Advanced 6502 Programming

301 Pages·1982·3.528 MB·English

by  Rodnay Zaks

ADVANCED
6502
PROGRAMMING
RODNAYZAKS
BERKELEY • PARIS • DDSSELDORF
SYM is a trademark of Synertek Systems, Inc.
KIM is a trademark of MOS Technology, Inc.
AIM65 is a trademark of Rockwell International, Inc.
"COMPUTEACHER" and "GAMES BOARD" are trademarks of Sybex, Inc.
Cover design by Daniel Le Noury
Technical illustrations by Guy S. Orcutt and j. trujillo smith
Every effort has been made to supply complete and accurate information. However,
Sybex assumes no responsibility for its use, nor for any infringements of patents or other
rights of third parties which would result. No license is granted by the equipment
manufacturers under any patent or patent rights. Manufacturers reserve the right to
change circuitry at any time without notice.
© 1982 SYBEXInc., 2344Sixth Street, Berkeley, CA94710. World rights reserved. No
part of this publication may be stored in a retrieval system, transmitted, or reproduced in
any way, including but not limited to photocopy, photograph, magnetic or other record,
without the prior agreement and written permission of the publisher.
Based on 6502 Games by Rodnay Zaks, © 1980 SYBEX Inc.
Library of Congress Card Number: 82-160235
ISBN 0-89588-089-X
Printed in the United States of America
10987654321
Contents
Preface vii
1
Introduction 1
Optional Hardware Support 2
Connecting the System 4
Ga.mes Bol\r.SJPtei;couuect ,,,..,..., 7
fr,~;}(~1~~~~~
13
2
Generating Square Waves (Music Player) 20
Introduction 20
The Rules 20
A Typical Game 22
The Connections 22
The Algorithm 22
The Program 23
3
PseudoRandomNumberGenerator (Translate) 41
Introduction 41
The Rules 41
A Typical Game 42
The Algorithm 43
The Program 43
4
Hardware Random Number Generator
(Hexguess) 59
Introduction 59
The Rules 59
A Typical Game 59
The Algorithm 60
The Program 60
5
Simultaneous Input/Output (Magic Square) 73
Introduction 73
The Rules 73
A Typical Game 76
The Algorithm 78
The Program 80
6
Simple Real Time Simulation (Spinner) 87
Introduction 87
The Rules 87
The Algorithm 88
The Program 89
V
7 Real Time Simulation (Slot Machine) 99
Introduction 99
The Rules 99
A Typical Game 100
The Algorithm 101
The Program 112
8 Real Time Strategies (Echo) 137
Introduction 137
The Rules 137
A Typical Game 139
The Algorithm 141
The Program 144
9 Using Interrupts (Mindbender) 162
Introduction 162
The Rules 162
A Typical Game 162
The Algorithm 165
The Program 167
10 Complex Evaluation Technique (Blackjack) 189
Introduction 189
The Rules 189
A Typical Game 190
The Program 194
11 Artificial Intelligence (Tic-Tac-Toe)) 218
Introduction 218
The Rules 218
A Typical Game 218
The Algorithm 224
The Program 247
Appendices 287
A. 6502 Instructions-Alphabetic 287
B. 6502 Instruction Set-Hex and Timing 288
Index 290
vi
Preface
This book has been designed to teach you advanced programming
techniques for the 6502 microprocessor in a systematic and progressive
way. Developing a program involves devising a suitable algorithm and
appropriate data structures, and then coding the algorithm. In the case
of a microprocessor such as the 6502, the design of the algorithm and the
data structures is generally constrained by three conditions:
1. The amount of memory available is often limited or must be
minimized; i.e., the program must be terse.
2. The highest possible execution speed may be required. Efficient
coding of the program into assembly level language instructions
then becomes an essential consideration. In particular, the use of
registers must be optimized.
3. The specific input/o utput design requires an understanding of the
input and output chips and their programming.
Thus, when evaluating designs for an algorithm and data structures,
the programmer must weigh the merits of the various techniques in terms
of his skill, the memory limitations, the requirec-1. speed of execution,
and the overall probability of success.
Advanced programming for the 6502, therefore, involves knowledge
of all the chips that may be affected by the program, in addition to the usual
programming skills concerned with the algorithm, the data structures,
and the efficient use of internal instructions and registers. This book
provides a comprehensive and complete overview of all the important
techniques required to program a 6502 system efficiently. The book has
been designed as an educational text. Each chapter introduces new con
cepts, chips, or techniques in turn. In the final chapters more complex
algorithms are presented, which integrate the techniques presented
throughout the book.
For clarity and consistency, this book uses a specific 6502-based
system on which all the programs will run. The details are presented in
Chapter 1. However, the programs and techniques presented here are
applicable to all 6502-based systems. Similarly, all the programs studied
in this book are presented in the form of realistic games involving success
ively all the techniques described. They cover most types of applications
ranging from simple input/ output techniques to sophisticated real-time
simulations, including the handling of interrupts and the design of com
plex data structures.
vii
ADVANCED 6502 PROGRAMMING
A case study approach is used, and each chapter contains the following:
1. A description of the concepts and techniques to be studied
2. The specifications of the program's behavior and a typical session
with the program, i.e., the problem to be solved
3. The algorithm(s): theory of operation, design, and trade-offs
4. The actual program: data structures, programming techniques,
specific subroutines, merits of alternative techniques, and a com
plete program listing.
Variations and exercises are also proposed in each chapter.
Thus, you will first study the definition of the problem, then observe
the expected program behavior, and then learn how to devise a possible
solution (algorithm plus data structures). Finally, you will design a
complete program for this algorithm in 6502 assembly level language,
paying specific attention to the required data structures, the efficient use
of registers, the input/o utput chips, and the techniques used for efficient
programming.
You will sharpen your skills at using input/o utput techniques including
timers and interrupts. But most importantly, you will be consistently
reminded of the trade-offs between ease in programming, use of
memory, efficiency of execution, and algorithmic improvements by use
of specialized hardware or software techniques.
In order to learn the advanced programming techniques presented in
this book, it is not necessary to build any actual hardware. However, it is
necessary to write programs on your own along the ten chapters of
this book. By showing you and explaining in detail the design of many
actual programs, the author hopes to facilitate your next step: actual
programming.
viii
Acknowledgments
The author would like to acknowledge the contributions of Chris Williams
and Eric Novikoff, who thoroughly checked all of the games programs and con
tributed numerous ideas for improvements.
The author is particularly indebted to Eric Novikoff for his valuable
assistance throughout all phases of the manuscript's production, and for his
meticulous supervision of the final text.
The author would also like to express his appreciation to Rockwell International
and in particular, to Scotty Maxwell, who made available to him one of the very
first system 65 development systems. The availability of this powerful develop
ment tool, at the time the first version of this book was being written, was a major
help for the accurate and efficient check-out of all the programs.

1. Introduction
In order to learn the techniques and study the program examples
presented in this book, no specific equipment is required. However, the
availability of a 6502-based system is a major advantage to develop and
test 6502 programs on your own. Bear in mind that each 6502-based
system will have a somewhat different input/ output configuration. The
techniques presented in this book are applicable to all, and the programs
can be easily adapted once you understand input/ output operations.
To read this book, you should be familiar with the 6502 instruction set
and basic programming techniques on the level of Programming the 6502.
A basic knowledge of input/o utput techniques is also recommended.
(This topic is covered in 6502 Applications.)
The programs presented in Chapters 2 through 11 range from simple
to complex. In order to implement these programs, algorithms will be
devised and data structures will be designed. This is the process any
disciplined computer programmer must go through when designing a
1