

4 0 0 BLACK

Default Colour Register Values

110 FOR REG=O TO 3

120 SETCOLOR REG,COLR+2*REG,12

130 NEXT REG

140 FOR DELAY=O TO 50:NEXT DELAY

150 NEXT COLR

160 GOTO 100

The colours will continue to cycle until you press

BREAK.

You can also use the POKE command to directly alter the contents of the colour registers. Add these lines to the last program and type GOTO 200 to cycle through all the colour and luminance combinations for the screen colour, by POKEing the colour register location (given in the table) with every number from 0 to 255.

200 REG=710

210 FOR COL=O TO 255

220 POKE REG,COL

230 FOR PAUSE=O TO lOO:NEXT PAUSE

240 NEXT COL

250 GOTO 250

When you change character colours, you will notice that the colour of the mode 0 window also changes, because this is controlled by one of the registers controlling character colours (register 2). This is a

114 The Atari XL Handbook bit distracting, and there is a way of removing the mode 0 window, allowing the entire screen to be given over to mode 1 or 2 displays. To achieve this simply add 16 to the mode number, so that mode 1 without a mode 0 window is obtained by typing:

GRAPHICS 1+16

10

20

25

30

35

40

50

60

65

70

80

Try changing line 20 in the previous program to see this. The penalty is that if the program is halted for any reason, the XL will return to mode 0 losing your display.

One drawback of being able to specify one of four colours for a mode 1 or 2 display is that only 64 characters can be represented by codes 0 to 255.

This restricts us to using only upper case letters in the previous examples, which is only half of the available character set. To display the other half of the character set, we must change the place from which the XL obtains its characters by PO KEing the character set pointer (location 7:56) with the number 226 (its default value is 224). Don't worry too much about what this means - all will become clear later in this chapter.

The following program, in a romantic vein, demonstrates the use of the other half of the character set in mode 2.

MODE=2+16

GRAPHICS MODE

SETCOLOR 0,3,8

POKE 756,226

FOR CHAR=2 TO 17

READ DATA

COLOR DATA:PLOT CHAR,4

NEXT CHAR

REM LOOP TO FLASH MESSAGE

FOR LUM=O TO 14 STEP 2

SETCOLOR 1,12,LUM

Characters 115

85

90

FOR DELAY=O TO 30:NEXT DELAY

NEXT LUM

100 GOTO 70

110 DATA 104,97,112,112,121,32,

32,118

120 DATA 97,108,101,110,116,105,

110,101

The screen full of hearts occurs because the character corresponding to space in the first half of the character set is a heart, so everywhere which would normally be blank contains a heart. The colour of the hearts is set in line 25, and the second half of the character set specified in line 30.

If you change line 30 to:

30 POKE 756,224 and run the program again, the first half of the character set will be used but the resulting display doesn't have the same romantic appeal!

CHARACTER DEFINITIONS

The shapes of all the displayable characters are defined by numbers held in a special area of memory. The XL allows you to create new symbols by entering new definitions for the characters to replace those held in memory. Each symbol in the character set is defined by the contents of eight memory locations, which correspond to a grid of eight rows; each row consisting of eight dots which may be set to the foreground or the background .

colour. (Each of these dots is called a pixel, which is a contraction of picture element.

The display is built up from 69120 of these pixels.) Each location defines one row of the character, and each bit of the number in the location represents one pixel. If the bit is set to 1, the pixel is displayed in the

116 The Ateri XL Handbook

Memory

Location:

Bit:

2

3 o

1

4

5

6

7

7 6 5 4 3 2 1 0

The Character Grid foreground colour; if the bi t is set to 0 the pixel remains in the background colour.

In normal use, the character definitions are held in

2K of read -only memory (ROM), beginning at the location determined by the contents of location 756.

The start address of the character set ROM is obtained by multiplying the contents of location

756 by 256. There are actually two sets of 128 characters, the first set including punctuation and maths symbols, numbers and letters and graphics characters. The second set is a copy of the first except that the 29 graphics symbols (normally obtained by holding down the CONTROL key and pressing a key) are replaced by a European character set, which contains characters like the £ sign. To switch between the two character sets use the command:

POKE 756,204 and to switch back:

POKE 756,224

Characters 117

INTERNAL CHARACTER CODES

The XL makes reference to the eight bytes defining a character by assigning a code, known as the

Internal Character Code to each one.

This is different from the ASCII code, so to find the eight bytes defining a character we must first convert the

ASCII code of that character to internal code. This is done with the following formula:

INTERNAL=ASCII+{{ASCII<32)*64)-

({ASCII>31 AND ASCII<96)*32)

What this means is that the internal code is the same as ASCII for codes greater than 95; for codes less than 32 the internal code is greater by 64, and for ASCII codes between 31 and 96, the internal code is 32 less than ASCII.

Having worked out the internal code of a character, to find the eight bytes which define it, we multiply the internal code by eight, and add the start address of the character set:

CHARACTER LOCATION =(INTERNAL

CODE*8)+{256*PEEK{756»

So the definition of the letter X (ASCII Code

Internal Code is to be found at location:

=

88;

=

56) in the standard character set

56*8+{256*{PEEK(756» = 57792

This short program will display the eight numbers defining any character.

1

5

10

REM DISPLAY CHARACTER DATA

OPEN#1,4,0,"K:"

PRINT"

118 The Atari XL Handbook

20

30

CSET = PEEK(756)*256

PRINT"ENTER ASCII CODE OF

CHARACTER" j

40

45

50

60

INPUT ASCII:PRINT"

PRINT "CHARACTER"jCHR${ASCII):

PRINT

CODE=ASCII+({ASCII<32)*64)-

({ASCII>31 AND ASCII<96)*32)

PRINT"ASCII CODE "jASCIlj"

INTERNAL CODE njCODE:PRINT

70

80

CHAR = CODE*8 + CSET

FOR Z=CHAR TO CHAR+7

90 PRINT PEEK{Z):NEXT Z

100 PRINT:PRINT "PRESS ANY KEY TO

CONTINUE"

110 GET#1,X:GOTO 10

For example, taking the letter X, the program should display the numbers 0,102, 102, 60, 60, 102,

102, Having found the eight numbers, we can.

128 64 32 16 8 4 2 1 o

102

102

60

60

102

102 o

The Character

X convert them to binary form and fill in the grid to produce a letter X.

Characters 119

Notice that the top and bottom lines of the character are left blank. This is to leave a space between lines of text on the screen.

REDEFINING CHARACTERS

You may be wondering how it is possible to create new characters when the standard character definitions are held in ROM - it's difficult to write new numbers into read-only memory! The answer is that we have to tell the XL to look somewhere else for the character shapes, and define the new characters in RAM. We have used this technique before to swap between Standard and European character sets. By POKEing the high byte of the start address of the character set into location 756 we changed the memory from which the character definitions were taken.

Location 756 is the place where the XL stores the high byte of the start address of character data (ie to find the address of the start of character data you must multiply the contents of location 756 by 256).

This information is used by the chip which controls all the display of the XL, known as the ANTIC chip. Location 756 is a copy of one of the registers on this chip, known as a shadow register.

The contents of the shadow register are copied to the chip every sixtieth of a second and used to dictate whereabouts in ROM the character set is to be found.

This pointer can just as easily be made to point to .

an area of RAM - try this:

LIST the program to show some characters on the screen, then type:

POKE 756,2

120 The Aieri XL Handbook

The program listing turns into garbage because the

ANTIC chip is taking data for characters from an area of RAM which contains system variables, not character data. Press RESET to return to normal viewing!

To make use of user defined characters you must first copy all the characters in a set into the RAM, and tell the ANTIC chip where to find them. You can then alter some (or all) of the characters to your own designs.

The first step in redefining characters then is to copy the entire character set into RAM, in order that we can modify some or all of the data. The following program does this for you.

50

60

70

75

10

20

30

40

REM COpy CHARACTER SET

POKE 106,PEEK(106)-4

GRAPHICS 0

FOR Z=O TO 124

PRINT CHR${Z)i

IF Z=26 THEN Z=31

NEXT Z

FOR DELAY = 0 TO 500:NEXT

DELAY

CSET=PEEK(756)*256

RAMSET = 256*PEEK(106)

80

90

100 POKE 756,PEEK(106)

110 FOR Z=O TO 1023

120 POKE RAMSET+Z,PEEK{CSET+Z)

130 NEXT Z

140 PRINT:PRINT "COPYING COMPLETE"

Line 20 reserves space in RAM for the character set; location 106 contains the page number of the top of memory, i.e.

(256*PEEK( 106))-1

Characters 121 is the address of the top of free RAM. By POKEing a value of four less into location 106, we protect the top four pages of memory (1024 bytes) from use by

BASIC. It is here that we will store our character set. Line 70 sets mode 0, and in doing so forces the computer to check location 106 to find out how much memory is available. Lines 40 to 70 put the characters set on the screen, omitting the control characters like screen clear and cursor characters.

After a short delay the character set pointer is moved to our reserved area of RAM by line 80, and the character data copied from ROM to RAM. As it is copied, you will see the garbage which used to be the character set return to normal. The computer will behave as usual after copying, the difference being that character data is now being taken from

RAM. Now that the character set is in RAM we can change it to suit ouselves. As an example, if you add these lines to the last program and then GOTO

1000, all the characters will be turned upside down

- you have redefined them!

200

1000

1010

1020

1030

1040

END

REM INVERT CHARACTERS

FOR Z=O TO 1023 STEP 8

FOR Y = 0 TO 7

POKE RAMSET+Z+Y,PEEK(CSET+

Z+(7-Y»

NEXT Y

Although this is a good example to illustrate redefined character sets, if you ever do want to invert the character set, there is a way of doing so' which requires much less programming effort:

POKE 755,6

This will invert the character set, and

122 The Atari XL Handbook

POKE 755,2 will return it to normal.

SPACE INVADERS

A more typical use of user defined characters would be to create a set of invader characters for a game, or a foreign character set such as Russian or Greek, or even a set of symbols for use in some specialist application.

Let's take the most exciting of these and create some invaders!

Firstly we take the eight by eight grid from the beginning of this chapter and draw our design Now

128 64 32 16 8 4 2 1 o

153

189

219

126

36

60

66

Space Invader Character to calculate the data statements required to specify the invader character, take one row at a time, and if a bit is set, write down the value of that bit (the numbers at the top of the grid). Continue for all eight bits in the row, and add up the numbers you

Characters 123 obtained. For example, in the bottom row, the sum

IS:

64 + 2 = 66 and for the top row where no bits are set the answer is O.

In this way, you can obtain eight numbers which define your character.

All that remains is to choose the character you wish to reprogram, find the start address of the data defining it, using the formula on page 117, and replace the data with your own.

If you add the following lines to the program on page 120, the @ symbol will be turned into an invader.

200 CHAR = 32*8+RAMSET

210 FOR Z=O TO 7

220 READ D

230 POKE CHAR+Z,D

240 NEXT Z

250 DATA 0,153,189,219,126,36,60,

66

From now on, until you press RESET, enter another graphics mode or redefine it, the @ symbol will appear as an invader.

While on the subject of invaders, it is quite easy to achieve their limited form of animation - by reprogramming several character sets, each containing a slightly different version of the character. To animate, simply move the character set data pointer (location 756) to each set in turn.

124 The Ateri XL Handbook

CHARACTER GENERATOR PROGRAM

This program allows you to design new characters on the screen and add them to the character set.

The characters are created by filling in a large grid on the screen, which is then used to program the character.

10

11

12

13

14

REM ***********************

REM * *

REM * CHARACTER GENERATOR *

REM * *

REM ***********************

15 REM

100 GOSUB 20000:REM SET UP

110 GOTO 10000:REM MAIN PROGRAM

190 REM

191 REM ***********************

192 REM * *

193 REM * REDEFINE CHARACTERS *

194 REM * *

195 REM ***********************

196 REM

200 PRINT" " "

210 PRINT "ENTER ASCII CODE OF

CHARACTER (0-127)":INPUT ASCII

220 IF ASCII<O OR ASCII>127 THEN

200

225 CODE=ASCII+«ASCII<32)*64)-

«ASCII>31 AND ASCII<96)*32)

230 CHARDATA=CODE*8+RAMSET

295 REM STORE CHARACTER DATA IN

ARRAY

300 PRINT" " " : GOSUB 900

310 FOR R=l TO 8:GOSUB 1000:NEXT R

395 REM DISPLAY CHARACTER

400 GOSUB 2000

410 X=I:R=l

500 B=PEEK(CC)

510 REM MODIFY CHARACTER

Characters 125

520

930

940

990

991

992

993

994

894

895

896

900

910

920

525

530

540

550

890

891

892

893

995

996

1000

1010

1020

1030

1040

1045

1050

1060

1070

1080

1090

1100

1990

1991

1992

POKE 764,255:GOSUB 3000:POKE

764,255

IF EXIT=l THEN EXIT=O:RETURN

IF CL=l THEN CL=O:GOTO 400

CC=GRIDST+X+40+R

GOTO 500

REM

REM *************************

REM

REM

REM

REM

*

*

*

DISPLAY CHARACTER SET

*

*

*

REM *************************

FOR Z=O TO 124

PRINT CHR(Z)i

IF Z=26 THEN Z=31

NEXT Z

RETURN

REM

REM ************************

REM

REM

*

* STORE CHARACTER DATA

REM

REM

*

REM ************************

*

*

*

N=PEEK(CHARDATA+R-l)

X=8

A(R,9)=N:REM ROW TOTAL

REM STORE BIT PATTERN

IF N/2=INT(N/2) THEN A(R,X)=O:

GOTO 1050

A(R,X)=l

N=INT(N/2):X=X-l

IF N>=l THEN 1040

FOR Z=X TO 1 STEP -1

A(R,Z)=O

NEXT Z

RETURN

REM

REM *********************

REM * *

126 The Ateri XL Handbook

1993

1994

1995

1996

2000

2010

2020

2030

2035

2040

2050

2060

2070

2075

2080

2990

2991

2992

2993

2994

2995

2996

3000

3010

3020

3030

3040

3050

3060

3080

3090

REM * DISPLAY CHARACTER *

REM * *

REM *********************

REM

PRINT

FOR R=1 TO 8

FOR X=l TO 8

IF A(R,X)=l THEN PIXEL=84:GOTO

2040

PIXEL=14

POKE GRIDST+X+(R*40),PIXEL

NEXT X

POSITION 18,6+R:PRINT A(R,9)

NEXT R

POSITION 2,20:PRINT"ESC

RETURN -

IN REVERSE

= + * CLEAR

PROGRAM":REM BOLD CHARACTERS

RETURN

REM

REM ***************

REM *

REM * MOVE CURSOR *

REM *

*

*

REM ***************

REM

REM CHECK KEYBOARD

K=PEEK(764):REM INTERNAL CODE

OF KEY PRESSED

REM FLASH CURSOR

CH=PEEK(CC):C=CH-«CH>128)*

128)+«CH<128)*128)

POKE CC,C:FOR PAUSE=O TO

10:NEXT PAUSE

REM CHECK KEY INPUT

IF K=255 THEN 3010

IF K=6 AND X>1 THEN POKE CC,B:

X=X-1:RETURN:REM LEFT

IF K=7 AND X<8 THEN POKE CC,B:

X=X+l:RETURN:REM RIGHT

Characters 127

3100

3110

3120

3130

3140

3150

3291

3292

3293

3294

3295

3296

3300

3310

3320

3325

3160

3170

3190

3191

3192

3193

3194

3195

3196

3200

3210

3290

3330

3990

3991

3992

3993

IF K=15 AND R<8 THEN POKE CC,B

:R=R+l:RETURN:REM DOWN

IF K=14 AND R>1 THEN POKE CC,B

:R=R-1:RETURN:REM UP

IF K=33 THEN A(R,X)=1-A(R,X):

GOTO 3200:REM CHANGE PIXEL

IF K=10 THEN GOSUB 4000:REM

REPROGRAM CHARACTER

IF K=118 THEN GOSUB 3300:REM

CLEAR CHARACTER

IF K=12 THEN X=1:R=1:POKE CC,B

: RETURN: REM TOP LH CORNER OF

GRID

IF K=28 THEN EXIT=I:RETURN

POKE 764,255:RETURN

REM

REM ****************

REM

REM

REM

REM

* *

* CHANGE PIXEL *

*

****************

*

REM

IF B=14 THEN POKE CC,84:RETURN

POKE CC,14:RETURN

REM

REM *******************

REM

REM

REM

REM

*

*

*

CLEAR CHARACTER

*

*******************

*

*

REM

FOR R=1 TO 8:FOR X=1 TO 9

A(R,X)=O

NEXT X

POSITION 18,R+6:PRINT"

; : PRINT A( R, 9) :NEXT R

CL=l:RETURN

REM

REM ***********************

REM

REM

*

* REPROGRAM CHARACTER

*

*

128 The Ateri XL Handbook

3994

3995

3996

4020

4030

4040

4050

4060

4065

4070

4080

REM * *

REM ***********************

REM

FOR RT=l TO 8

FOR XT=l TO 8

D=D+(2A(8-XT»*A(RT,XT)

NEXT XT

POKE CHARDATA+RT-l,D

A(RT,9)=D

4090

9990

9991

D=O

POSITION 18,RT+6:PRINT"

E--E--E--E--" ; :

PRINT A( RT, 9 ) : NEXT RT

RETURN

REM

REM *************

9992

9993

9994

9995

9996

10000

10010

REM * *

REM * MAIN MENU *

REM

REM

*

REM *************

*

PRINT""" :CLOSE#l

POSITION 9,1:PRINT"CHARACTER

GENERATOR"

10020 POSITION 2,5:PRINT"CHANGE

CHARACTERS":REM BOLD

CHARACTERS IN REVERSE

10030 PRINT:PRINT:PRINT"LOAD

CHARACTER SET FROM TAPE"

10040 PRINT:PRINT:PRINT"SAVE

CHARACTER SET TO TAPE"

10050 PRINT:PRINT:PRINT"ESC TO END

PROGRAM"

100600PEN#1,4,0,"K:"

10065 GETil,K

10070 IF K=67 THEN GOSUB 200:GOTO

10000:REM C

10080 IF K=76 THEN GOSUB 11000:GOTO

10000:REM L

10090 IF K=83 THEN GOSUB 12000:GOTO

10000:REM S

10100 IF K=27 THEN END:REM ESC

Characters

10110 GOTO 10065

10990 REM

10991 REM

10992 REM

**********************

* *

*

LOAD CHARACTER SET

*

10993 REM

10994 REM

10995 REM

*

**********************

*

10996 REM

11000 CLOSE#l: PRINT""" :OPEN#l, 4,

0, "C: "

11010 FOR BYTE=Q TO 1023

11020 GET#1,TEMP

11030 POKE RAMSET+BYTE,TEMP

11040 NEXT BYTE

11050 CLOSE#1:RETURN

11990 REM

11991 REM

11992 REM

11993 REM

11994 REM

11995 REM

11996 REM

**********************

*

*

SAVE CHARACTER SET

*

*

*

**********************

*

12000 CLOSE#1:PRINT""":OPEN#1,8,

0, "C: "

12010 FOR BYTE=O TO 1023

12020 PUTI1,PEEK(RAMSET+BYTE)

12030 NEXT BYTE

12040 CLOSE#1:RETURN

11990 REM

11991 REM

11992 REM

11993 REM

*

***********************

*

11994 REM *

11995 REM

SETTING UP ROUTINES

*

*

*

***********************

11996 REM

20000 DIM A(8,9)

20010 POKE 106,PEEK(106)-4

20020 GRAPHICS 0

20025 SETCOLOR 2,2,6:SETCOLOR

1,2,0:SETCOLOR 4,2,8

20026 POSITION 7,5

129

130 The Atari XL Handbook

20030 PRINT"RELOCATING CHARACTER

SET"

20035 POSITION 5,22:PRINT "Please wait approx.

25 seconds"

20040 CSET=256*PEEK(756)

20050 RAMSET=256*PEEK(106)

20060 FOR BYTE=O TO 1023

20070 POKE RAMSET+BYTE,PEEK

(CSET+BYTE)

20080 NEXT BYTE

20085 POKE 756,PEEK(106)

21000 DLIST=PEEK(560)+256*PEEK(561)

21010 SCRNST=PEEK(DLIST+4)+256*

PEEK(DLIST+5)

21020 GRIDST=SCRNST+245

25000 RETURN

The program allows you to redefine any character, and to save character sets on tape and reload them.

You can use the new character sets with other programs.

The first thing the progam does is create some space for the character set and then copy the character set into RAM. Lines 20020 to 20080 do this. The rest of the lines in the setting up routine find the start of screen memory (SCRNST) and decide whereabouts on the screen the, character grid is to be placed (GRIDST).

Lines 10000 to 10110 then display the menu, giving you the option to change a character or to load or save a character set. Pressing the ESC key stops the program.

-

The redefining of characters is controlled by the routine from 200 to 550. When you input the code of the character you wish to alter, the line 310 calls the routine at lines 1000 to 1100 to store the data of

Characters 131 the character in an array. The character is then displayed on the screen by the routine at 2000.

The modification of the character is performed by the section of program at 3000. You may move the cursor around the character grid wi th the cursor keys and use the space bar to set or clear the pixels of the character. A menu of the options open to you is displayed at the bottom of the screen. The ESC key returns you to the main menu, while the

RETURN key moves the cursor to the top left corner of the grid.

To erase all the data for a character, hold down SHIFT and press the CLEAR key.

When you have finished designing the new character, pressing P will store the new definition in character memory. Lines 4000 to 4090 perform this function.

Saving a character set onto tape is performed by the subroutine at 12000. The character sets are reloaded into the machine by the routine at 11000.

WARNING

Do not reset the XL by using the RESET key or the new characters will be lost.

MULTICOLOUR CHARACTERS

There are two more graphics modes which display characters, and wi th these the colour of the individual pixels which make up a character can be .

specified. This means you can have up to four different colours within each character.

The two multicolour character modes are modes 12 and 13, and are obtained in the usual way. A four line mode 0 window is obtained but as with modes 1 and 2, adding 16 to the mode number in a

132 The Atari XL Handbook

GRAPHICS

command will allow the entire display to be used for multicolour characters.

Mode 12 allows 40 columns by 24 rows of characters - the same as mode o.

Mode 13 allows 40 columns by 12 rows of multicolour characters, the characters being twice as tall as those in mode 12.

Type in the following program to see some multicolour characters.

10

20

GRAPHICS 12

PRINTit6i"MULTICOLOUR

CHARACTERS"

You may just be able to discern the message - it is certainly multicoloured, but hardly legible!

This is because in mode 12 (and mode 13) the information defining a character is interpreted differently by the computer.

A multicolour character is still defined by eight bytes of character memory, but each pixel comprising the character is represented by two bits rather than one.

This means that multicolour characters are only four pixels wide, but the pixels are double the normal width so characters are the same size as in other modes. The extra information available in two bits is used to select the colour of that pixel.

The multicolour characters can be designed on a similar eight by eight grid to that used for standard characters, as shown opposite.

There are four possible combinations of two bits,

00, 01, 10 and 11, and each of these represents a different colour or rather specifies a different colour register.

Press the RESET key (to return the colour registers

to

their default values) and type in the following

Characters 133

Memory

Location: o

1

2

3

4

5

6

7

:

:

: :

Bit: 7 6 5 4 3 2 1 0

The Multicolour Character Grid program,which puts a letter Q in the centre of a mode 13 screen. Mode 13 is used for clarity, the principles apply equally to mode 12.

10

20

30

GRAPHICS 13

POSITION 18,4

PRINT#6;"Q"

The Q is just recognisable, and if you look closely comprises three different colours. To see how these colours originate, let's look at the eight bytes defining the letter Q in character memory.

The diagram shows how the data for letter Q is stored in the XL. In mode 0, each pixel is represented by a single bit. In modes 12 and 13 .

however, the computer treats this data as four groups of two hits, each specifying the colour register from which information concerning that pixel is to he taken.

If we take one row (one byte) and examine it in more detail we will see how colour information is

134 The Afar; XL Handbook

2

5

6

7

3

4 o

1

The Character Q derived. Take row 6 as it has an example of each of the four possible bit combinations.

o o o o

Row 6 of the character Q

The first bit pair is 00, which means the pixel represented by that bit pair takes its colour from register 4.

The second pair, 11, specifies register 2, the third register 0 and the fourth register-L. So the four pixels comprising the seventh row of the letter Q in our example should each be in the default colours of those four registers. If you look closely at the screen you will find this to be the case.

Characters 135

A slight variation on this is that if the characters placed on a naode 12 or 13 screen are in inverse video (i.e. if bit seven of the character code is set), the colour of pixels specified by the bit pair 11 is taken from register 3 rather than register 2.

To see this try typing:

GRAPHICS 12:PRINT#6;"ABCDEFGABCDEFG" with the second half of the message in inverse video. Areas of the first half of the message appearing blue (register 2 default colour) are violet in the second half of the message (the register 3 default colour).

You will have realised by now that to make full use of mode 12 and 13 requires a special character set which will have to be defined in the usual way, but taking into account the extra considerations of colour register requirements, The applications are wide ranging and this is an economical way (in terms of memory requirements) to achieve colourful displays.

CHAPTER 13

GRAPHICS

In addition to the five text displaying modes described in the last chapter, the Atari XL has eleven graphics modes. The different modes offer many variations in the number of colours available, and in resolution (the number of rows and columns in the display). The same commands are used to control displays in all the graphics modes.

Graphics Modes

The modes are selected by the command

GRAPHICS

followed by the number of the desired mode. The display modes are numbered from 0 to

15; modes 0, 1, 2, 12 and 13 display text, as described in Chapter 12; modes 3 to 11 and 14 and

15 are the graphics displays. Try the command

GRAPHICS 5

You will see the screen turn black, except for a four line high blue area at the foot of the screen. The black area is for drawing graphics, and the blue region is a text 'window' - a smaller version of the mode 0 text display. As in modes 1 and 2, you can remove the text window by adding 16 to the number of the mode following' the

GRAP-HICS

command, but you cannot control windowless displays directly from the keyboard as the XL will snap back into mode 0 as soon it has obeyed the

GRAPHICS

command so that it can display the

(Ready' message.

136

Graphics 137

Colour in Graphics Modes

The different graphics modes use differing numbers of colours, but they are all controlled in the same way. As described in Chapter 12, there are five colour registers which control the display colours.

The SETCOLOR command is used to select the colour produced by each register.

The colours of objects displayed on the screen are set by indicating which colour register is to control their colour.

The command COLOR selects the colour to be used. The command is followed by a number which indicates the colour selection, but the number is not always that of the required colour register, which can be a little confusing.

In mode 5, four colors are used. The background colour is controlled by colour register 4, register 3 is not used, and registers 0, 1 and 2 hold the

(foreground' colours.

The colours to be used are selected by COLOR as follows:

COLOR 0 Selects the background colour.

COLOR 1 Selects the register 0 colour.

COLOR 2 Selects the register 1 colour.

COLOR 3 Selects the register 2 colour.

When you first enter mode 5, the colour selected in the background colour register 4. Type

(COLOR 0) held in colour

.

GRAPHICS 5

COLOR 1 so that you are ready to start drawing.

138 The Atari XL Handbook

Points and lines

To light up a point (or pixel) on the screen the command PLOT is used. The command is followed by two numbers - the row and column in which the point lies.

PLOT 20, 10 lights a pixel on the screen in the colour selected by the previous CO1.,0 R command. Try PLOTting a few points around the screen.

Mode 5 has 80 columns, numbered from 0 to 79, and 40 rows - 0 to

39. If you use numbers which are too large the XL will print

tERROR - 141';

than zero the Atari prints if you use numbers less

tERROR -

3'.

Try changing the colour of the points you PLOT by using

COLOR

2 or

COLOR

3.

COLOR 0

will give invisible points as it selects the background colour.

This means that you can erase pixels by returning them to the background colour.

When you have finished experimenting with the

PLOT command clear the screen again by typing:

GRAPHICS 5

The command DRA WTO draws lines on the display. Set a pixel by typing:

COLOR 1

PLOT 5,5 and then try:

DRAWTO 60,5

DRAWTO 60,35

DRAWTO 5,5

Graphics 139 the XL draws a line from the position of the last

PLOT

or

DRAWTO

to the point specified by the coordinates after the DRA

WTO

command.

You now know enough to produce line drawings of all kinds. Try a few experiments.

This program produces a lacy pattern by drawing lines close together on the mode 7 display, which has twice as many rows and columns as the mode 5 display but is otherwise the same.

10

20

30

40

50

60

70

REM LACE

GRAPHICS 7

COLOR 1

FOR X=O TO 159 STEP 2

PLOT 79, 0

DRAWTO X, 79

NEXT X

Filling in the Colours

As well as drawing points and lines on the screen, you can fill in areas of colour. The command used is

XIO,

but to fill in an area you must first define the outline of the shape. The sequence is as follows:

1

PLOT

the bottom right-hand corner of the shape to be filled.

2

3

DRAWTO

the top right-hand corner.

DRAWTO

the top left-hand corner.

4 Use

POSITION

to define the bottom left-hand corner of the shape.

5 POKE

the number of the colour you are using into location 765.

140 The Ateri XL Handbook

6 Then use the command XIO 18, #6,0,0, "S:".

This program draws a solid rectangle on the mode 5 display in colour number 1, with its corners at (5,

5), (75, 5), (5, 75) and (75,75):

10 REM FILL WITH XIO

100 GRAPHICS 5

110 COLOR 1

120 PLOT 75, 35

130 DRAWTO 75, 5

140 DRAWTO 5, 5

150 POSITION 5, 35

160 POKE 765, 1

170 XIO 18, #6, 0, 0, "S:"

You can fill shapes other than rectangles, within certain limits:

1 The bottom of the shape must be horizontal, and

2 The top must be horizontal, or slope down to the right.

This is because of the way the shapes are filled.

The XL fills in the colour by drawing horizontal lines to the right from a line between the top lefthand corner and the bottom left-hand corner, as far as the line defining the right-hand edge.

This means that if the top of your shape slopes up to the right it won't all get filled. If the bottom left-hand coner is higher than the bottom right-hand corner then not all the shape is filled. If the bottom. lefthand corner is lower than the bottom right-hand corner then the colour spills out across the full width of the screen.

The next program uses XIO to fill random blocks on the screen, producing instant abstract art. The

Graphics 141 program uses mode 10, which is a nine-colour mode. Mode 10 is explained later in the chapter.

10

20

30

40

50

60

REM ABSTRACT XIO

GRAPHICS 10

XMAX=79: YMAX=191

POKE 704, 56

POKE 705, 24

POKE 706, 168

70

80

POKE 707, 88

SETCOLOR 0, 4, 10

90 SETCOLOR 1, 10, 8

100 SETCOLOR 2, 12, 4

110 SETCOLOR 3, 13, 14

120 SETCOLOR 4, 14, 6

200 X

210 Y

= RND(O)

= RND(O)

*

*

220 X2 = X+RND(O)

230 Y2 = Y+RND(O)

240 C = RND(O)

250 COLOR C

*

XMAX

YMAX

8

* (XMAX - X)

*

(YMAX - Y)

260 PLOT X2, Y2

270 DRAWTO X2, Y

280 DRAWTO X, Y

290 POSITION X, Y2

300 POKE 765, C

310 XIO 18, #6, 0, 0, "5:"

320 GOTO 200

The final graphics command is

LOCATE.

This command moves the invisible graphics cursor to a point on the screen and reads the colour of the pixel at that point into a variable.

LOCATE X, Y, C stores the colour (the colour register number) at X,

Y into

C.

The next program plots a point in a

142 The Ateri XL Handbook randomly selected colour, and then finds its colour using

LOCATE.

10

20

30

40

40

60

REM LOCATE DEMONSTRATION

GRAPHICS 3

COLOR RND(0)*3

PLOT 30, 15

LOCATE 30, 15, COL

PRINT COL

This program uses mode 3, which is similar to mode

5 but with only half the number of rows and columns.

THE GRAPHICS DISPLAY MODES

So far we have used only modes 3,5 and 7. We will now describe in detail all the graphics modes available on the XL.

You can use

PLOT,

DRAWTO, LOCATE, XIO

and

COLOR

commands in all the modes, the only differences being in the available colours, and in the resolution of the display.

Mode3

This is a four colour mode. The background colour is held in colour register 4 and selected by COLOR

1, the other three colours are held in registers 0, 1 and 2 and are selected by COLOR 1, COLOR 2 and

COLOR 3 respectively.

The display resolution is 40 columns and 24 rows, or 20 rows if the text window is shown.

Mode4

This mode displays 48 rows of 80 columns, or 40x80 if the text window is shown.

Graphics 143

Two colours may be used; the background colour is controlled by colour register 4 and selected by

COLOR

0, the second colour is controlled by colour register 0 and selected as

COLOR

1.

Mode 5

Four colours are available in this mode, controlled in the same way as those in mode 3.

The resolution is the same as that of mode 4 - 48 rows of80 columns, or 40x80 with the text window.

Mode6

This mode has two colours, controlled as in mode 4.

The resolution is 96 rows of 160 columns, or 80x160 if the text window is displayed.

Mode7

This mode has four colours, controlled as in mode 3, with the same resolution as mode 6.

ModeS

Mode 8 has the highest resolution of all the display modes, 192 rows of 320 columns are allowed, or

160x320 with the text window.

The colour available is similar to that used in mode

0: only one colour can be used, with two possible luminances. The colour and the luminance of the background are controlled by colour register 2, and selected by

COLOR

O. The second luminance is controlled by the luminance of colour register 1, and may be selected by

COLORI.

Because this mode uses the colour registers in the same way as the mode 0 text display, the

144 The Ater! XL Handbook background does not appear black when you select this mode, but instead retains the colour of the mode 0 screen.

Mode9

This mode, like mode 8, uses only one colour.

However, unlike any other display mode, this mode gives 16 different luminances.

The colour is controlled by colour register 4, but the luminance is controlled by the

COLOR

command and the luminance set in register 4 has no effect.

The background is set to a luminance ofO.

The resolution in this mode is 192 rows of 80 columns. The text window may not be displayed in this mode.

The example program shows all 16 luminances of colour number 3 on the screen.

10

20

30

40

50

60

70

GRAPHICS 9

SETCOLOR 4, 3, 0

FOR LUM=O TO 15

COLOR LUM

FOR Z=O TO

PLOT 0,

11

LUM*12 + 2

DRAWTO 79, LUM*12 + 2

80 NEXT Z

90 NEXT LUM

100 GOTO 100

Mode 10

This mode has a similar resolution to mode 9; 192 rows of 80 columns. Again the text window may not be displayed.

Nine colours are displayable, each in eight luminances. Five of the colours are specified by the colour registers, and can be set by

SETCOLOR,

the other four are controlled by registers which are usually reserved for use with

Graphics 145

Player-Missile graphics (see Chapter 15). These registers are memory locations 704, 705, 706 and

707, and cannot be set using

SETCOLOR;

instead you must use PO KE to place a value for colour and luminance into the register. The value to use is:

(Colour Number

*

16) + Luminance

Eight of the nine colours can then be selected by the ninth sets the colour of the border of the screen and can not be used for drawing. The registers and their

CO LO

R codes are:

COLOR

No.

Register

2

°

1

5

6

3

4

7

Border

Location 705

Location 706

Location 707

Colour Register 0

Colour Register 1

Colour Register 2

Colour Register 3

Colour Register 4

Location 704

Mode 11

This mode is the (opposite' of mode 9, giving sixteen colours but only one luminance, instead of one colour at sixteen luminances.

The resolution is again 192 rows of 80 columns, and the text window is once again unavailable.

To control the colours you must first set the colourin register 4 to 0, or the full set of sixteen colours will not be displayed.

The colours are selected using

COLOR

and the colour number.

This example program draws the Atari logo in sixteen colours on a mode 11 display.

146 The Ateri XL Handbook

10

20

30

40

50

60

70

80

90

100

110

120

200

REM ATARI LOGO

GRAPHICS 11

FOR Y=O TO 191

COLOR Y/16

X

= SQR{191-Y}*2

PLOT X,Y

DRAWTO X+7, Y

PLOT 36, Y

DRAWTO 43,Y

PLOT 72-X, Y

DRAWTO 79-X, Y

NEXT Y

GOTO 200

Modes 12 and 13

These modes display text and are described in

Chapter 12.

Mode 14

Mode 14 is similar to modes 4 and 6, but has higher resolution.

There are two colours, used in the same way as those in modes 4 and 6: the background is controlled by register 4 and is selected by COLOR 0, the foreground is controlled by register 0 and is selected by COLOR 1. A text window may displayed with colours controlled as in mode o.

The resolution of this mode is 192 rows of 160 columns, or 160x160 if the text window is used.

Mode 15

This mode is a four colour version of mode 14, with the colours controlled as in modes 3,5 and 7.

Graphics 147

Memory Use

The various displays require different amounts of memory, with the higher resolution modes needing as much as 8k of storage. This may cause problems if you have a long program, particularly if you have a 600XL with only 16k of RAM. Be careful when writing long programs, and keep checking the available store using FRE(O). For example, if you want to run a program with a mode 8 display, you will need at least 7200 free bytes in mode modes in Appendix 5.

o.

There is a table showing the memory used by the display

JOYSTICKS AND PADDLES

The Atari

XL

can accomodate two joysticks, or four paddles, which are plugged into the ports on the right-hand side of the computer. Both joysticks and paddles may easily be used to control BASIC programs.

There are two BASIC commands used to read joysticks. The joystick ports are numbered 0 and 1, not 1 and 2 as is stamped on the computer's casing, with port 0 at the front. To find the position of a joystick, use the function

STICK( ),

specified port. A second function, which returns a value for the position of the joystick in the

STRIG()

gives a value of 0 if the Fire button of the selected joystick is pressed, or 1 if it is not. The values returned by

STICK( )

are illustrated in the diagram below.

Paddles are used in a similar way. The a pair of" paddles may be plugged into each port; the paddles in port 0 are numbered 0 and 1, those in port 1 are numbered 2 and 3.

The function P ADDLE( ) returns a value of between 0 and 224 depending on the rotation of the

148 The A tar; XL Handbook

(0

The Joystick Positions given

by

STICK( )

controller of the specified paddle. PTRIG() is used to read the Fire button of a paddle.

The program listed below uses a joystick to control the drawing of lines on a graphics display.

10

20

REM JOYSTICK ARTIST

REM SET UP VARIABLES AND

LABELS

100 POKE 752,1: PRINT

" i i .SKETCHPAD"

110 GOSUB 20000

192 REM FIND JOYSTICK PORT

200 PRINT"

START" i i i i .PRESS FIRE TO

210 PORT=O

220 IF STRIG(PORT)=l THEN PORT=

PORT=O: GOTO 220

992 REM PICTURE

1000 GRAPHICS 7

1010 X=O:Y=O

1020 GOSUB MESSAGE

1030 POKE 82,0

1040 GOSUB COLOUR

Graphics 149

2092

2100

2110

2120

2200

2210

2220

2230

2240

1092

1100

1110

1120

1130

1140

1150

1190

1200

1210

1220

1230

1240

1250

1260

1300

1992

2000

2010

2020

2030

2040

2050

REM JOYREAD

JOY=STICK(PORT)

IF JOY=15 THEN GOTO JOYREAD

XNEW=X+DXY(JOY,O)

YNEW=Y+DXY(JOY,1)

IF XNEW<O OR XNEW>159 OR

YNEW<O THEN GOTO JOYREAD

IF YNEW>79 THEN GOSUB TEXT:

GOTO JOYREAD

REM

IF STRIG(PORT)=O THEN GOTO

1250

COLOR BACKGR

PLOT X,Y

COLOR COL

LOCATE XNEW,YNEW,BACKGR

PLOT XNEW,YNEW

X=XNEW:Y=YNEW

GOTO JOYREAD

REM TEXT

COLOR BACKGR

PLOT X,Y

X=INT(X/4):Y=0:IF X<2 THEN X=2

POKE 752,0:REM TURN ON CURSOR

POKE 656,Y:POKE 657,X:POKE

658,0

;

REM CHECK JOY

IF STRIG(PORT)=O THEN GOSUB

SWITCH

JOY=STICK(PORT)

IF JOY=15 THEN GOTO CHECKJOY

XNEW=X+OXY(JOY,O)

YNEW=Y+DXY(JOY,1)

IF XNEW<O OR XNEW>39 OR YNEW>3

THEN GOTO CHECKJOY

IF YNEW<O THEN GOTO SCREEN

POKE 656,YNEW:POKE 657,XNEW

150 The Atari XL Handbook

2245

2250

2255

2260

2292

2300

2310

2320

2330

2492

2500

2510

2520

2530

2540

2550

2592

2600

2610

2620

2630

2650

2992

3000

3010

3020

3030

X=XNEW:Y=YNEW

FOR T=l TO 10:NEXT T

GOTO CHECKJOY

REM SCREEN

X=X*4+2:Y=79

POKE

PLOT X,Y

RETURN

REM SWITCH

IF X<7 THEN COL=O:GOSUB

MESSAGE:GOSUB CHCURSOR:RETURN

IF X>8 AND X<15 THEN COL=1:

GOSUB MESSAGE:GOSUB CHCURSOR:

RETURN

IF X>16 AND X<23 THEN COL=2:

GOSUB MESSAGE:GOSUB CHCURSOR:

RETURN

IF X>24 AND X<31 THEN COL=3:

GOSUB MESSAGE:GOSUB CHCURSOR:

RETURN

IF X>32 AND X<38 AND Y=1 THEN

POP:COL=2:GOTO PICTURE

RETURN

REM CHCURSOR

POKE 656,Y

POKE 657,X

POKE 752,0

RETURN

;

REM MESSAGE

POKE 752,1

POKE 82,0

PRINT" " "

PRINT" ERASE

BLUE CLEAR"

RED YELLOW

150 The Atari XL Handbook

2245

2250

2255

2260

2292

2300

2310

2320

2330

2492

2500

2510

2520

2530

2540

2550

2592

2600

2610

2620

2630

2650

2992

3000

3010

3020

3030

X=XNEW:Y=YNEW

FOR T=l TO 10:NEXT T

GOTO CHECKJOY

REM SCREEN

X=X*4+2:Y=79

POKE

PLOT X,Y

RETURN

REM SWITCH

IF X<7 THEN COL=O:GOSUB

MESSAGE:GOSUB CHCURSOR:RETURN

IF X>8 AND X<15 THEN COL=l:

GOSUB MESSAGE:GOSUB CHCURSOR:

RETURN

IF X>16 AND X<23 THEN COL=2:

GOSUB MESSAGE:GOSUB CHCURSOR:

RETURN

IF X>24 AND X<31 THEN COL=3:

GOSUB MESSAGE:GOSUB CHCURSOR:

RETURN

IF X>32 AND X<38 AND Y=1 THEN

POP:COL=2:GOTO PICTURE

RETURN

REM CHCURSOR

POKE 656,Y

POKE 657,X

POKE 752,0

;

RETURN

REM MESSAGE

POKE 752,1

POKE 82,0

PRINT" " "

PRINT" ERASE

BLUE CLEAR"

RED YELLOW

150

The Ateri XL Handbook

2245

2250

2255

2260

2292

2300

2310

2320

2330

2492

2500

2510

2520

2530

2540

2550

2592

2600

2610

2620

2630

2650

2992

3000

3010

3020

3030

X=XNEW:Y=YNEW

FOR T=1 TO 10:NEXT T

GOTO CHECKJOY

REM SCREEN

X=X*4+2:Y=79

POKE

PLOT X,Y

RETURN

REM SWITCH

IF X<7 THEN COL=O:GOSUB

MESSAGE:GOSUB CHCURSOR:RETURN

IF X>8 AND X<15 THEN COL=1:

GOSUB MESSAGE:GOSUB CHCURSOR:

RETURN

IF X>16 AND X<23 THEN COL=2:

GOSUB MESSAGE:GOSUB CHCURSOR:

RETURN

IF X>24 AND X<31 THEN COL=3:

GOSUB MESSAGE:GOSUB CHCURSOR:

RETURN

IF X>32 AND X<38 AND Y=1 THEN

POP:COL=2:GOTO PICTURE

RETURN

REM CHCURSOR

POKE 656,Y

POKE 657,X

POKE 752,0

;

RETURN

REM MESSAGE

POKE 752,1

POKE 82,0

PRINT" " "

PRINT" ERASE

BLUE CLEAR"

RED YELLOW

Graphics 151

3040 IF COL=O THEN POKE 656,1:POKE

657,2:PRINT"ERASE":REM TEXT IN

REVERSE

3050 IF COL=l THEN POKE 656,1;POKE

657,9:PRINT n RED n:REM TEXT IN

REVERSE

3060 IF COL=2 THEN POKE 656,l;POKE

657,l7:PRINT"YELLOW":REM TEXT

IN REVERSE

3070 IF COL=3 THEN POKE 656,1;POKE

657,25:PRINT" BLUE n:REM TEXT

IN REVERSE

3100 RETURN

3992 REM COLOUR

4000 SETCOLOR 0,3,8

4010 SETCOLOR 1,1,12

4020 SETCOLOR 2,9,4

4030 SETCOLOR 4,0,12

4100 RETURN

19992 REM SET UP

20000 DIM DXY(15,1)

20010 FOR J=5 TO 15

20020 FOR K=O TO 1

20030 READ DXY

20040 DXY(J,K)=DXY

20050 NEXT K

20060 NEXT J

20070 COL=2

20092 REM DEFINE LABELS

20100 PICTURE=1000

20110 JOYREAD=1100

20120 TEXT=2000

20130 CHECKJOY=2100

20140 SCREEN=2300

20150 SWITCH=2500

20160 CHCURSOR=2600

20170 MESSAGE=3000

20180 COLOUR=4000

152 The Atari XL Handbook

21000 RETURN

22000 DATA 1,1,1,-1,1,0,0,0,-1,1,-1,

-1,-1,0,0,0,0,1,0,-1,0,0

To use this program you will need a joystick, which may be plugged into ei ther of the ports.

The program is controlled entirely by the joystick, with no keyboard actions at all.

After the setting up routines have been run, pressing the Fire button on the joystick will display a blank graphics 7 screen, with a text window. The cursor will be visible as a yellow spot in the top left corner of the screen. Use the joystick to move the cursor around the screen, and hold down the Fire button while moving the cursor to draw lines.

To change colour or clear the screen, move the cursor down into the text window, position it over the command you require, and press Fire. There are four colours to choose from: red, yellow and blue, and the background colour (selected by

ERASE).

If you find that the display is too fine in mode 7, you can change the program to run in mode 5 by altering line 1000, when specifies the mode, and also by altering the tests in lines 1140 and 1150 which check whether the cursor is running off the edge of the display. You will also need to alter line

2020, which calculates the cursor position in the text window after it crosses from the graphics display, and line 2300, which calculates the cursor position in the graphics display as the cursor leaves the text window.

CHAPTER 14

ADVANCED GRAPHICS

This chapter deals with the way in which the XL generates the various displays discussed in the last three Chapters. It describes how the computer controls the TV and how you can alter this mechanism to produce yet more varied displays.

THE ANTIC AND GTIA CHIPS

The Atari uses two special chips to handle the requirements of generating a display, called

ANTIC and GTIA. The ANTIC chip takes the data concerning a display from display memory and passes it to the GTIA chip in the correct form for the display type currently selected.

The GTIA chip uses the data to control the TV and so create the display. To understand how this process works, you must understand some of the principles used to generate TV pictures.

TV PICTURES

The pictures on a TV screen are created by an electron beam which is directed at the phosphor coated inner surface of the screen.

Where this beam strikes the screen, the phosphor glows. To create a full screen picture, the electron beam

scans

across the screen in rows, varying in intensity as it goes. This variation in intensity is dictated by information from the TV transmitter and produces a corresponding variation in the intensity with which the phosphor glows. When the electron beam

153

154 The Ateri XL Handbook reaches the edge of the screen it has completed one

Scan Line and is turned off and moved back to a posi tion just below the last starting position, before the process begins again. This scanning continues until the electron beam reaches the bottom of the screen, at which point it starts allover again at the top. This process must happen many times a second to create a picture that doesn't flicker.

To create a colour picture, the screen is coated with three different types of phosphor which emit the colours red, blue or green when struck by the electron beam.

The different phosphors are distributed in tiny dots or blocks over the screen and the colour TV signal must contain information about which of these points to direct the electron beam at, as well as luminance information.

In creating its displays the XL uses the same principle as the TV transmitter, with the ANTIC chip feeding data to the GTIA chip, which controls the electron beam and supplies colour and luminance information for each dot on the display.

THE DISPLAY LIST

The ANTIC is a microprocessor, like the 6502, and has its own program in RAM telling it how the data in screen memory is to be organised to create a display. This program is called the display list, and is placed in memory (immediately below screen memory) by the 6502 processor. The contents of the display list depend upon the graphics mode currently selected.

The Atari display is divided into groups of scan lines known as display blocks.

The number of scan lines per display block depends upon the graphics mode. In mode 0 a display block is eight scan lines high, since a graphics 0 character is eight scan lines high. In mode 8 a display block is only one

Advanced Graphics 155 scan line high, resulting in the 192 line vertical resolution in mode 8.

The size of a display block governs the way in which ANTIC reads data from the display memory, and hence the way in which it is displayed. Let's look at the display list and see how it works. Type in the following program which will print the display list on the screen.

10

20

30

40

50

60

70

GRAPHICS 0

DL=PEEK{560}+256*PEEK(561)

FOR Z=O TO 31

POSITION 2+{{Z>15)*18),Z-

({Z>15*16)

PRINT DL+Z,PEEK{DL+Z)

NEXT Z

GOTO 70

The location of the start of the display list is stored in two memory locations: 560 and 561. The program calculates the address of the start of the display list and then prints the contents of the next

32 bytes of memory which comprise the display list.

(Line 40 ensures that all the information is displayed clearly on the screen.)

If you run the program you will see a display like the one below.

The five figure numbers in columns one and three of the display are memory locations in which are stored the display list instructions given in the second and fourth columns.

15392

15393

15394

15395

15396

112

"112

112

66

64

15408

15409

15410

15411

15412

2

2

2

2

2

156 The Ateri XL Handbook

15397 60

15398 2

15399 2

15400 2

15401 2

15402 2

15403 2

15404 2

15405 2

15406 2

15407 2

15413

15414

2

2

15415 2

15416 2

15417 2

15418 2

15419 2

15420 2

15421 65

15422 32

15423 60

The ANTIC processor reads the display list from top to bottom and the instructions in it control the display in that order.

The three 112 instructions at the start of the list instruct ANTIC to generate three eight-line display blocks of border colour.

Of the next three bytes, only the first (66) is an instruction; this tells ANTIC that display memory starts at the location given in the next two bytes of the display list. In a 16k machine these numbers will be as shown and tell ANTIC that display memory starts at

64

+

256*60 or 15424.

To see that this is the start of display memory, stop the program and type: -

POKE 15424, 1 (or whatever the address is for your machine)

You will see an exclamation mark (!) appear in the top left hand corner of the screen.

Advanced Graphics 157

Following this instruction there are 23 t2's.

This is the ANTIC instruction for a mode 0 display block.

When the XL interpreted the

GRAElHICS

0 command in the program, it allocated sufficient memory for the display, and created the display list accordingly. The 23 (2's indicate that the display should comprise 24 display blocks of 8 lines each.

There is a discrepancy here, since there are only 23 code 2s but mode 0 is a 24 line display. There is another code 2 at the beginning of the display list but it isn't obvious. We said earlier that the 66 code indicated that the next two bytes in the list are to be read as a 16 bit address. In fact the code for this is 64 and this has been added to the code 2 specifying the first mode 0 display block of the screen (the top line of the display), in much the same way as you addd 16 to a

GRAPHICS

instruction in BASIC to modify its effect. The 64 added to the (2' instruction is called a modifier and is discussed more fully later in this chapter.

Only the first of the last three bytes of the display list is an instruction. The code 65 tells ANTIC to jump back to the location specified in the following two bytes - the start of the display list.

The display list we have examined is fairly short and simple, being for a mode 0 display. For mode 8 it would be much longer, containing 192 instructions specifying one line display blocks, and some other codes

A list of the display list instructions is given in

Appendix 9.

Armed with this information we can now make changes to the display list which will allow us to create more varied displays than have previously

158 The Atari XL Handbook been possible. -One of the things we can do is to scroll the display in all directions.

SCROLLING

We have met the term scrolling in Chapter 3 as the name given to the way in which characters on the screen move upwards and disappear off the top - as happens when a large program is listed.

However, scrolling is not restricted to this one direction. We can scroll the display both vertically and horizontally from wi thin a program, by changing the address which ANTIC uses as the start of display memory.

As mentioned earlier, the address of the start of display memory is held in the fifth and sixth bytes of the display list.

To make the display scroll horizontally we must add to (to scroll to the left) or subtract from (to scroll to the right) the data stored in these bytes.

5

10

20

30

40

50

60

70

80

The following program demonstrates horizontal scrolling; use the to scroll right.

+ key to scroll left and the

* key

REM LEFT AND RIGHT SCROLLING

GRAPHICS 0

DIM MESSAGE$(40):MESSAGE$=

"LEFT AND RIGHT SCROLLING"

DISPLIST = PEEK(560)+256*

PEEK(561)

DISPMEM = PEEK(DISPLIST+4)+

_

256*PEEK(DISPLIST+5)

POSITION O,ll:PRINT MESSAGE$

OPEN #: 1 , 4 , 0 , "K : "

GET #:l,X

IF X=43 THEN DISPMEM=DISPMEM+ l:GOSUB 100

Advanced Graphics 159

90 IF X=42 THEN DISPMEM=DISPMEMl:GOSUB 100

95 GOTO 70

100 DISPHI = INT(DISPMEM/256)

110 DISPLO = DISPMEM-(DISPHI*256)

120 POKE DISPLIST+4,DISPLO

130 POKE DISPLIST+5,DISPHI

140 RETURN

This program makes the display scroll because it alters the address from which the ANTIC chip reads display data, but it does not alter the addresses into which the BASIC routines write the picture data. In normal use, the pointer to display memory is set by the operating system to the address at which BASIC begins storing display data when the graphics mode is set, or the machine is switched on.

When you run this program, you will notice that as you scroll the display to the right, a series of graphics and other characters appear at the top of the screen. This is the lower end of the display list, and appears because to scroll to the right we subtracted one from the display memory pointers.

This tells ANTIC to take display data from the address one lower than that where display memory starts, and as the display list is always stored immediately before display memory, it is treated as screen data and displayed.

Vertical scrolling can be achieved in much the same way, but the display memory pointers must.

be incremented by 40, as the following program demonstrates.

5

10

20

REM UP AND DOWN SCROLLING

GRAPHICS a

DIM MESSAGE$(40): MESSAGE$=

"WHAT GOES UP MUST COME DOWN!"

160 The Atari XL Handbook

30

40

45

50

55

60

65

70

75

80

85

90

100

110

120

130

140

DISPLIST = PEEK(560)+256*

PEEK(561)

DISPMEM = PEEK(DISPLIST+4)+256

* PEEK(DISPLIST+5)

DISPMAX = DISPMEM+880:DISPMIN

=DISPMEM

POSITION 0,23:PRINT MESSAGE$

REM SCROLL UP

DISPMEM = DISPMEM+40

IF DISPMEM

>

DISPMAX THEN 80

GOSUB 100:GOTO 60

REM SCROLL DOWN

DISPMEM = DISPMEM-40

IF DISPMEM

<

DISPMIN THEN 60

GOSUB lOO:GOTO 80

DISPHI = INT(DISPMEM/256)

DISPLO = DISPMEM-(DISPHI*256)

POKE DISPLIST+4,DISPLO

POKE DISPLIST+5,DISPHI

RETURN

This technique is simple to use but has the disadvantage that the scrolling is (coarse', that is the motion is not smooth because the display is moved by a whole character height or width of eight pixels at a time. With a bit more programming, we can cause the display to scroll by a single pixel at a time within a single character space. A combination of both techniques will allow smooth scrolling over the entire screen.

SMOOTH SCROLLING

Smooth scrolling is achieved with the use of two

GTIA registers:

VERTICAL SCROLL

HORIZONTAL SCROLL

Advanced Graphics 161

To cause an area of the screen to scroll smoothly, we must modify the display list for the area of the screen we are interested in. To do this, a modifier instruction must be added to the display list opcode for each display block which is to be smooth scrolled.

The modifiers are:

HORIZONTAL SCROLL add 16

VERTICAL SCROLL add 32

The modifier indicates to ANTIC that a display block is to be treated differently from normal.

When such a modifier is encountered, ANTIC checks the appropriate scroll register and uses the number it finds to determine how many (if any) scan lines of that display block to display.

An example will make this clear.

Suppose we wanted to vertically smooth scroll just one line of a mode 0 display - the following steps are required:

Add 32 to the display list opcode for that display block, to indicate to ANTIC that this block is to be vertically scrolled.

Write a number between 0 and 7, corresponding to .

the number of scan lines of that block to be displayed initially, into the vertical scroll register.

A value of7 would display the entire block whereas o would result in none of the block being displayed.

Increment or decrement the scroll register to smooth scroll that display block up or down.

162 The Atari XL Handbook

The following program illustrates this technique by scrolling two lines of text in the middle of the screen.

5

10

20

REM SMOOTH SCROLLING

VERTSCROLL=54277

DISPLIST=PEEK(560)+256*

PEEK(561)

30

40

50

60

GRAPHICS 0

FOR Z=l TO 10

PRINT "NON-SCROLLING LINE "jZ

NEXT Z

70

80

90

PRINT "HIDDEN LINE 11"

PRINT "S-M-O-O-T-H S-C-R-O-L-

L-I-N-G-!"

PRINT "HIDDEN LINE 13"

100 FOR Z=14 TO 23

110 PRINT "NON-SCROLLING LINE "jZ

120 NEXT Z

200 POKE DISPLIST+15,2+32:POKE

DISPLIST+16,2+32

300 FOR Y=O TO 7

310 POKE VERTSCROLL,Y

320 FOR DELAY=Q TO 100:NEXT DELAY

330 NEXT Y

400 FOR Y=7 TO 0 STEP-l

410 POKE VERTSCROLL,Y

420 FOR DELAY=O TO 100:NEXT DELAY

430 NEXT Y

440 GOTO 300

The program works like this:

Lines 10 to 120 create a display to illustrate the scrolling mechanism.

.

Line 200 adds a modifier of 32 to the display list codes for lines 11 and 12 of the graphics 0 screen to indicate vertical scrolling. (The code for a graphics o display block is 2, as described earlier). Line 13, which has no modifier, is initially hidden to allow

Advanced Graphics 163 compensation for the disappearing scan lines of line 11 when scrolling starts. This prevents the whole of the rest of the screen from moving up with line 11.

Lines 300 to 330 and 400 to 440 increment and decrement the vertical scrolling register, with a delay, to gradually increase or decrease the vertical offset of lines 12 and 13 on the screen and produce the effect of smooth scrolling.

This technique can be applied to any area of the screen, and also works for modes 1 and 2.

GRAPHICS MODE

It is possible to modify the entire display list to create a display mode which the BASIC command

GRAPHICS cannot give. If you look at the table of ANTIC instruction codes in Appendix 9, you'll notice that code 3 isn't used. This is used to select another text mode which is very similar to mode 0, but is composed of display blocks which are 10 scan lines tall.

The only way to use this mode is to modify the display list, and the following program will do just that:

5

10

20

30

40

50

60

REM MODE 0.5

DL=PEEK(560)+256*PEEK(561)

POKE DL+3,67

FOR Z=6 TO 23:POKE DL+Z,3

NEXT Z

FOR Z=24 TO 26:POKE DL+Z,

PEEK(DL+Z+5)

NEXT Z

List the program, then try running it. You will see the screen 'stretch', leaving more space between the lines of the listing then usual. This is because each display block is now 10 scan lines high instead

164 The Atari XL Handbook of the normal 8. The number of lines on the screen is reduced to 19.

N ow try typing some lower case characters; you will observe that some of them appear slightly different from normal - they have the pixel normally in the top row of the character displayed on the bottom.

This is because in this mode,

ANTIC takes character data from memory in a slightly different order from usual, but only for characters with ASCII codes between 96 and 128.

You may be wondering what use there is for this it allows you to create a more legible character set, in which characters like g, y and j can have descenders (the part of the character that normally appears below the line of the text). This involves reprogramming the character set as described in

Chapter If you want to try this you should be aware that the data in the top two rows of the character grid is displayed in the bottom two rows on the screen, so to create a letter p with a descender, your character grid should look like this:

The Character p as Stored in

Memory

ANTIC will rearrange the data so that when displayed, the character appears like this:

Advanced Graphics 165 rO-l-o-r-o,--OTo--ro-l-o-r--,

-.._-

------

_.

_

..

_.

-.-._--I

I

2 'EXTRA' SCAN

LINES

The Character p as Displayed in Mode 0.5

and will insert two blank scan lines before the character is displayed.

You will need to use the previous program as a subroutine to set up the display list in any program you write to use this mode.

MIXING MODES

With more programming, it is possible to mIX several graphics modes in the screen at once. You have seen examples of such mixed modes when a mode 0 text window is displayed at the foot of a graphics display, and also in the displays used in self-test mode.

As an introduction, let's examine the display list of a mode 2 screen, using the following program.

10

20

30

40

50

60

DIM LST(25)

GRAPHICS 2

DL=PEEK(560)+256*PEEK(561)

FOR Z=O TO 23

LST(Z)=PEEK(DL+Z)

NEXT Z

166 The Afar; XL Handbook

70

80

90

GRAPHICS 0

FOR Z=O TO 23

PRINT LST(Z)jU uj:NEXT Z

The display instructions are stored in the array

LST( ), and are displayed on a mode 0 screen. This is what you should see (the data is shown here in a list for clarity - not as it appears on the screen):

2

2

2

65

88

62

7

7

7

7

7

7

7

7

7

112 8 blank scan lines

112 8 blank scan lines

112 8 blank scan lines

71

112 Display memory at

62

7 +

112

64 - 1 mode 2 display block

+ 62*256

66

96

63

Mode 2 display block

Mode 2 display block

Mode 2 display block

Mode 2 display block

Mode 2 display block

Mode 2 display block

Mode 2 display block

Mode 2 display block

Mode 2 display block

2

Display memory at

96

+

+

64 - 1 mode 0 display block

63*256

Mode 0 display block

Mode 0 display block

Mode 0 display block

Jump back to

88 + 62*256 at Vertical Blank

This display list is more complex than that examined previously, but you should be able to see that it specifies 10 mode 2 display blocks, followed by four mode 0 display blocks. The thing to note is that there are two different pointers to display

Advanced Graphics 167 memory. At the start of the list, ANTIC is told that display memory is at 112

16k machine), and this is where data for the mode 2 display is stored. After completing the 10 mode 2 display blocks, ANTIC is directed elsewhere in memory, to 96

+

+

63*256

62*256

=

= 15984 (for a

16224 (for a 16k machine), where it finds the data for the mode 0 display.

You can apply the same principle to creating your own mixed mode displays.

As an example, the following program alters the display list of a mode 0 screen to cause any data appearing on the lower half of the screen to appear in multicolour character mode (mode 12).

10

15

20

30

40

GRAPHICS 0

SETCOLOR2,3,14:SETCOLOR4,3,14

DL=PEEK(560)+256*PEEK(561)

FOR Z=18 TO 29

POKE DL+Z,4:NEXT Z

Line 10 sets up the mode 0 display list and the contents of colour registers 2 and 4 are set equal so that the background of both displays appears the same.

If you run the program, it will turn the entire screen pink. If you now list the program all will seem normal until you list it again - the second listing on the lower half of the screen will be in .

mode 12!

This is a fairly straightforward and not particularly useful demonstration of mixed mode displays.

A better but more complex one is to create a mode 8 graphics display, and label it in a combination of text modes. The following program

168 The Ateri XL Handbook

100

110

120

130

140

199

200

210

15

20

30

40

50

60

99 creates a display comprising a picture in mode 8, wi th ti tles in modes 2 and o.

5

10

215

220

230

240

299

300

310

320

330

340

499

500

510

REM MIXED MODES DEMO

DIM Tl$(20),T2$(20),T3$(40),

A$(40)

GRAPHICS 8+16

ST=PEEK(560)+256*PEEK(561)

DM=PEEK(ST+4)+256*PEEK(ST+5)

SETCOLOR 4,3,12

SETCOLOR 1,0,2

SETCOLOR 2,3,12

REM ALTER DISPLAY LIST

POKE ST+3,71

POKE ST+6,7:POKE ST+7,2

POKE ST+160,PEEK(ST+199)

POKE ST+161,PEEK(ST+200)

POKE ST+162,PEEK(ST+201)

REM SET UP TITLES

TI$=" THE CUBE

T 2 $ =

II mmmmmmmmmm

"

"

REM m IN REVERSE VIDEO

T3$="

REM 20 SPACES

"

A$=Tl$:GOSUB 500:MEM=MEM+20

A$=T2$:GOSUB 500:MEM=MEM+20

A$=T3$:GOSUB 500

REM DRAW CUBE IN MODE 8

COLOR 1

.

.

PLOT 100,50:DRAWTO 150,50:

DRAWTO 150,100:DRAWTO 100,100:

DRAWTO 100,50

DRAWTO 125,25:DRAWTO 175,25:

DRAWTO 150,50

PLOT 150,100:DRAWTO 175,75:

.

DRAWTO 175,25

GOTO 340

REM TRANSLATE TITLES

FOR Z=I TO LEN(A$)

IF A$(Z,Z)=" " THEN A$(Z,Z)=

CHR$(O)

Advanced Graphics

520 IF ASC(A$(Z,Z»<>O THEN

A$(Z,Z)=CHR$(ASC(A$(Z,Z»-32)

530 NEXT Z

549 REM POKE INTO DISPLAY MEMORY

550 FOR Z=l TO LEN(A$)

560 POKE DM+MEM+(Z-l),ASC(A$(Z,Z»

570 NEXT Z

580 RETURN

The program will create a display comprising:

2 lines of mode 2 text +

1 line of mode 0 text + many lines of mode 8 graphics looking like this:

169

2 LINES OF MODE 2 TEXT

1 LINE OF MODE a

TEXT

TH E MODE 8 DISPLAY

The Mixed Mode Display

170 The Atet! XL Handbook

How The Program Works

Lines 10 to 60 dimension arrays for the titles, select mode 8 wi th no text window, and find the start of the display list (ST) and display memory (DM).

The colour registers for background, border and foreground are also set up.

Lines 100 to 140 alter the display list as follows:

100 Creates a mode 2 display block.

The code 71 is actually 64 + 7 - this is the first display block in the list, and so contains the modifier pointing to the display memory address which is 64 .

110 Creates the second mode 2 display block, followed by a mode 0 display block.

120-140 Look to the end of the display list, where the (jump at vertical blank' instruction which sends ANTIC back to the start of the display list is found, and move this instruction and the following 16 byte address further up the display list. This reduces the effective length of the display list by 37 scan lines. The reason for this is that in adding the three text display blocks to the top of the list, we added an extra 37 scan lines, because the first three instructions in the list would normally have specified 3 mode 8 display blocks, which are only one scan line high. To ensure that no more than

192 scan lines are specified by the display list, we must (chop off' the extra scan lines now at the bottom of the list.

If you don't take this precaution, some strange effects will result (something for you to tryl).

Advanced Graphics 171

Lines 200 to 240 set up the characters for the titles, which are converted to internal code and IJO KEd into display memory by subroutine 500.

The characters are put into display memory starting at the top, and the variable MEM keeps track of the position in memory of each string of characters.

Finally, the picture is drawn in the usual way by lines 300 to 340.

You can use the principles covered here to create your own mixed mode displays, by observing the following rules.:

1 Start with the display list of the mode which uses the most memory (in our case this was mode 8).

See Appendix 5 for memory requirements of the various modes.

2 Change the codes to create text for labelling in the appropriate part of the screen.

3 Calculate the number of scan lines required for the new display, and if greater than 192 transfer the jump from the end of the display list to a point such that the display requires no more than 192 scan lines.

4 Calculate the memory requirements for each text mode display block, and POKE the internal codes for the characters into display memory.

5 Draw the mode 8 picture in the normal way.

These rules and some patience are all that's needed to create a very complex mixed mode display.

CHAPTER 15

PLAYER-MISSILE GRAPHICS

The 16 graphics modes of the XL allow you to create superb displays, but there is one thing missing; it is very difficult to produce animated displays using the normal graphics facilities. The

Atari has a second kind of graphics feature to overcome this problem: player-missile graphics.

Consider the problems of producing an animated display, even if you have only one object moving over a fixed background. As well as redrawing the moving object every time its position changes, you must redraw the background, or the moving object will leave a trail of damage in its wake. Atari have solved this problem by providing movable graphics objects - called players and missiles, which can easily be moved across the screen without any risk of upsetting the background image.

Players and missiles are similar in nature.

A player can extend over the full height of the screen and be up to eight pixels wide; a missile is the same height but only two pixels wide. Four players and four missiles are available, or the missiles may be combined and used as a fifth player. The players and missiles are completely independent of the background display, and can be used in conjunction with displays in any of the XL's graphics modes.

A further feature, which is useful in many applications, is that the Atari hardware (the GTIA chip) automatically detects the 'col lisions' between pairs of players, players and missiles, and players or missiles and background features when they are overlapping each other on the display. You can in

172

Player-Missile Graphics 173 addition control the relative priorities of the players, missiles and the background to control which will show when two objects overlap, and thus obtain three-dimensional effects.

This simple program shows one player, which appears as a vertical band on the mode 0 screen.

10

20

30

40

50

90

REM SET ADDRESSES FOR PLAYER 0

POGRAPH=53261

POHPOS=53248

POCOL=704

POSIZE=53256

REM SHOW PLAYER

100 POKE POGRAPH, 255

110 POKE POCOL, 54

120 POKE POSIZE, 3

130 POKE POHPOS, 120

The player (Player 0) is set to a vertical red band by this program.

The properties of the player are controlled by the contents of the four registers. The pattern of bits in location 53261 controls the appearance of the player - the program sets it to

255 to give a solid vertical band (255 is 11111111 in binary). Try:

POKE 53261, 170

This give four thin stripes down the screen (170 is

10101010 in binary only the positions corresponding to the Is are shown).

The colour is controlled by location 704, which works in the same way as the colour registers for normal graphics.

The number in the register indicates the colour and luminance as follows:

Number = (Colour No.*16) + luminance

174 The Ateri XL Handbook

Gold is colour 1, so for a bright gold player, try

POKE 704, 16*1 + 12

The value in location 53256 controls the width of the player. A value of 0 gives normal width, 1 gives double width, and 3 quadruples the player's width.

To shrink the player a li ttle, try:

POKE 53256, 1

The fourth control register has the most dramatic effect.

Location 53248 controls the horizontal position of the player on the screen. The value may range from 0 to 255, and the corresponding player posi tions range beyond the edges of the screen on both sides, allowing players to be (hidden'.

Try adding these lines to the first example program:

130 FOR

140

160

X=O

150 NEXT X

TO 255

POKE POHPOS,

GOTO 130

X

The player will move across the screen from left to right as the value in the horizontal position register is increased.

CONTROLLING THE SHAPES OF PLAYERS

The four players may all be controlled in the same way as Player o.

The registers to be used for the players are shown in the table oppositee These registers all control the GTIA (the special purpose chip which controls the display).

So far you have seen nothing very impressive.

Moving bands, yes, but these are hardly the stuff of which exciting video games are made. To find out

Player-Missile Graphics 175

REGISTER

M2HPOS

M3HPOS

POSIZE

P1SIZE

P2SIZE

P3SIZE

MSIZE

POGRAPH

Pl GRAPH

P2GRAPH

P3GRAPH

MGRAPH

PMOCOL

PM1COL

PM2COL

PM3COL

POHPOS

P1HPOS

P2HPOS

P3HPOS

MOHPOS

M1HPOS

EFFECT

Player/Missile 0 colour

Player/Missile 1 colour

Player/Missile 2 colour

Player/Missile 3 colour

Player 0 position

Player 1 position

Player 2 position

Player 3 position

Missile 0 position

Missile 1 position

Missile 2 position

Missile 3 position

Player 0 size

PIayer 1 size

Player 2 size

PIayer 3 size

Size of all missiles

Player 0 graphics

Player 1 graphics

Player 2 graphics

Player 3 graphics

Graphics for all missiles

Player-Missile Control Registers

LOCATION

53253

53254

53255

53256

53257

53258

53259

704

705

706

707

53248

53249

53250

53251

53252

53260

53261

53262

53263

53264

53265 how to turn players into spaceships, we must study the GTIA in a bit more detail.

176 The Atari XL Handbook

As you have seen, the data in the GRAPH register controls the appearance of the player. As the GTIA builds up the video pictures, scanning across the screen from left to right, it is constantly checking the HPOS registers to see if there are players to be switched on. When the GTIA finds it has reached the specified horizontal position for a player,the data in the appropriate GRAPH register is used to generate the next part of the horizontal line instead of the normal data in the screen memory.

This process is repeated as each line of the display is scanned: as the GTIA reaches a player's HPOS setting, it takes data from the GRAPH register for that player to generate the next part of the line.

Because the display is built up a line at a time, if we could change the contents of the GRAPH register between lines, we could create much more interesting shapes for the players.

It would be impossible to modify the registers quickly enough in BASIC, but it can be done. The

Atari has another special chip, the ANTIC, which is dedicated to feeding the GTIA registers wi th the right information at the right time. All we have to do is set up a table in memory of the data for each line of a player, and tell the ANTIC chip to use this data to feed the GRAPH register of the GTIA. The next program uses this technique to turn Player 0 into a sunflower.

10

20

30

40

50

60

70

REM SUNFLOWER

PMBASE = 54279

POCOL = 704

POSIZE 53256

POHPOS 53248

SDMCTL = 559

GRACTL = 53277

100 START = INT(PEEK(742)/8)-1

110 FOR X=l TO 25

Player-Missile Graphics 177

120 READ F: POKE START*2048 + 1024

+ 120 + X, F

130 NEXT X

150 POKE POCOL, 28

160 POKE POSIZE, 1

170 POKE PMBASE, START * 8

180 POKE SDMCTL, 58

190 POKE GRACTL, 2

200 FOR X=20 TO 180

210 POKE POHPOS, X

220 FOR T=1 TO 10: NEXT T

230 NEXT X

500 DATA 30, 42, 42, 93, 93, 93,

93, 42, 42, 3D, 8, 8, 8, 8, 8,

8, 8, 8, 8, 8, 8, 107, 42, 42,

28

Three new control registers are used by this program: PMBASE (54279), SDMCTL (559) and

GRACTL (53277).

PMBASE is a control register in the ANTIC chip, telling it where in memory to find the data tables for the players and missiles. SDMCTL controls the memory access of the ANTIC, and GRACTL is a

GTIA register, again controlling memory access.

The GRACTL (graphics control) register is used as follows:

Bit 1 is set to 1 to use memory tables for player graphics.

Bi t 0 is set to 1 to use memory tables for missile graphics.

The other bits ofGRACTL have no meaning.

SDMCTL is more complicated:

178 The Ateri XL Handbook

Bit 5 must be set to 1 to switch ANTIC on otherwise there will be no display at all.

Bit 4 is set to 1 to give players and missiles a vertical resolution of 1 screen line to each data line; and set to screen lines to each line of data.

Bit 3 is set to 1 to enable player memory access.

Bits 1 and

°

to give a resolution of two control the width of the background screen. Set them to 0,0 for no display, 0,1 for a width of 32 columns in graphics mode 0, 1,0 for

40 columns (the normal setting) and 1,1 for 48 columns.

The value in PMBASE should be the high byte of the beginning of the player-missile graphics data area. The data area is laid out as shown below.

PMBASE + 2048 (1024)

Player 3 data

PMBASE + 1792 (896)

Player 2 data

PMBASE + 1536 (768)

Player 1 data

PMBASE + 1280 (640)

Player 0 data

PMBASE + 1024 (512)

Missile Missile Missile Missile

3 data 2 data 1 data o data

PMBASE + 768 (384)

Not used for graphics

PMBASE

Player-Missile Data Area

There are two possible configurations of the data area; which is used depends on the resolution indicated by bit 4 of the SDMCTL register. If a

Player-Missile Graphics 179 resolution of 1 data line per screen line is indicated,

256 bytes are needed to store the data for each player, one byte for each screen line.

If the resolution selected is 2 screen lines for each data line then only half as much memory is needed, 128 bytes for each player, and the whole table can be fi tted into half the memory.

The address indicated by PMBASE must be a multiple of2048 (1024 in reduced resolution mode).

If PMBASE was an eight-bit register you would expect the value to be the high byte of an address, which could therefore be any multiple of 256. In fact, not all the bits of PMBASE are used.

In the sunflower program, the higher resolution of

1 screen line to each data line was used. PMBASE was set up by finding the high byte of the address of the top of available memory, (stored in 742), finding the next lowest multiple of 2048 and subtracting a further 2048 to find a safe place for

PMBASE.

Defining Players

The method of defining a player is similar to that used for defining characters (see Chapter 12). The symbol is drawn on a grid, eight pixels wide, and the values of the bits added up to give a data byte.

The difference with players is that instead of using eight lines, you can use up to 256. The sunflower uses only 25 lines of the player to reduce the amount of typing needed.

Missiles

Missiles are defined in a similar manner to players, except that a missile is only two pixels wide instead of eight. This means that one register of the GTIA, the MGRAPH register, can be used to specify the

180

...

::::=

-x-;

.'.

:::

......

==::::

::::::

..

"

-,

..

..., ....

......

.........

....

."."

::

::::::

::::::

The Ateti XL Handbook

PMBASE+l024+120

::::::

....

:.:

":.:-

":.:.

:::::

.'.

::

==::::

:=::::

.......

.....

PMBASE+l024+120+24

The Sunflower data graphics for all four missiles at once. The register is used like this:

Bits 7 and 6 define the graphics for Missile 3

Bits 5 and 4 define the graphics for Missile 2

Bits 3 and 2 define the graphics for Missile 1

Bits 1 and 0 define the graphics for Missile 0

Because there is only one register on the GTIA for all four missiles, only one data table is needed to specify the appearance of all four missiles.

Each missile usually takes the colour specified in the colour register for the player of the same number.

The missiles use horizontal position registers in the same way as players.

Missile size is controlled by one register MSIZE

(53260). The width of missile 0 is controlled by bits o and 1, that of missile 1 by bits 2 and 3, and so on.

Player-Missile Graphics 181

The values for the two bits are the same as those used in the player width registers.

It is possible to use the four missiles as a fifth player by careful use of the missile horizontal position registers. If bit 4 of the priority register

GPRIOR is set to 1, the missiles will all appear the same colour - that indicated in display colour register number 3.

PRIORITIES

With a possible four players and four missiles moving around the screen, and passing over graphics displays, some rules are necessary for deciding what should be shown where two players overlap, or where a player or missile overlaps the other graphics. Another GTIA register is used to controls the relative priorities of players, missiles, and graphics of different colours. This register is called GPRIOR, and is at address 623. The four lowest bits of this register (bits 0 to 3) are used to select among four different orders of precedence for the colours. These are shown in the table overleaf.

Notice that the priorities are given in terms of colour registers. The players and missiles are not shown separately as the colour registers are shared, and it is the colour register used which determines the priority of a graphics item. Points to note are that colour register 4 is always used for the background colour of a graphics display, and colour register 3, which is not used in most .

graphics modes, may be used as the colour register for a fifth player ifbit 4 of GPRIOR is set to 1.

If you try setting more than one of the four priority controlling bits of GPRIOR at the same time, you will get very strange results, as the order of precedence will not then be clearly defined.

182 The Atari XL Handbook

Bit 3 set Bit 2 set Bit 1 set Bit 0 set

COLO

COL 1

COLO

COl1

PCOlO

PCOl1

PCOlO

PCOl1

PCOlO

PCOl1

PCOl2

PCOL3

COl2

COL 3/PCOLS

PCOlO

PCOl1

COLO

COL 1

COl2

COL 3/PCOlS

PCOl2

PCOl3

COLO

COl1

COl2

COL 3/PCOlS

PCOl2

PCOl3

PCOl2

PCOl3

COl2

COL 3/PCOLS

COL4 COL4 COl4 COl4

(background) (background) (background) (background)

Player-Missile and Colour Priorities

Try running the example program below, and watch what happens as the priorities are changed.

The progam shows the four players: player 0 is pink, player 1 is white, player 2 is green and player

3 is light blue. The players move around each other and over graphics blocks coloured by three colour registers: colour register 0 is orange, colour register 1 is green and colour register 2 is dark blue.

10 REM PRIORITIES

90 REM SET UP PLAYERS

100 POKE 704, 56: POKE 705, 14:

POKE 706, 198: POKE 707, 122

110 FOR GRAPH=53261 TO 53264: POKE

GRAPH, 255: NEXT GRAPH

120 FOR SIZE=53256 TO 53259: POKE

SIZE, 0: NEXT SIZE

138 REM

140 REM BEGIN DEMO

Player-Missile Graphics 183

142 REM LOOP FOR EACH PRIORITY

144 REM

150 FOR PR=O TO 3

155 GRAPHICS 5

159 REM SET PRIORITY

160 POKE 623, 2APR

170 PRINT "PRIORITY BIT "i PRi "

SET"

178 REM MOVE PLAYERS RIGHT

180 FOR HP=O TO 3: POKE 53248+HP,

60+HP*15: NEXT HP

200 FOR HP=O TO 3

210 FOR X=60+HP*15 TO 170+HP*10

220 POKE 53248+HP, X

230 FOR D=l TO 10: NEXT D

240 NEXT X

250 NEXT HP

290 REM DRAW RECTANGLES

300 FOR C=l TO 3

310 COLOR C

320 X=(C-l)*20

330 Y=(C-l)*15

340 PLOTX+I0, Y+I0

350 DRAWTO X+I0, Y

360 DRAWTO X, Y

370 POSITION X, Y+I0

380 POKE765, C

390 XIO 18, #6, 0, 0, "S:"

400 NEXT C

490 REM MOVE PLAYERS LEFT

500 FOR HP=O TO 3

510 FOR X=170+HP*10 TO 30 STEP -1

520 POKE 53248+HP, X

530 FOR D=l TO 10: NEXT D

540 NEXT X

550 NEXT HP

184 The Ateri XL Handbook

600 'NEXT PR: REM GO BACK FOR NEXT

PRIORITY

One final point is that in the text mode, mode 0, and in text windows in other graphics modes, the text colour is controlled by colour register 2, but the luminance of the text is controlled by colour register 1. If a player passes over the text, the text colour changes to the colour of the player, but its luminance is unaltered. This means that the text may disappear if the luminances of the player and of the text are the same.

COLLISIONS

The final feature of player-missile graphics is that the GTIA chip can detect collisions between players, between a player and a missile, and between players or missiles and the graphics objects on the main display.

These collisions, which occur when two graphics objects overlap, are automatically detected and recorded in various registers.

The collision indication registers are shown in the table opposite. The contents of the registers have the following meanings:

Missile/Display and Player/Display collisions

Bit 0 set indicates a collision with a display item coloured by colour register O.

Bit 1 set indicates a collision with a display item coloured by colour register 1.

Bit 2 set indicates a collision with a display item coloured by colour register 2.

coloured by colour register 3.

.

Bit 3 set indicates a collision with a display item

Missile/Player and Player/Player collisions

Bit 0 set indicates a collision with Player o.

Bit 1 set indicates a collision with Player 1.

Player-Missile Graphics 185

REGISTER

MODISP

M1 DISP

M2DISP

M3DISP

PODISP

P1 DISP

P2DISP

P3DISP

MOPL

M1PL

M2PL

M3PL

POPL

P1PL

P2PL

P3PL

HITCLR

MEANING

Missile O/Display

Missile 1/Display

Missile 2/Display

Missile 3/Display

Player O/Display

Player 1/Display

Player 2/Display

Player 3/Display

Missile O/Player

Missile 1/Player

Missile 2/Player

Missile 3/Player

Player O/Player

Player 1/Player

Player 2/Player

Player 3/Player

Clear collisions

Collision Registers

LOCATION

53256

53257

53258

53259

53260

53261

53262

53263

53248

53249

53250

53251

53252

53253

53254

53255

53278

Bit 2 set indicates a collision with Player 2.

Bit 3 set indicates a collision with Player 3.

(A player cannot collide with itself.)

Note that the addresses of these registers are the same as those of some of the control registers we have already described. This does not matter, as you will only be reading from the collision registers and writing to the control registers.

The Atari hardware directs the signals to two different

186 The Atari XL Handbook registers depending on whether data is being read or written.

It is not possible to clear the collision detection registers by writing zeros to them. The registers are cleared by POKEing to a special address called

HITCLR, at 53278. It does not matter what value you write this location, the effect is to clear all sixteen collision registers.

The final example programs demonstrate the detection of collisions between players, and between a player and an object on the display. The first program demonstrates the detection of collisions between two players.

10

20

30

40

50

60

70

80

Note:

REM PLAYER-PLAYER COLLISIONS

POKE 53261, 255: POKE 704, 87

POKE 53262, 255: POKE 705, 55

FOR X=O TO 255

POKE53248, X: POKE 53249, 255-

X

IF PEEK(53260)=2 THEN PRINT

"{BELL}"

POKE 53278, 0

NEXT X

The (BELL' character is CONTROL-2

The second program shows the detection of collisions between a player and a coloured rectangle drawn on the mode 5 display.

10

20

30

40

50

60

70

REM PLAYER-DISPLAY COLLISIONS

GRAPHICS 5

POKE 53261, 255: POKE 704, 87

COLOR 1: PLOT 30, 30:

DRAWTO 30, 10: DRAWTO 20, 10:

POSITION 20, 30: POKE 765, 1:

XIO 18, #6, 0, 0, "S:"

Player-Missile Graphics 187

80

90

IF PEEK(53252)=1 THEN PRINT

"{BELL}"

NEXT X

Player-missile graphics are one of the most powerful features of the Atari, making the creation of impressive animated displays an easy task.

CHAPTER 16

PERMANENT STORAGE

The XL is able to store programs on cassette tapes or floppy disks, and to LOAD and RUN programs from tape, disk or plug-in cartridge. Tapes and disks may also be used to store data for future use.

PROGRAMS ON TAPE

There are three BASIC commands you can use to load programs from the cassette unit into the XL, and there are three corresponding commands to allow you to save programs.

eSA VE and eLOAD

To save a program using eSA VE, type:

CSAVE and press RETURN

The TV speaker will (beep' twice, reminding you to position the tape and press the RECORD and

PLAY buttons on the cassette unit.

Pressing

RETURN a second time will save the program on the tape. At the end of the process (which you can monitor by turning up the volume on the TV) the tape will stop and the READY prompt will be displayed.

Your program, and any variables it created when RUN, will be saved on the tape.

To load a type:

e

SA VEd program back into the XL,

CLOAD and press RETURN

188

Permanent Storage 189

The TV speaker will beep once, indicating that you should position the tape and press the PLAY key on the cassette uni t. When you press RETURN again the tape will begin to move an the first program the

XL finds on the tape will be loaded.

The two other groups of loading and saving commands are less commonly used.

SA

VE

and LOAD

This pair of commands are more general in their application, and you must specify to which device you wish to save your program. For the cassette unit the syntax is:

SAVE "C: " unit.

the C indicating the cassette

Any program saved in this way must be reloaded using the

LOAD

command, the syntax of which is:

LOAD "C:"

To confuse things slightly, it is also possible to

eLOAD

a program which has been saved with the

SA

VE command, but it is not possible to

LOAD

a

eSA

VEd program.

LIST and ENTER

As we said earlier, these commands are less commonly used but can be very useful.

LIST

will save a program to the cassette in a different form from that used by eSAVE.

SA

VE and

Both SAVE and eSAVE save programs in tokenised form, that is, any BASIC command within the program are saved as single character

190 The Ateri XL Handbook tokens; for example,

SETCOLOR

would be saved as a one byte number, not as the characters SET C o

LOR. This has the advantage of saving both time and space on the tape.

LIST

however, saves the program in exactly the form it appears when you character by character. The syntax of used to save programs is:

LIST

it to the screen,

LIST

when

LIST "C:"

You can use the

LIST

command to save sections of programs on tape by specifying the line numbers of the section to be saved in the command:

LIST "C:" 10,100 will save all the program lines between 10 and 100.

This means that you can keep a tape of often used subroutines on tape for incorporation into your programs as you write them.

Programs saved with into the XL with the syntax being:

LIST

can only be loaded back

ENTER

statement, the

ENTER "C:"

AUTO RUN PROGRAMS

There is another command,

RUN "C:'

RUN "C:",

will only load and

CSA

VEd.

which allows you to automatically load and run a program in one step.

run programs which have been

You can use this facility to chain programs, that is to have one program load and execute another from the tape. If you use this technique, bear in

Permanent Storage 191 mind that the variables of the first program will be destroyed when the second is loaded.

PROGRAMS ON DISK

There are two types of command for loading and saving programs on disks.

LOAD and SAVE

LOAD and SA VE operate on programs in tokenised form, much as their counterparts used with the cassette unit. Their syntax is:

SAVE "D: PROG.BAS"

LOAD "D: PROG.BAS" where the program is given the name PROG and is a BASIC program.

It is possible to automatically load and run a program from disk with the RUN command:

RUN "D:PROG.BAS" will load and run the BASIC program PROG.

LIST and ENTER

LIST and ENTER deal with programs in ASCII form, and can be used to save and load sections of program by specifying line numbers, as with cassettes.

LIST "D: PROG.BAS"

ENTER liD: PROG.BAS"

192 The Ater! XL Handbook

By adding line numbers at the end of these commands, sections of program can be merged wi th the program already in memory.

PROGRAMS ON CARTRIDGES

Programs on cartridges are the easiest of all to use.

First, you must switch off the XL, or it may be damaged. Then plug the cartridge into the socket on the top of the machine and switch on again.

Follow the instructions enclosed with the cartridge to start the program.

FILE HANDLING

As well as loading and saving programs, the Atari

XL can use the cassette unit to store and reload data generated by programs. The data is stored in files, and can be in numerical or string form. To create a file, the command OPEN is used:

OPEN Ie, A, F, D

The parameter C is the channel number a reference number used to identify a channel through which communication to a peripheral device is made.

A is the direction in which the data is to travel on the channel, for the cassette unit, a value -of 4 indicates that data is to be read from the cassette, while a value of 8 indicates writing data to the cassette.

The F parameter indicates file type and for our purposes this is O.

Permanent Storage 193

Finally, D specifies the device type where ((C:" indicates the cassette unit.

Having opened a channel, data is sent to the specified device using either the PRINTI command or the PUT command.

The syntax of the PRINrrl command is:

PRINT #Ci"DATA"

PRINT #Ci"123456"

The

PRINTI

characters to the tape, via the channel specified by the parameter command prints a string of ASCII

C.

The PUT command sends a single item of data to the cassette and takes the form:

PUT #C,x where X is the numeric data and C specifies the channel.

Reading data from the tape involves the two commands

INPUTI

and GET.

The INPUT command is used to retrieve numeric or string data, depending upon the variable given in the command. Its syntax is:

INPUT IC,A

INPUT #C,A$

GET is used to read single bytes of data from the tape, its format is:

GET Il,X

194 The Atari XL Handbook

Closing Files

After use, a file must be closed, with the command

CLOSE IC where C is the channel number.

The following short program shows the use of

OPEN,

PRINT#, INPUT# and

5

10

20

30

40

DIM A$(40)

OPENll, 8, 0, "C:"

PRINTll, "THIS IS A TEST FILE"

CLOSEll

STOP

100 OPENll, 4, 0, "C:"

110 INPUT#l, A$

120 CLOSE#l

130 PRINT A$

Put a blank tape into the tape unit and wind it forwards past the transparent leader tape. RUN the program, and the TV speaker will sound twice as happens when saving programs. Start the tape, and the file will be created and the data stored.

The program will stop with the message

AT LINE 40".

Rewind the tape and type GOTO 100.

The TV speaker sounds once. When you set the tape going the data file will be opened and read, and the data printed on the screen.

This technique may be used to store any amount of data onto tape.

Commas and semi-colons may be used with

PRINT# as with PRINT. Without punctuation,

Permanent Storage 195

PRINT# puts a carriage return character after each item of data written to the tape, and these carriage returns are used by the INPUT# statement to distinguish between adjacent items of data. Using commas or semi-colons suppresses the carriage returns with the result that the INPUT# statement will not read the data properly. This is where GET# is useful, reading single bytes from the file without regard for carriage returns or any other markers. If you need to store single byte numbers, then writing them to the tape with

PUT#; and reading them with GET# makes very efficient use of the tape, as all the space is taken up by data with no separating characters.

This method is used in the Character Generator program in Chapter 12 to save character sets onto tape.

OTHER PERIPHERAL COMMANDS

STA TUS, is a system variable which provides information about tape files.

The variable has three possible meanings, which are shown in the table:

STATUS

1

3

8

MEANING

OK

CLOSE TO END OF FILE

ERROR CODES

CHAPTER 17

ADVANCED TECHNIQUES

The previous chapters cover all you need to write many programs for your XL. This chapter decribes some further BASIC commands which you will find useful as you wri te more ambi tious programs, and rounds off wi th some general advice on how to write good programs.

THE POP COMMAND

This strange looking command

IS used In connection with BASIC subroutines.

Suppose you have a program with subroutines nested one within the other, one of which checks which keys are being pressed. You may wish to respond to one of these keys - ESC perhaps - by abandoning the current routine and returning to the start of the program.

This could mean

RETURNing through several levels of subroutine, which would be tedious to program, requiring IF ...

THEN RETURN instructions at each level. The

XL allows a simpler way of doing this, using the

POP command

Each time a GOSUB command is executed, the XL stores the location of the instruction after the

GOSUB on a stack in the reserved area of memory.

At any time the stack contains RETURN addresses for all GOSUBs for which RETURN commands have not yet been performed. The POP command removes the most recently added

RETURN address from the stack, so that the next

RETURN sends the computer back to the GOSUB

196

Advanced Techniques 197 before that most recently added. Try this example program.

10

20

GOSUB 100

PRINT "MAIN PROGRAM"

30 END

100 GOSUB 200

110 PRINT "SUBROUTINE 100"

120 RETURN

200 GOSUB 300

210 PRINT "SUBROUTINE 200"

220 RETURN

300 RETURN

If you run this program, the XL will follow the

GOSUBs to line 300 and then RErrURN progressively through each subroutine, printing the message. If you now change line 300 to read:

300 POP:POP:RETURN and run the program again, the program will only print (MAIN

PROGRAM'

and stop.

ERROR HANDLING

The XL has a command which allows you to take corrective action should an error occur as a program is running - TRAP.

The syntax is:

TRAP N where N is the line number to which the program is directed when an error is encountered.

TRAP must be executed before the error occurs, so you should put it at the start of any program in which you wish to use it.

198 The Afar; XL Handbook

In use, if an error is encounterd, the normal mechanism whereby program execution is stopped and an error message displayed is suspended.

Instead, program execution continues at the line specified by the TRAP command.

At this line number you must include code to handle any possible errors. You can determine what type of error has occurred with the statement:

ERR = PEEK(195) since location 195 contains the code number of the error (see Appendix 2). The line number at which the error occurred can be calculated with a statement like:

LINE = PEEK(186)+256*PEEK(187)

The following program uses the TRAP command to good effect by using it to display full error messages rather than error codes which must be looked up in a table.

1 GOSUB 32000:TRAP 32500

32000 DIM ERROR$(400):DIM

MESSAGE$(20)

32020 RESTORE 32100

32030 FOR ITEM =2 TO 21

32040 MESSAGE$="

":REM 20 SPACES

32050 READ MESSAGE$

32060 ERROR$«ITEM-2)*20+1)=MESSAGE$

32070 NEXT ITEM

32080 RESTORE

32090 RETURN

32100 DATA MEMORY INSUFFICIENT,

ILLEGAL VALUE,TOO MANY

VARIABLES,STRING LENGTH,OUT OF

DATA

Advanced Techniques 199

32110 DATA LINE NO TOO LARGE, INPUT

TYPE ERROR,DIMENSIONING

ERROR,STACK OVERFLOW,

OVER/UNDER FLOW

32120 DATALINE NOT FOUND,NEXT

WITHOUT FOR,LINE TOO LONG,

GOSUB/FOR DELETED,RETURN

WITHOUT GOSUB

32130 DATA GARBAGE, INVALID STRING,

LOAD PROG TOO LONG, DEVICE

NUMBER, LOAD FILE ERROR

32500 LINE=PEEK(186)+PEEK(187)*256

32510 CODE=PEEK(195)

32520 PRINT" {BELL}ERROR "iCODE;" IN

LINE "iLINE

32530 P=(CODE-2)*20+1

32540 PRINT ERROR$(P,P+19)

32550 END

If you save this program on tape using the LIST command it can be reloaded to form a temporary part of the program you are developing without disturbance.

MACHINE CODE SUBROUTINES

The XL's microprocessor does not understand

BASIC programs without some assistance; it understands a much cruder set of instructions called machine code or machine language.

BASIC programs can be run only because the XL has some machine code programs permanently stored in .

read-only memory (ROM) which interpret the

BASIC. Writing programs in machine code is more difficult than using BASIC, but can be very worthwhile as machine code programs run very much faster. Machine code is beyond the scope of this book, but there are several books available

200 The Afar; XL Handbook about the XL's microprocessor (the 6502) as it is very common in personal computers.

You can enter machine code routines from BASIC using the command

USR,

giving the start address in memory of the machine code routine.

For example:

USR (69316) will perform the same function as pressing the

RESET key, and:

USR (69319) will call the power-on setting up routines which clear the memory and print the start-up message.

BE

CAREFUL

using this - it will destroy any program you have in the XL at the time.

The

USR

command may also contain one or more arguments, separated by commas, after the address of the machine code routine. When the command is executed, the address of the current BASIC instruction is placed on the stack, followed by each of the arguments, followed by the number of arguments included in the command. The machine code routine can then use the arguments and return them to the stack, for use by BASIC.

Machine code subroutines may be stored as string variables, and another command, AD

ADR

is:

R,

used to determine the start address in memory of the string, hence the start address of the machine code subroutine. The syntax of x =

ADR(A$)

Advanced Techniques 201

DESIGNING

GOOD

PROGRAMS

It is easy to write simple BASIC programs, and after some practice you will find it fairly easy to write quite complicated routines. However, unless you plan your programs carefully, a long program can get very messy and it can be very difficult to find all the mistakes you are bound to make.

To write complex programs successfully you must design them carefully, following a few simple rules.

The phrase structured programming is often used to describe these rules ; all this means is that programs designed in accordance wi th these rules have a clear and logical structure, and the flow through the program is easy to follow . You may be put off by hearing people say that BASIC is not a suitable language for structured programming.

Don't let this discourage you. While BASIC may lack the elegance of some other languages, it is still possible to use it to write good programs by following the simple guidelines set out here:

1 Always plan out the program on paper. Decide what you want the program to do, identifying the various tasks it will have to perform, and divide the program into sections which correspond to the tasks.

For example, you might have one section of program to get data from the keyboard, another to sort the data, a third to display results, and so on.

2 It is often useful to draw sketches of the way the tasks fit together.

When you have got the overall structure of the program sorted out, work out in more detail

202 The Ateri XL Handbook

Key not valid

Sketch programs before writing the

BASIC

what each section should be doing, and how it should be done.

4 Write the BASIC routines for each section of the program - writing them on paper and typing straight into the XL. Try to write each section of the program so that it is as independent as possible of the other sections, and arrange the routines with a clear sequential flow from beginning to end: avoid jumping around with

GOTO.

not

Use a different set of line numbers for each section, beginning each at a round number of so many thousands or tens of thousands.

5 Now type the program into the XL.

6 The program will not work! At this stage you can sort out all the errors easily. If you have designed the program well, with different

Advanced Techniques 203 sections doing different tasks, it will be much easier to find the more subtle errors, as you will be able to isolate the faulty bit of the program without difficulty.

If you follow these guidelines, you will save a lot of time in getting your programs to work, and you will have more time for actually using them.

ABS (N)

ADR(C$)

AND

ASC (C$)

ATN(N)

BYE

CHR$(N)

APPENDIX 1

BASIC COMMANDS

Returns the absolute value of a n umber (removing the minus sign).

Chapter 8

Returns the address of a string in memory.

Chapter 17

Logical operator. Returns the value TRUE if both operands are true (1).

A = BANDCAisTRUEifB and C are both TRUE.

Chapter 9

Function. Returns the ASCII code of a character, or of the first character of a string.

Chapter 7

Function. Gives the angle, in radians or degrees, whose arctangent is N.

Chapter 8

Causes exit from BASIC to

Self-Test mode.

Converts ASCII codes to characters in string form.

Chapter 7

204

CLOSE (N)

CLOAD

CLOG

CLR

COLOR

COM

CSAVE

CONT

BASIC Commands 205

Closes a channel, N, to a peripheral device (disk, printer, etc)

Chapter 15

Loads a program from the cassette unit.

Chapter 16

Returns base 10 logarithm of a number.

Chapter S

Clears variables, arrays, etc, from memory, and makes the memory available to BASIC programs.

Chapter 16

Specifies colour register to be used in subsequent PLOT commands. In modes 0 to 2 selects character and colour register for PLOT.

Chapter 12

Identical to DIM.

Saves a program on the cassette unit.

Chapter 16

Restarts program after pressing the BREAK key or executing a STOP command.

ChapterS

206

COS (N)

DATA

DEG

DIMA(L,M)

DIM A$(L)

DOS

DRAWTOX,Y

ENTER

END

EXP (N)

The Atari XL Handbook

Gives cosine of angle, which may be given in radians or degrees.

Chapter 8

Marks .u

list of string or numeric data written into a program. The items must be separated by commas.

Chapter 6

All subsequent calculations to be performed in degrees.

Chapter 8

Dimensions arrays and strings and allocates memory space for them.

Chapter 4

Displays DOS menu if disk in use, otherwise has same effect as BYE.

Draws a line from last cursor position to specified position.

Chapter 13

Loads a program which has been listed to cassette or disk.

Chapter 16

Ends program. Program"may be restarted using CONT.

Chapter 5

Function returning exponential of N (eN).

Acts as natural antilog function.

Chapter 8

FOR

FRE (0)

GET#(N)

GOSUB

GOTO

BASIC Commands

207

Begins loop. For example:

FOR N = A TO B STEP C

All lines as far as NEXT command are repeated wi th value of N increased each time from A to B in steps of

Chapter 5

C.

STEP may be omitted, in which case the variable is increased by 1.

Returns the amount of memory at present unused by

BASIC.

Chapter 10.

Reads a single character from the specified file or device and assigns it to a variable.

Device may be keyboard.

Chapter 15

Program branches to a subroutine at the specified line, returning to instruction after

GOSUB when RETURN is encountered.

Line number may be specified by a variable.

Chapter 5

Program branches to the specified variable.

line.

The line number may be specified by a

Chapter 5

208

GRAPHICSM

The Atari XL Handbook

IF (condition)

(action)

INPUTN

INPUT ((data";N

INPUT #(N)

INT (N)

Selects graphics mode M.

M

+

16 removes text window,

M + 32 sets mode clearing screen.

without

Chapters 12 and 13

If the condi tion is true the action after THEN is carried out, otherwise the program continues at the next line.

Chapters 5 and 9

Prompts the operator for an input and assigns it to a variable. The variable may be a number or string: the input data must correspond.

Chapter 4

Retrieves data from the specified file number. Data is in the form of strings up to 80 characters in length, delimited by CHR$(13) or , or

; or: .

Chapter 15

Returns integer component of real number. For example:

INT ( 3 • 75) returns 3.

Chapter 8

Gives the number of characters in the string.

Chapter 7

LET

LIST

LOAD

LOCATE

X,Y,Z

LOG(N)

LPRINT

NEW

NEXTN

NOT

BASIC Commands

209

Optional. May be used when assigning values to variables:

LET P

P = 5

=

5 and have the same effect.

Lists the specified lines of the program on to the screen or to a peripheral device.

Chapter 4

Reads program file from peripheral device.

Chapter 15

Returns the value of the data at screen location X,Y in variable Z.

Chapter 13

Returns the natural log arithm of a number.

Chapter 8

Outputs data to the printer.

Clears a program and its variables from memory.

Chapter 4

Marks end of loop begun by.

FOR. The variable must be specified.

ChapterS

Logical operator.

Reverses truth of expression.

Chapter 9

210

NOTE

ON

N

GOSUB

ON

N

GOTO

OR

PEEK (LOC)

PLOTX,Y

The Atari XL Handbook

OPEN

#N,O,O,D

Finds location of next byte to be accessed from disk.

Program branches to Nth subroutine in list.

If N is larger than number of items in list no branch occurs.

ChapterS

Program branches to Nth destination in list.

If N larger than number of items in list no branch occurs.

ChapterS

Opens a channel N to a peri pheral device specified by

D.

Chapter 16

Logical operator.

Returns value TRUE if either or both operands are true. Thus:

A = B OR C

A is true ifB or

Chapter 9

C is true.

Gi ves the contents of memory location LOC.

Chapter 15

Puts point or character on screen at location X,Y.

Chapter 13

POINT

POKE LOC,N

POP

BASIC Commands 211

Selects position on disk at which next byte is to be wri tten or read.

Puts value of N into memory location LOC. N must have value 0 - 255.

Chapter 15

Removes one address from the stack.

Chapter 17

POSITION

X,Y

PRINT

Positions the cursor at X,Y for subsequent PRINT operation.

Chapter 12

Puts data, numbers or characters on the screen.

Chapter 3

PRINT#N

Writes data to the specified file.

Chapter 15

PUT#D

RAD

Outputs a single byte to specified device.

Chapter 16

All subsequent calculations to be in radians.

Chapter 8

212

READ

REM

RESTORE

RETURN

RND (N)

RUN

The Atari XL Handbook

Copies items from DATA statements into variables.

Chapter 6

Allows remarks to be inserted in programs as an aid to clarity. Remarks are ignored when program is run.

Chapter 4

Returns READ pointer to first DATA item. May be followed by a line number to indicate the DATA item required.

Chapter 6

Marks end of subroutine.

Program returns to the instruction after the GOSUB instruction which called the subroutine.

ChapterS

Returns a random number between 0 and 1, N is a dummy variable.

Chapter 8

Begins execution of BASIC program.

All variables are cleared.

Chapter 4

BASIC Commands 213

SAVE

Stores the program currently in memory onto tape or disk.

Device number specifies tapetfl) or diskrD).

Chapter 15

SETCOI...OR

R,C,L Sets colour register R ro colour C at luminance L.

Chapter 13

SGN(N)

Returns 1, 0 or -1 according to whether number is positive, zero or negative.

Chapter 8

SIN

Function. Returns the sine of a number in radians.

Chapter 8

SQR

STATUS

STICK(N)

Returns the square root of the argument.

Chapter S

Returns status of the specified device as an error code.

Chapter 8

Returns posi tion of specified joystick.

Chapter 13

STOP

Stops execution of program.

A message AT

LINE xxx' is displayed.

Program may be restarted usingCONT.

ChapterS

214 The Ateri XL Handbook

STRIG(N)

STR$ (N)

THEN

TO

Returns 0 if fire button of specified joystick is pressed, 1 ifnot.

Chapter

13

Converts numbers to strings of numeric characters.

Chapter

7

See

IF.

ChapterS

See

FOR.

Chapter

5

TRAPN

Causes jump to specified line number when an error occurs.

Chapter

17

USR (ADDR,[X,Y...

])Calls machine code routine at address specified by

The optional arguments values X, Y etc. are placed on the 6502 hardware stack,

Chapter 17

ADDR.

together with a count of the arguments and the address of the next BASIC instruction.

VAL (C$) Converts string of number characters to number.

Chapter 7

XIO T,C,pl,p2,D Mainly used for filling blocks of colour on screen.

Chapter 13

APPENDIX 2

BASIC ERROR MESSAGES

If the XL encounters a command which it is unable to execute, or a number it cannot handle, an error message will be displayed.

If the error occurred while running a program, the program will stop, and a message of the form

ERROR

***

AT LINE XX will

be

displayed, where

*** represents the error code, and XX is the program line at which the error has occurred.

The program is retained in the computer, as are the values assigned to all variables at the time of the error.

In immediate mode, the error message takes the form

ERROR -

** and is embedded in the line which is in error.

The following descriptions explain the error codes, and the possible reasons for them:

2 OUTOFMEMORY

Either the program is too large, or you have used too many variables, too many FOR ... NEXT loops,

215

216 The A tar; XL Handbook too many GOSUBs or allocated too much memory space with a DIM command.

3

A number outside the allowable range was encountered, or a negative value has been used as an argument for a function requiring a positive argument.

4

TOO

MANY V

More than 128 variable names have been used.

5 STRING LENGTH

ERROR

A program tried to manipulate string data outside the range for which the string was dimensioned, or zero was used as a character position reference in a string handling command.

6 OUTOFDATA

There were insufficient data items for the READ command.

7 LINE

NUMBER

A negative line number, or one greater then 32767 has been referenced.

8 INPUT ERROR

An attempt to input a string value into a numeric variable was made.

BASIC Error Messages 217

9 ARRAY OR STRING DIMENSION

Either an undimensioned string or array has been referenced, or an attempt has been made to redimension a string or array.

10 ARGUMENT STACK

An expression is too large or the program contains too many GOSUBs.

11 FLOATING POINT OVER/UNDER

A number less than IE -99 or greater than IE 98 has been encountered, or an attempt has been made to divide by zero.

12 LINE NOT FOUND

A non existent line number has been referenced.

13 NO MATCHING R

A NEXT statement with no corresponding FOR was encountered.

14 LINE

TOO LONG

The line entered is greater than 140 characters.

15 GOSUB OR FOR DELETED

The line containing a GOSUB or FOR statement has been deleted.

16 RETURN ERROR

There is no corresponding GOSUB statement for a

RETURN.

218 The Afar; XL Handbook

17 GARBAGE ERROR

Random error.

18

INVALID STRING CHARACTER

Either an invalid character is present in a string, or the. argument of a VAL command is nonnumeric,

19 LOAD

PROGRAM TOO LONG

The program currently being loaded is too large for the available memory.

20 DEVICE NUMBER ERROR

A device number less than 0 or greater then 7 was used.

21 LOAD FILE

ERROR

The LOAD command was used to load a program which was CSAVED or LISTed.

128

BREAK ABORT

The BREAK key was pressed during an I/O operation.

129 CHANNEL

OPEN

An attempt has been made to use a channel which is already open.

130 NON - EXISTENT DEVICE

The device has not been specified In an I/O operation.

BASIC Error Messages

219

131

IOCB

WRITE

ONLY

An attempt has been made to read from a file opened for a write operation.

132

INVALID COMMAND

An illegal command has been used In an I/O operation.

133

DEVICE/FILE NOT

OPEN

An attempt has been made to reference an un-

OPENed channel.

134 ILLEGAL

IOCB NUMBER

A number less than 0 or greater than 7 was used to access a channel.

135

IOCB

READ ONLY ERROR

An attempt has been made to write to a device or file which was opened only for read operations.

136

END OF

FILE

The end offile marker has been reached.

137

TRUNCATED RECORD

The INPUT command has been used to access a file created wi th the PUT command, or the record is larger than allowed.

138 DEVICE TIMEOUT

The specified device did not respond within the time allotted by the XL for communication between the comuter and peripherals.

220 The Ateri XL Handbook

139 DEVICE NAK

No response from peripheral device.

140 SERIAL FRAl\1E ERROR

Indicates faulty computer or peripheral.

141 CURSOR OUT OF RANGE

The cursor is outside the allowable range for a graphics mode.

142 SERIAL BUS RUNOUT

Fault in serial bus between computer and peripherals.

143

CHECKSUM ERROR

Data being transferred from peripheral is being corrupted.

144

DEVICE DONE ERROR

An attempt has been made to write to a writeprotected disk.

145 BAD SCREEN MODE ERROR

An error in the screen handler has occured.

146 FUNCTION NOT IMPLEMENTED

An attempt has been made to use a device in a manner which is not possible.

BASIC Error Messages

221

147 INSUFFICIEN'r RAM

Not enough memory for the graphics mode selected.

160 NUMBER

An incorrect number has been used to specify the disk drive.

161

'roo

MANY

The limit imposed on the number of disk files open at one time has been reached.

All sectors of the disk are used.

163 SYS'rf-i:M 1/0 ERROR

An error is present in the DOS or on the disk.

Incorrect use of the POINT command - the sector specified was not included in the open file.

Illegal filename.

166 1)0 I Nfl' IlA'I'A I{ RO R

Incorrect use of the POINT command the specified byte n umber did not exist within the specified sector.

222

The Afar; XL Handbook

167

FILE LOCKED

An attempt was made to access a locked file.

168

INV

ALIDDEVICE

COMMAND

An illegal device command was used.

169 DIRECTORY

No more than 64 files can exist on one disk.

170

FILENO'rFOUND

An attempt has been made to access a file which is not on the disk.

171

POINT INVALID

Incorrect use of POINT command - an attempt has been made to use the POINT command with an incorrectly opened file.

172 ILLEGAL APPEND

The wrong DOS has been used to append a file.

173

BAD FORMAT

Bad sectors have been found on the disk while formatting - discard the disk.

APPENDIX 3

SPEEDING UP PROGRAMS

There are several things you can do to increase the running speed of your BASIC programs.

Unfortunately, this is usually at the expense of clarity, and you may find it useful to keep a 'slow', but easy-to-follow version of your program should you wish to amend it at a later date.

1

2

3

4

5

6

Remove all unnecessary spaces andREMs from the program.

A small speed increase will result, because BASIC will not have to skip over redundant spaces to find executable commands.

Always use variables instead of constants.

The XL can handle variables much more rapidly than numbers.

This is especially important in FOR ... NEXT loops.

Use as many statements per line separated by ':' as possible.

Re-use the same variables whenever possible.

Use the zero elements of arrays when possible.

Assign often used variables early on in the program. The XL stores all variables in a table. The first declared variables are the first in the table and are found more quickly.

223

224

7

8

The Alar; XL Handbook

Put all subroutines 'near the start of the program. The computer searches through the whole program for a subroutine each time a

GOSUB command is executed, and subroutines having low line numbers will be found and executed more quickly than those at the end of the program.

Programs involving large amounts of calculation but not requiring the screen display may be speeded up by turning off the screen, wi th a line such as :

100 A=PEEK(559):POKE 559,0

The screen can be turned back on to display the results of the calculations with a statement:

200 POKE 559,A

APPENDIX 4

NUMBERING SYSTEMS

Computers store and operate upon numbers in a different way from humans - they use a numbering system known as binary notation.

Binary notation is a means of representing quantities with groups of Is and Os. We are more used to a system called decimal notation, in which quantities can be represented by combinations of up to ten symbols (the numbers 0 to 9).

Computers use the binary system because they are able to recognise and differentiate between only two states -

ON

and

OFF.

These two states can conveniently be represented by 1

(ON)

and 0

(OFF).

A single 1 or 0 is called a BInary digiT, or BIT, and computers store data in the form of groups of eight of these bits, known as

BYTES.

In the computer memory, one memory location is able to store one Byte of data (eight bits).

A collection of 1024 of these bytes is called a

KILOBYTE,

or k for short. We can get an idea of the data storage capacity of a computer from the number ofk of memory it has (48k, for example, is

48

*

1024

*

8 bits).

We have described a byte as a collection of eight bi ts, like this:

11111111

225

226 The Ateri XL Handbook

This is an 8-bit binary number, which represents

255 in decimal notation. To see how this is so, we must first examine the decimal number and see what it means.

H T U

2 5 5 means:

2*100 + 5*10 + 5*1

In other words, each digit is worth 10 times the one to its right.

Binary notation uses this same 'place value' principle, except each bit in a binary number is worth double that to its right.

We can assign values to the eight bits in the same way as the

'hundreds, tens and units' assigned to the digi ts of a decimal number.

128 64 32 16 8 4 2 1

1 1 1 1 1 1 1 1

By adding up, we can see why this number represents 255:

1

1

+1

1

1 *128

1

*

64

1

1

*

32

*

16

*

8

*

4

*

2

*

1

255

You'll notice that 255 is the biggest number we can represent with an 8-bit binary number. Hence this

Numbering Systems

227 is the largest number we can store In a single memory location.

As a further example, let's take the decimal number 170. To find its binary representation, we continuously divide by two, and the remainder becomes a bit in the binary number.

170/2

85/2

42/2

21/2

10/2

5/2

2/2

1/2

=

85

42

21

10

5

2

1 o remainder o

1 o

1 o

1 o

1 gi ving us the binary number

10101010 (reading upwards)

ADDITION OF BINARY NUMBERS

Binary numbers can be added together in the same way as decimal numbers. An example will make this clear.

.

To perform the sum:

105

+ 19

124 we add up the digits in each column to form that digit of the answer. If the result of this addition is greater than 9, we generate a carry into the next column.

This principle also applies to binary numbers. Let's perform the same calculation with the binary forms of 105 and 19:

228 The Atari XL Handbook

01101001

+00010011

01111100

In the case of binary addition we generate a carry when adding 1 to 1 (in columns 1 and 2 in our example).

NEGATIVE BINARY NUMBERS

You might have wondered how the computer can recognise negative numbers, since it can only tell the difference between on and off. This is achieved by a method known as 'two's complement' notation, which uses one bit of the binary number (the most significant bit, often labelled bit 7) as a sign bit.

To subtract two numbers in binary, we form the two's complement of the number to be subtracted, then add it to the other number.

As an example we'll subtract 50 from 100:

100

-50

01100100

00110010

50 00110010

(the answer we want).

To perform the sum in binary, first we find the two's complement of 50, by changing all the Os to Is and all the Is to Os, then adding 1.

50 in binary is:

Change Is to Os:

Adding 1 gives its two's complement:

00110010

11001101

11001110

Now add this to the binary for 100:

Numbering Systems

229

01100100

11001110

1 00110010

The result is binary 50 - the method worked.

Notice that a carry was generated, indicating that the answer is positive. A consequence of using bit 7 as a sign bit is that the range of numbers we can represent with an 8-bit number is restricted to

-128 to + 127

HEXADECIMAL

Manipulating numbers in binary is a lot easier for a computer than it is for a human, and one way in which binary numbers can be made more digestible is by representing them in hexadecimal notation, or hex.

Hex is a system of counting in base 16, using the symbols 0 to 9 and A to F as follows

DECIMAL 0 1 ••• 9 10 11 ••• 15 16 17

HEX 0 1 ••• 9 A B... F 10 11

Thus FF (hex) represents 255 (decimal) and

11111111(binary).

You will frequently encounter references to hex, usually as memory addresses, because it is so convenient. (Which of these is easiest to recognise? .

1111111111111111 or 65535 or #FFFF).

APPENDIX 5

GRAPHICS MODES

MODE TYPE COlS.

ROWS ROWS

SPLIT FUll

COLOURS

MEM REO. MEM REQ.

SPLIT FUll

0

TEXT

40 24

1

TEXT 20 20 24

1

5

992

674 672

2

3

TEXT

GR

20

40

10

20

12

24

5

4

424 420

434 432

4

5

6

GR

GR

GR

80

80

40

40

160 80

48

48

96

2

4

694 696

1174 1176

2 2174 2184

7 GR 160 80 96 4 4190 4200

8 GR 320 160 192 1 8112 8138

9 GR 80 192 1

10 GR 80

11 GR 80

192 9

192 16

12

TEXT 40 20 24 5

\

13

TEXT 40 10' 12 5

14 GR 160 160 192 2

15 GR 160 160 192 4

-

-

8138

8138

8138

1154 1152

664 660

4270 4296

8112 8138

230

APPENDIX

6

COLOUR CODES

COLOUR

GREY

GOLD

ORANGE

RED/ORANGE

PINK

PURPLE

BLUE/PURPLE

BLUE

BLUE

LIGHT BLUE

TURQUOISE

GREEN/BLUE

GREEN

YELLOW/GREEN

ORANGE/GREEN

LIGHT ORANGE

CODE

7

8

9

10

11

12

13

14

15/

3

4

5

0

1

2

6

231

/

0

1

2

3

4

5

6

7

8

9

(

)

*

+

APPENDIX 7

CHARACTER CODES

SET 1

SPACE

1#

!

#

$

%

&

[]

<

=

SET 2 0 1 2 3

[IJ

[B

0

32

33

34

35

36

37

38

4

5

6

2

3

0

1

160 128

161 129

162 130

163 131

164 132

165 133

166 134 [2J

39 7 167 135

40 8 168 136

C.

41 9 169 137

IitJ 42 10 170 138

43 11 171 139

IJ

[B

[] t

[B

B

EB

II

44 12 172 140

D

45 13 173 141

D

46 14 174 142

47 15 175 143

48 16 176 144

49

50

51

52

17

18

19

20

177 145

178 146

179 147

180 148

[]

EB

53

54

55

21

22

23

181 149

182 150

183 151

56

57

58

24

25

26

184 152

185 153

186 154

59

60

27

28

187 155

188 156

61 29 189 157

232

SET 1

I

J

K

L

M

N

0 p

D

E

F

A

B

C

G

H

>

?

@

Y

Z

[

-,

]

1\

Q

R

5

T

W

X

U

V

Character Codes

C d e f

9 h i

I m n

J k

0

U v w

P q r t

S

X

, I b

+ a

..

SET 2

..

0

62

1

30

2

190 158

3

63

64

31

96

191 159

192 224

65 97 193 225

66 98 194 226

Y z

73

74

75

76

77

78

67

68

69

70

71

72

79

. 80

89

90

91

92

93

94

95

85

86

87

88

81

82

83

84

99 195 227

100 196 228

101 197 229

102 198 230

103 199 231

104 200 232

105 201 233

106 202 234

107 203 235

108 204 236

109 205 237

110 206 238

111 207 239

112 208 240

113 209 241

114 210 242

115 211 243

116 212 244

117 213 245

118 214 246

119 215 247

120 216 248

121 217 249

122 218 250

123 219 251

124 220 252

125 221 253

126 222 254

127 223 255

233

APPENDIX 8

DISPLAY LIST INSTRUCTIONS

In,. order to create borders, ANTIC has a set of instructions which create up to eight scan lines in either screen or border colour:

ANTIC CODE

80

96

112 o

16

32

48

64

NUMBER OF BLANK LINES

1

7

8

4

5

6

2

3

Another group of instructions select graphics modes:

ANTIC CODE BASIC MODE

2

10

11

8

9

6

7 2

3

4

5

6 o

1

234

Display List Instructions 235

5

12

14

3

13

15

15

15

15

4

9

10

11

12

13

14

15

7

8

Antic can be instructed to jump, in which case it treats the next two bytes in the display list as the address to which it shouldjump:

JUMP TO ADDRESS 1

JUMP AT VERT BLANK 65

Most instructions can be modified as follows: add 16 HORIZONTAL SCROLL add 32 VERTICAL SCROLL add 64 LOAD MEMORY SCAN

(REFRESHES SCREEN

MEMORY POINTERS)

APPENDIX

9

USEFUL MEMORY LOCATIONS

82

83

84

85,86

87

88,89

93

94,95

128,129

656

657,658

144,145

186,187

195

212,213

251

559

LOCATION

16

18,19,20

65

694

DESCRIPTION

POKE with 64 and POKE 53774 with 64 to disable BREAK key.

Real time clock.

Poke with 0 to hear sound from cassette during loading.

Poke wi th 1 to stop sound.

Left margin of mode 0 text display.

Right margin of mode 0 display.

Current cursor row.

Current cursor column.

Current display mode.

Start of screen memory.

Contains a copy of the character underneath the cursor.

Current cursor address.

Start of BASIC memory.

Cursor row in text window.

Cursor column in text window.

Top of BASIC memory.

Line number of error found by TRA P.

Error number of error found by

TRAP.

Value returned by USR function.

0= Radians mode.

6

=

Degrees mode.

POKE with 0 to disable display. See

Chapters 14, 15.

0= Keyboard gives normal characters.

Non-zero = Keyboard gives inverse video characters.

236

702

703

704-707

708-712

741,741

743,744

752

755

756

763

764

765

53279

54018

58484

58487

Useful Memory Locations 237 o =

128

No shift locks.

64 = CAPS lock on.

=

CONTROL lock on.

24 == Text window off.

4 == Text window on.

Player-Missile colour registers 0 to 3.

Graphics colour registers 0 to 4.

Top of free RAM.

Start of free RAM.

I

0= Cursor on.

=

Cursor off.

Controls appearance of cursor and characters.

Setting bit 0 makes the cursor opaque.

Setting bit 1 makes the cursor visible.

Setting bit 3 inverts the displayed characters.

224

226

=

Standard character set.

=

Alternate character set.

ASCII code of last key pressed.

Internal code of last key pressed.

Holds colour data for XIO command.

Bi t 0 is zero if START is pressed.

Bit 1 is zero if SELECT is pressed.

Bit 2 is zero if OPTION is pressed.

POKE 53279, 8 retracts loudspeaker cone.

POKE 53279, 0 extends loudspeaker cone.

PO KE wi th 52 to start cassette motor.

POKE with 60 to stop cassette.

X

X

=

USR(58484) gives RESET.

=

USR(58487) gives switch-on reset.

APPENDIX 10

MUSIC FREQUENCIES

C

C#,Db

D

D#,Eb

E

F

F#,Gb

G

G#,Ab

A

A#,Bb

B

C

C#,Db

D

D#,Eb

E

F

NOTE PERIOD NOTE PERIOD

255

239

227

211

203

191

179

171

159

151

143

135

127

119

113

105

101

95

F#,Gb

G

G# ,Ab

A

A# ,Bb

B

C

C#,Db

D

D#,Eb

E

F

F#, Gb

G

G# ,Ab

A

A#,Bb

B

57

53

50

47

68

64

63

89

81

76

72

45

42

40

37

35

33

31

\

238

APPE·NDIX 11

MUSICAL NOTATION

This appendix will not teach you all about music, but it contains the basic information you need to translate sheet music into XL programs.

Music is written by positioning symbols which represent the length of notes on a framework

(called a stave) representing the pitch.

G

E

C

A

F

D

B

G

F

D

B

A

F

D

B

G

E

C

A

F

G

E

C

Atari 3rd octave

Atari 2nd octave

Atari 1st octave

The lengths of notes are indicated by the note shape:

A Semibreve 0 is twice as long as a Minim a Crotchet a Quaver

) which is twice as long as which is twice as long as which is twice as long as

""

239

The Ateri XL Handbook 240 a Semiquaver )

Tails on notes may go up

V or down i.

The feathers on quavers and shorter notes may be joined where they appear in groups:

A dot after a note means that it is made half as long again as a normal note:

J. =J j

The mark are joined together.

is a tie which means the notes

J_J =J

Volume is indicated by markings below the stave.

ff Very loud f

Loud mf

Moderately loud mp

Moderately soft p

Soft pp

Very soft

<:::::::

Get louder

Get softer

Musical Notation 241

Speed is indicated by markings which may be above or below the stave. Examples are:

Presto

Allegro

Allegro moderato Moderately fast

Moderato

Andante

Largo which means Fast

Quite fast

Medium pace

Slow

Very slow

Unfortunately, there are many other Italian words and phrases which may be used. The best thing to do is to adjust your XL program until the speed sounds right, and not worry too much about what is written on the music.

Two other markings which may appear next to notes are # and b.

# (sharp) means that the note should be raised by one semitone and b (flat) means that the note is lowered one semitone.

All other markings which may appear on sheet music (and there are many of them) can be ignored.

LAND OF HOPE AND GLORY

Here is the beginning of 'Land of Hope and Glory', one of the examples used in Chapter 11. The notes shown In this music correspond to the notes

242 The Ateri XL Handbook represented by the first item of every group of two in the.

DA'rA statement in the program.

The length of the note, indicated by its shape, corresponds to the second number of each group.

Note the (sharp' symbols (#) on the lines of the stave corresponding to the note F. These mean that all F's throughout the music are sharpened.

APPENDIX 12

MEMORY MAP

Operating system in ROM

Not Used

ANTIC registers

Input/Output registers

POKEY registers

Not Used

GTIA registers

BASIC ROM

Cartridge Slot or RAM

Screen Memory

------------------------

Display List

------------------------

BASIC Program and

Variables

Free RAM

Operating system and

BASIC Storage

65535

55296

54784

54272

54016

53760

53504

53248

40960

32768

NOTE

Locations 16784 to

32767 are not used in the 600XL.

1792

1536

o

243

INDEX

DELETE key

DIM

DRAWTO

11

24, 52

138

ABS

ADR

Alternative char. set

AND

Animation

Arrays

Arithmetic

Assembly Language

ASC

ASCII code

ATN

BASIC

Commands

Binary numbers

Bit

Boolean algebra

Branching

BYE

Byte

Editing

64 END

200 ENTER

114 Error messages

71 EXP

123,172

24 Files

12,21 Filter

199 FOR

54 FRE

53 Functions

65

11

41

188

215

66

192

102

31

76

63

GET

1 GET#

204 GOSUB

225 GOTO

47

193

38

34

225 GRAPHICS

71 Graphics Symbols

108

15

34

204 Hexadecimal numbers 225

225 Hierarchy of operations 13

Cassette unit

Cartridges

Channels

Character sets

CHR$

Clock

CLOAD

CLOG

CLOSE

CLR

Collision detection

Colour

COLOR

COM

CONT

COS

CSAVE

Cursor

Cursor keys

DATA

DEG

4 IF

192 Immediate mode

192 INPUT

115 INPUT#

53 INSERT key

38 INT

188

67 Joysticks

194 Jumps

76

184 Keyboard

105 Keywords, BASIC

111, 137

205 Languages

41 LEN

65 LET

188 LIST

8 LOAD

9 Loading programs

LOCATE --

45 LOG

65 Loops

244

1

52

209

19

188

188

141

67

31

34

8

27

193

11

64

147

34

8

204

Index 245

Lower case

LPRINT

Machine code

Memory map

Memory use

Microprocessor

Monitor

Music

Nested loops

NEW

NEXT

NOT

Numbering systems

ON ... GOSUB

ON ... GOTO

OPEN

Operati ng System

OR

PADDLE

Pauses

PEEK

Pixel

PLOT

POKE

POP

POSITION

PRINT

PRINT #

Program

PTRIG

PUT

RAD

RAM

Random numbers

READ

Real variables

Register

REM

Reserved words

Reset key

RESTORE

RETURN

Reverse video

RND

14 ROM

209 RUN

74

19

199 SAVE

243 Saving programs

74 Scrolling

199 SETCOLOR

7 SGN

77 SIN

189

188

10, 158

105

65

65

79

32

20

31

SOUND

SQR

STATUS

STEP

73 STICK

225 STOP

Stopping programs

42 STRIG

41 String variables

63

195

31

147

41

41

147

23,52

192 STR$ 54

243 Structured programming201

71 Subrouti nes 39

147 TAB key

38 Television

10

4

74 THEN

115 TO

111, 138 TRAP

74

34

31

197

196 Upper case 14

49 User defined characters 115

8,18,48

193

18 VAL

148

193

USR

Variables

200

55

21

XIO 139

65

74

67

45

23

75

28

204

11

46

39

14

67

Also available from Century Communications

Dictionary of New Information Technology

A.

J.

MEADOWS, M. GORDON and

A. SINGLETON

What to Buy for Business: A Handbook of New Office

Technology

JOHN DERRICK and

PI-IILLIP OPPENHEIM

The Electronic Mail Handbook

STEPHEN CONNELL and

IAN A. GILBRAITH

The Way the New Technology Works

KEN MARSH

Century Computer Programming Course

PETER MORSE and

IAN ADAMSON

Computers and Your Child

RAY HAMMOND

International Dictionary of Graphic Symbols

JOEL ARNSTEIN

Microman

GORDON P ASK and SUSAN CURRAN

Microphobia

MARTIN HONEYSETT

The Really Easy Guide to Home Computing: the ZX Spectrum

SUE BEASLEY and RUTH CLARK

Bobby Catches a Bug

RAY HAMMOND and MICHAEL COLE

Bobby Meets a Pirate

RAY HAMMOND and

MICHAEL COLE

Century Microguide to the ZX Spectrum

Century Microguide to the BBC Micro

Century Microguide to the Dragon

Century Microguide to the Commodore 64 edited by

PETER MORSE

Assembly Programming Made Easy for the BBC Micro

IAN MURRAY

Microwars on the Commodore 64

HUMPHREY WALWYN

In association with 'Personal Computer World'

Microcomputing for Business: A User's Guide edited by

DICK OLNEY

The Microcomputer Handbook: A Buyer's Guide edited by DICK OLNEY

The Spectrum Handbook

TIM LANGDELL

The Intimate Machine

NEILFRUDE

35 Educational Programs for the BBC Micro

IAN MURRAY

Educational programs for the Dragon 32

IAN MURRAY

Educational Programs for the Spectrum

IAN MURRAY

Information Technology Yearbook edited by

PHILIP HILLS

The Database Primer

ROSE DEAKIN

Computer Gamesmanship

DAVID LEVY

Best ofPCW: Software for the BBC Micro

Best ofPCW: Software for the Dragon

Best ofPCW: Software for the Spectrum

The ORIC Handbook

PETER LUPTON and

FRAZER ROBINSON

The Commodore 64 Handbook

PETER LUPTON and FRAZER ROBINSON

35 Programs for the Dragon 32

TIM LANGDELL

CENTURY SOFTWARE

VOUCHER

The major programs in this book are available on cassette from the publishers.

Please cut out this voucher (do not copy the voucher) and send it together with a cheque/postal order for £6, payable to George Philip Ltd:

George Philip Services Ltd,

Arndale Road,

Wick,

Littlehampton,

West Sussex.

BN177EN.

VOUCHER

r-------------------------------------------,

Please send me one cassette of the major programs in the AlARI XL HANDBOOK at the special price of £6 including postage and packing.

NAME .

I

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I

I

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I

I L

ADDRESS

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