Table Of ContentStarting FORTH
Starting FORTH
An Introductionto the FORTH
Languagaen dO peratingS ystem
for Beginnersa nd Professionals
LeoB rodie,
FORTH. Inc.
+
With a foreword by
Charles H. Moore
Prentice-Hall, Inc., Englewood Cliffs, NJ 07632
library of CongressC atalogingi n PublicationD ata
BRODIE, LEO.
Starting FORTH.
I. FORTH (Computer program language) I. Title.
QA76.73.F24B76 001.64'24 81-11837
ISBN 0-IJ-842930-8 AACR2
ISBN 0-13-842922-7 (pbk.)
Publisher's credits:
Editorial/production supervision: Kathryn Gollin Marshak
Manufacturing buyer: Gordon Osbourne
Paper cover design: FORTH, Inc. and Alon Jaediker
Case cover design: Edsal Enterprises
The pages of this book were reproduced
from camera ready copy
designed and prepared by FORTH, Inc.
© 1981 by FORTH, Inc., Hermosa Beach, CA 90254
All rights reserved. No part of this book
may be reproduced in any form or by any means
without permission in writing from the publisher.
Printed in the United States of America
19 18 17 16 15 14 13 12
Prentice-Hall International, Inc., London
Prentice-Hall of Australia Pty. Limited, Sydney
Prentice-Hall of Canada, Ltd., Toronto
Prentice-Hall of India Private Limited, New Delhi
Prentice-Hall of Japan, Inc., Tokyo
Prentice-Hall of Southeast Asia Pte. Ltd., Singapore
Whitehall Books Limited, Wellington, New Zealand
ABOUT THE AUTHOR
Leo Brodie's inability to express even the most complex
technical concepts without adding a twist of humor comes from an
early love of comedy. He specialized in playwriting at UCLA and
has had several comedies produced there and in local theater.
He has also written freelance magazine articles and has worked as
a copywriter for an ad agency. When a company he was working
for installed a computer, he became inspired to try designing a
microprocessor-based toy. Although he never got the toy running,
he learned a lot about computers and programming. He now works
at FORTH, Inc. as a technical and marketing writer, where he can
play on the computers as the muse determines without having to be
a fanatical computer jockey, and is allowed to write books such
as this.
Leo's other interests include singing, driving classic Volvos, and
dancing to 50's music.
V
FOREWORD
The FORTH community can celebrate a significant event with the
publication of Starting FORTH. A greater effort, talent, and
commitment have gone into this book than into any previous
introductory ~anual. I, particularly, am pleased at this evidence
of the growing popularity of FORTH, the language.
I developed FORTH over a period of some years as an interface
between me and the computers I programmed. The traditional
languages were not providing the power, ease, or flexibility that
I wanted. I disregarded much conventional wisdom in order to
include exactly the capabilities needed by a productive
programmer . The most important of these is the ability to add
whatever capabilities later become necessary.
The first time I combined the ideas I had been developing into a
single entity, I was working on an IBM 1130, a "third-generation"
computer. The result seemed so powerful that I considered it a
"fourth -generation computer language." I would have called it
FOURTH, except that the 1130 permitted only five-character
identifiers . So FOURTH became FORTH, a nicer play on words
anyway.
One principle that guided the evolution of FORTH, and continues
to guide its application, is bluntly: Keep It Simple. A simple
solution has elegance. It is the result of exacting effort to
understand the real problem and is recognized by its compelling
sense of rightness." I stress this point because it contradicts
the conventional view that power increases with complexity.
Simplicity provides confidence, reliability, compactness, and
speed. ·
Starting FORTH was written and illustrated by Leo Brodie, a
remarkably capable person whose insight and imagination will
become apparent. This book is an orig inal and detailed
prescription for learning. It deftly guides the novice over the
thresholds of understanding that all FORTH programmers must
cross.
Although I am the only person who has never had to learn FORTH,
I oo know that its study is a formidabl e one. As with a human
language, the usage of many words must be memorized. For
beginners, Leo's droll comments and supe rbly cast characters
appear to make this study easy and enjoyable. For those like
myself who al ready know FORTH, a quick reading provides a
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delightful trip and fresh views of familiar terrain. But I hope
this book is not so easy and enjoyable that it seems trivial. Be
warned that there is heavy content here and that you can learn
much about computers and compilers as well as about programming.
FORTH provides a natural means of communication between man and
the smart machines he is surrounding himself with. This requires
that it share characteristics of human languages, including
compactness, versatility, and extensibility. :t: cannot imagine a
better language for writing programs, expressing algorithms, or
understanding computers. As you read this book, I hope that you
may come to agree.
Charles H. Moore
Inventor of FORTH
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ABOUT THIS BOOK
Welcome to Starting FORTH, your introduction to an exciting and
powerful computer language called FORTH.
If you're a beginner who wants to learn more about computers,
FORTH is a great way to learn. FORTH is more fun to write
programs with than any language that I know of. (See the
"Introduction for Beginners.")
If you are a seasoned professional who wants to learn FORTH, this
book is just what you need. FORTH is a very different approach
to computers, so different that everyone, from newcomers to old
hands, learns FORTH best from the ground up. If you're adept at
other computer languages, put them out of your mind, and remember
only what you know about computers. (See the "Introduction for
Professionals, '1
)
Since many people with different backgrounds are interested in
FORTH, I've arranged this book so that you'll only have to read
what you need to know, with footnotes addressed to different
kinds of readers. The first half of Chap. 7 provides a background
in computer arithmetic for beginners only.
This book explains how to write simple applications in FORTH. It
includes all standard FORTH words that you need to write a
high-level, single-task application. This word set is an
extremely powerful one, including everything from simple math
operators to compiler-controlling words. (See Appendix 3,
"FORTH-79 Standard.")
Excluded from this book are all commands that are related to the
assembler, multiprogrammer, printing and disking utilities, and
target compiler. These commands are available on some versions
of FORTH such as polyFORTH. (See Appendix 2, "Further Features
of polyFORTH. ")
I've chosen examples that will actually work at a FORTH system
with a terminal and disk. Don't infer from this that FORTH is
limited to batch or string -handling tasks, since there is really
no limit to FORTH's usefulne ss.
Here are some features of this book that will make it easy to use:
All commands are listed twice: first, in the section in which the
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