The Sinclair ZX81: Programming for Real Applications

170 Pages·1981·13.9 MB·English

by  Randle Hurley (auth.)

The SINCLAIR ZX81
Programming for Real Applications
Macmillan Computing Books
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TheAlien, Numbereater, and OtherProgramsforPersonalComputers - withnoteson
how they werewritten John Race
BeginningBASIC Peter Gosling
ContinuingBASIC Peter Gosling
Program Your Microcomputerin BASIC Peter Gosling
Digital Techniques Noel Morris
Microprocessor and Microcomputer Technology Noel Morris
Understanding Microprocessors B.S.Walker
Codesfor Computers andMicroprocessors P.Gosling and Q.Laarhoven
Z80AssemblyLanguageProgrammingfor Students Roger Hutty
The
SINCLAIR ZX81
Programming for Real Applications
Randle Hurley
M
© Randle Hurley 1981
Allrights reserved. No part ofthispublication may be reproduced or
transmitted, inanyformorbyanymeans, withoutpermission.
Published 1981 by
THE MACMILLAN PRESS LTD
Londonand Basingstoke
Companies and representatives
throughout theworld
ISBN978-0-333-32973-3 ISBN978-1-349-06315-4(eBook)
DOI 10.1007/978-1-349-06315-4
The paperback editionofthis bookissoldsubject tothe conditionthat it
shallnot,bywayoftrade orotherwise,belent,resold, hiredout, or
otherwisecirculated withoutthepublisher'sprior consent inanyform
ofbindingorcoverotherthan that inwhichitispublished andwithout
asimilarconditionincludingthiscondition beingimposedonthe
subsequent purchaser.
Contents
I Aims and Assumptions
2 Programming
3 Saving Time and Space
4 Word Juggler
5 Money
6 Personal Finances
7 Banking
8 Bulk Storage
9 Rank Order
10 Exam Result Analysis
II Hardware Modifications
I would like to express my gratitude to all the ZXBO and ZXBl
enthusiasts who have contributed towards this book. Some made
their contributions directly but the majority will probably be
oblivious of having helped. During countless conversations
centring round the machines, chance remarks have sparked off the
ideas which led to the finished programs in this work. Steve Adams
made many of the direct contributions and his idea on modifying
the 3K RAM pack is included in the chapter on hardware. However
you made your contribution, thanks.
Particular thanks must go to my wife, Linda. The original idea
was that she should read and criticise the material from a com
plete beginner's point of view but she learned so fast that she
became the major bug finder and editor as well. Thanks, love, I
couldn't have got half way through without you.
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Aims and Assumptions
When I sat down first to work my material into a book I had to
make some assumptions about the sort of reader that I was writing
for. The person that I had in mind when writing was not a computer
expert. My target reader had finished all the material in the
Sinclair manual and probably did not really understand those bits
about Procrustean Assignment and Unary Functions. He or she will
probably have read a book or two of games programs and perhaps
will have been interested in some of the ideas presented in the
pages of magazines devoted to the ZX81. My reader may have shared
my frustration that these ideas, while fascinating, are given in
isolation and seldom related to any real task the ZX81 may be
called on to do. The person I am writing for may be a teacher like
myself, hoping to use their machine for academic organisation or
as an educational tool. On the other hand, I may be writing for
someone interested in using the ZX81 to help run a small business
or a social club. All these possible needs have been borne in
mind during the writing.
The program listings shown in these chapters not only work,
they are workhorses, doing real jobs in a variety of environments.
Each of the ideas presented has been followed through to a real
conclusion and not used simply to illustrate a nice programming
point. Lack of finance for a larger machine meant that the ZX81
simply had to do my data processing work. Necessity forced me to
consider alternative ways of using the ZX81's BASIC and these
alternatives have proved to be very useful. None of the ideas
used in the programs shown is highly technical and so they will
all be quite easy for even a relatively inexperienced programmer
to understand. Expect to find a logical extension of the material
in the handbook and some interesting twists in the way that it
has been put to work.
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While we'are still on the subject of assumptions, I have
assumed that the reader has a fully expanded machine. Some of the
programs occupy the whole of the 16K RAM pack and all of them are
too big for the 1K ZX81. The ZX printer has been considered and
commands for it have been written into some of the programs. In
all but one case the programs will work satisfactorily without
this extra. The word processing package, however, really does rely
on the availability of a printer for most of its uses. Towards the
end you will find a chapter on some simple hardware modifications.
Anyone who has built the kit will find no difficulty in making the
modifications but, while they more than repay the small amount of
work involved, these are in no way necessary for the smooth run
ning of the programs.
Games programs teach a lot about how the computer works and
about the facilities available on the machine but even the most
avid games player will eventually tire of the material on offer
and will look for alternatives. And then there are those awkward
people who keep on asking the classic question: "that's interest
ing, but what is it forT' A glance through the contents of the
book will be enough to provide several answers to the askers of
this question and at the same time dispute the idea that the ZX81
is a toy computer, suitable only for keeping the children quiet
on wet afternoons and for drilling them in their arithmetic. The
central processor of the ZX81 is the same Z80A chip found in many,
much larger machines which run commercial organisations. If this
chip can run a wages and stock control system or a word processor
when built into other computers then there must be a lot of un
tapped potential in the ZX8l and my aim has been to exploit this.
I hope that those readers who have tired of the normal diet
offered to ZX81 users will share in the great satisfaction that I
have had from coaxing the little machine to take on tasks normally
thought to demand a much larger computer.
A quick flip through the pages which follow might give the
impression that the contents are much the same as to be found in
all the other books of programs; large chunks of not very intel
ligible code and hardly the thing for someone who has just closed
the pages of the handbook for the first time.
Well, look again. The chunks are all quite small, seldom more
than about ten lines. The reader has very little to key into the
machine at anyone time and the likelihood of a line being missed
from a long listing is reduced considerably. Now look closer
still. All the code is accompanied by little bits in English.
Every line which is the slightest bit difficult to understand is
explained there and then. Readers are not expected to have infal
lible memories, if the same idea crops up in a later line then
the explanation is given again. The information in English that
accompanies the code, the documentation, is not limited to ex
plaining the hard bits. All the GO TO instructions are accompanied
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by notes on the material to be found at the line that the computer
is to jump to. GO SUB lines carry reminders of what it is that
each subroutine does. On the occasions that PEEK and POKE com
mands are used, information on the identity of the value being
"peeked at" and the purpose of the number being "poked" is given
in full. Where the logical structure of the program is spread
over a large number of lines and is difficult to grasp as a whole
then the explanatory notes are supplemented by a flow chart.
The first chapter after this introduction is on programming.
This is not intended to be a course in BASIC. It is a supplement
to the handbook material and looks at ways of making the fullest
possible use of the ZX81 facilities. In places, this chapter
reads a little like a list of household tips. Even such small
matters as the type of exercise books to consider and the benefits
of shelf lining paper are covered. Later, the cost of all the com
mands and the functions is investigated in terms of both memory
use and running time. These investigations are built up into some
general observations on programming the ZX81 more effectively.
The main aim of the book is to develop in the reader an inter
est in pushing the ZX81 computer much further than anyone expected
it to go when it was first launched. The programs will show you
how to store more numbers than there are memory bytes in the 16K
machine and then access this idea in many different ways after
wards. You will find the necessary information on how to reduce
the likelihood of your programs crashing the moment they are
handed over to an inexperienced user. Ways of storing large
amounts of text which remain available for editing and ways of
storing numbers as text to improve storage efficiency will be made
available to you. More important than all this, however, after
reading the material you should have discarded the belief that the
ZX81 is too small for any real computing work. The programs given
here are just the beginning; the techniques are flexible enough
to be worked into your own programs to fill your own data process
ing needs.
The material can be approached at two levels. On the one hand,
there are large, powerful, "off the peg" programs which can be
put to work at once or modified to suit particular needs. Alter
natively, you can use the programs as working examples to illus
trate the programming ideas given in the early part of the book.
The ideas and the examples can then be used as a basis for the
development of entirely new and different programs for completely
different applications. I hope that most readers will approach
the material from both angles at different times. Teachers, by
the way, may find ideas of interest in the non-educational programs
and I hope that those of you without an involvement in education
will enjoy the programming ideas in the educational material.
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