Atari 130XE Machine Language for the Absolute Beginner

164 Pages·2010·29.07 MB·English

ATARI 130XE MACHINE
LANGUAGE FOR THE
ABSOLUTE BEGINNER
Kevin Bergin
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MELBOURNE HOUSE
PUBLISHERS
© 1985 Beam Software
All rights reserved. This book is copyright and no part may
be copied or stored by electromagnetic, electronic,
photographic, mechanical or any other means whatsoever
except as provided by national law. All enquiries should be
addressed to the publishers:
IN THE UNITED KINGDOM -
Melbourne House (Publishers) Ltd
Castle Yard House
Castle Yard
Richmond, TW10 6TF
IN AUSTRALIA -
Melbourne House (Australia) Pty Ltd
2nd Floor, 70 Park Street
South Melbourne, Victoria 3205
ISBN 0 86161 200 0
Edited by Richard Woolcock
Printed and bound in Great Britain by Short Run Press Ltd, Exeter
Ed,tlon: 7 6 5 4 3 2 I
Prinling: FED C B A 9 8 7 6 5 4 3 2 I
Year: 9089 88 87 86 85
Contents
Chapter 1 - Introduction to Machine Language 1
Using a machine language program. 2
Memory addressing. 2
Using memory directly from BASIC 3
Machine language as a subroutine 3
Summary 5
Chapter 2 - Basics of Machine Language Programming. 7
Using memory from machine language 7
The registers . 7
The accumulator 7
Addressing modes 8
Simple program input 10
Assembly language 11
Screen memory 12
Printing a message 14
Summary 16
Chapter 3 - An Introduction to Hexadecimal 19
Uses of Hexadecimal. 19
Binary . 20
Why hexadecimal? 22
Hex and Binary maihematically 23
Absolute addressing 24
Converting hexadecimal to decimal. 25
Summary 26
Chapter 4 - Introduction to ALPA + Disassembler. 29
To get ALPA running . 31
Using ALPA 31
ALPA commands 32
Memory Usage in ALPA 33
Summary 35
Chapter 5 - Microprocessor Equipment 37
Storing numbers 37
The carry flag 37
Adding numbers 39
Two byte addition 41
Subtracting numbers . 45
An exercise . 45
Summary 46
Chapter 6 - Program Control . 47
Player Missile Graphics . 47
Moving Player-Missile Graphics. 48
Looping using JMP 48
ALP A label name addressing 49
Infinite loops 50
Comparing numbers 51
Branch instructions 52
Zero Flag. 52
Relative addressing 55
Summary 56
Chapter 7 - Looping and Pointing . 59
Counting to control a loop .. ..... .. . 59
Counting using the accumulator 59
Counting using memory 60
The X and Y registers 61
Using the X register as a counter . 62
Moving blocks of memory . 63
Implied addressing 64
Relative addressing . 64
Absolute addressing 64
Indexed addressing . 64
Using the X register as an index. 64
Non-symmetry of commands 65
Searching through memory . 65
Using more than one Index 68
Zero page indexed addressing 68
Summary 69
Chapter 8 - Using Information Stored in Tables .... .. . . . 71
Displaying characters as graphics . 71
Graphics memory 72
Copying the character sets from ROM . 73
Indirect indexed addressing. . . . . . . . . . 74
Register transfer instructions . 76
Indirect addressing 77
Summary 78
Chapter 9 - Processor Status Codes . . . . . . . . . 81
BCD representation . 82
Summary 85
Chapter 10 - Logical Operators and Bit Manipulators 87
Changing bits within memory 87
The logical AND 88
The logical OR . 89
The logical Exclusive OR . 90
The bit instruction 91
Rotating Bits within a byte 91
Rotation with carry 93
Rotating to the right 94
Clever multiplication 95
Summary 98
Chapter 11 - Details of Program Control . 99
The Program Counter 99
Storing into the Program counter 100
The Program counter and subroutines 100
The stack control structure 101
Subroutines and the Stack . 102
The Stack and interrupts 106
Summary 107
Chapter 12 - Dealing with the Operating System 104
The Kernal 104
CLI 110
RTI 110
Summary 112
Appendices
Appendix 1 - 6502 Instruction Codes . 113
Appendix 2 - Hexadecimal to Decimal Conversion Table 121
Appendix 3-Relative Branch and Two's Complement Numbering Tables 123
Appendix 4 - Atari 130XE Memory Map 124
Appendix 5 - The Screen Chip 125
Appendix 6 - The Sound Chip 130
Appendix 7 - Memory Usage Directory 133
Appendix 8 - Table of Screen Codes 136
Appendix 9 - Current Key Pressed . 138
Appendix 10 - ALPA and Disassembler 139
Index 153
Forevvord
So, you feel you've had enough of BASIC and want to learn more
about your machine.
Maybe you use your computer to run some professionally written
software, like word processing, accounting systems, educational
software or games.
You may have wondered what it is that makes these programs so
different from the ones you have written in BASIC. These
professional programs seem to be able to do many tasks at the
same time, including functions which you may have not realised
that your computer can do.
Apart from the size of the programs and the amount of time
spent in writing them, the one major difference between your
programs and most of the programs that you will buy in a store,
is that most professional programs are written wholly or partly
in machine language.
Machine language is a must for the really serious programmer.
Most games, useful utilities and interfaces are written in
machine language.
This book attempts to give you an introduction to the world of
machine language, the other side of your 130xE.
You will be led through the microprocessor's instruction set
slowly at first, practising each instuction learned using the
monitor/program entry program ALPA (Assembly Language
Programming Aid).
As we work through the instruction set you will meet the new
concepts and features of your computer, some of which you may
not have known it possessed.
You are encouraged throughout the book to check that the
computer's output is what you would logically expect it to be.
Keep a pen and paper close at hand LO copy on paper what the
microprocessor is doing, to get its answers, and to see if your
answers agree.
Chapter 1
Introduction to Machine Language
One advantage of machine language (M.L.) is that it allows the
programmer to perform several functions not suited to BASIC.
The most remarkable advantage of machine language, however, is
its speed. On the 130XE you can carry out approximately one
hundred thousand M.L instructions per second. BASIC commands
are several hundred times slower.
This is due to the fact that BASIC is written in machine
language and one single BASIC command may be a machine language
program of hundreds of instructions. This is reflected in the
capabilities of each of the languages.
Machine language instructions, as you will see as you work your
way through this book, are extremely limited in what they can
do. They perform only minute tasks and it takes many of them
to achieve any 'useful' function. They perform tasks related
to the actual machinery of the computer. They tell the
computer to remember some numbers and forget others, to see if
a key on the keyboard is pressed, to read and write data to the
cassette tape and to print a character on the screen.
Machine language programs can be thought of as subroutines
like a subroutine in BASIC - a program within another program
that can be used anywhere in the program and returns to where
it was called from when finished. You use the commands COSUB
and RETURN to execute and then return from a subroutine.
10 COSUB 8000
8000 RETURN
1
This wouldn't be a very useful subroutine because it doesn't do
anything but it does show how a subroutine works!
Using a machine language program
To call a machine language subroutine from a BASIC program you
use the command 'A=USR (address)' where A is a dummy variable.
Just as with the COSUB command you must tell the computer where
your routine starts. 'COSUB 8000' calls the subroutine at line
number 8000. Similarly A=USR (8000) calls the machine language
subroutine at memory address 8000.
NOTE here that memory address 8000 is very different to line
number 8000. A memory address is not a program line number, it
is the 'address ' of an actual piece of memory in the computer.
Memory addressing
Each piece of memory in the computer can be visualised as a box
which can contains one character, one piece of information.
With over 65,000 separate boxes, the computer must have a
filing system to keep track of them, so that it can find each
separate piece of information when it needs it. The filing
system it uses gives each box an 'address', which is like the
address of your house. You use addresses to find the dne
particular house you are looking for anywhere within a busy
city. You use this address to visit a house, send it mail or
to pick up a parcel from it. The computer, like us, sends
information and moves from one place (subroutine) to another
using its system of addresses.
The computer's system of addressing is simpler than ours in
its terms, anyway - as it starts at one end of memory and calls
this address zero. It then counts through the memory 'boxes ~,
giving each of them a number as it goes - from zero at one end
to 65535 right at the other end of memory. For us this would
be very difficult to remember, but for the computer it is the
logical way to do things. These numbered boxes can be thought
of as post office boxes. If you put something in the box at
address number one, it will stay there until you replace it
with something else.
Each box can hold only one thing at a time. When you put
something in a box, what was originally there wi 11 be lost
forever.
The command 'A=USR (8000) ' tells the BASIC to execute a machine
language subroutine whose first instruction is stored in the
box at address 8000.
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