How Computers Work
Processor and Main Memory
Copyright 2001, 2002, 2009 by Roger Young.
All rights reserved.
This is a tutorial web book. All 152 pages of the large paperback book with 96 diagrams are on 38 web pages here.
Even if you know nothing about electronics, you have come to the right place.
If you are wondering how microprocessors work, you have come to the right place. A microprocessor is a small processor.
If you already know something about electronics, don't be put off by the fact that the book starts out very simple and uses relays instead of transistors. You will get through the first chapter quickly. See web page 24, below, for just how complex it gets.
If you have a very fast connection (DSL or cable modem), then you can click the following link to see an easier to read (PDF) version of the book. It is 783 kilobytes, so it will take a while.
.doc Microsoft Word version
With a regular (56K) modem, it will take about 3 or 4 minutes to be displayed.
The pdf does not include the addendum on the 'registers on bus' computer architecture.
If you print this book ('How Computers Work: Processor and Main Memory') out, it will be easier to follow when it gets to large, relatively-complex diagrams.
Please email questions or comments to me (Roger Young) at:
This book has been used as a supplementary text in 300 level computer science courses.
Last updated: January 2, 2018
About The Author
This page can be reached indirectly by going to the easy-to-remember URL howcomputers.com in case you want to tell anyone about it.
Computers are the most complex machines that have ever been created. Very few people really know how they work. This book will tell you how they work and no technical knowledge is required. It explains the operation of a simple, but fully functional, computer in complete detail. The simple computer described consists mainly of a processor and main memory. Relays, which are explained, are used in the circuitry instead of transistors for simplicity. This book does not cover peripherals like modems, mice, disk drives, or monitors.
Did you ever wonder what a bit, a pixel, a latch, a word (of memory), a data bus, an address bus, a memory, a register, a processor, a timing diagram, a clock (of a processor), an instruction, or machine code is? Though most explanations of how computers work are a lot of analogies or require a background in electrical engineering, this book will tell you precisely what each of them is and how each of them works without requiring any previous knowledge of computers or electronics. However, this book starts out very easy and gets harder as it goes along. You must read the book starting at the first page and not skip around because later topics depend on understanding earlier topics. How far you can get may depend on your background. A junior high school science background should be enough. There is no mathematics required other than simple addition and multiplication. This is a short book, but it must be studied carefully. This means that you will have to read some parts more than once to understand them. Get as far as you can. You will be much more knowledgeable about how computers work when you are done than when you started, even if you are not able to get through the whole text. This is a technical book though it is aimed at a non-technical audience. Though this book takes considerable effort to understand, it is very easy for what it explains. After you have studied this book, if you go back and read it, it will seem simple. Good Luck!
CONTENTSClick the page number to view the page.
Each page is a few book pages.
BASICS . 2 . 3 . 4 . 5 . 6
MEMORY . 7 . 8 . 9 . 10 . 11
INSTRUCTIONS . 12 . 13 . 14 . 15 . 16
PROCESSOR . 17 . 18 . 19 . 20 . 21 . 22 . 23 . 24
PROGRAMMING . 25 . 26 . 27 . 28 . 29 . 30 . 31 . 32 . 33 . 34 . 35 . 36 . 37
MISCELLANEOUS . 38
Starting on April 17, 2009 the second edition of this book is ALSO available in 8" X 10" paperback book form for only $4.47 from Amazon.com (my favorite book and software supplier) or Direct to book at Amazon.com . You can read reviews of the book there.
Related Suggested Reading
For a great, easy-to-read, wonderfully-illustrated, introductory book on how computers work that covers all parts of a computer but isn't nearly as in depth about the processor and doesn't have any circuit diagrams, read the book 'How Computers Work' by Ron White, which is available at most libraries.
For how many other things work, see:
For more free technical books on the web: Free Tech Books
For many more free net computer books: Free Computer Web Books
For over 18,000 free online books, see: The Online Books Page from the University of Pennsylvania.
For computer architecture information, see the WWW Computer Architecture Page from the University of Wisconsin-Madison.