Atari XL User manual

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 1
2
Setting up your XL 3
3
First Steps 8
4
Programming
18
5
Program Control
31
6
Data and Programs
45
7
Pieces of Strings 52
8
Functions 63
9
Logical Thinking
70
10
Memory Managemen t74
11
Sound and Music
77
12
Characters
105
13
Graphics 136
14
Advanced Graphics
153
15
Player-Missile Graphics
172
16
Permanent Storage 188
17
Advanced Techniques 196
APPENDICES
1
BASIC Commands 204

2 BASIC Error Codes
3
Speeding Up Programs
4N 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 wri ting
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' AII-
purpose 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

2The 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 Monitor Power aNI
socket socket socket OFF
The Atari XL Connectors - Rear View
00
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

4The 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 alow 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.
Y ou 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 uni t. 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 uni t.
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 swi tch on the TV.

6The Atari XL Handbook
Second, swi tch 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
swi tch 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
prin ter, 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
1 Check that the aerial lead is connected.
7
2
Make sure the computer is connected to the
mains and switched on. The red power
indicator should be on.
3
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 ARI
XL' appear on the screen, and the word 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

PRINT "HELLO!"
First Steps 9
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.