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Code: The Hidden Language of Computer
Hardware and Software
Charles Petzold
Copyright © 2009
Microsoft Press books are available through booksellers and distributors worldwide. For further information about international editions,
contact your local Microsoft Corporation office or contact Microsoft Press International directly at fax (425) 936-7329. Visit our Web
site at mspress.microsoft.com. Send comments to mspinput@microsoft.com.
Macintosh is a registered trademark of Apple Computer, Inc. Microsoft, MS-DOS, and Windows are either registered trademarks or
trademarks of Microsoft Corporation in the United States and/or other countries. Other product and company names mentioned herein
may be the trademarks of their respective owners.
Images of Charles Babbage, George Boole, Louis Braille, Herman Hollerith, Samuel Morse, and John von Neumann appear courtesy of
Corbis Images and were modified for this book by Joel Panchot. The January 1975 cover of Popular Electronics is reprinted by
permission of Ziff-Davis and the Ziff family. All other illustrations in the book were produced by Joel Panchot.
Unless otherwise noted, the example companies, organizations, products, people, and events depicted herein are fictitious. No association
with any real company, organization, product, person, or event is intended or should be inferred.
Preface to the Paperback Edition
Code rattled around in my head for about a decade before I started writing it. As I was contemplating
Code and then writing it, and even after the book was published, people would ask me, "What's the
book about?"
I was always reluctant to answer this question. I'd mumble something about "a unique journey through
the evolution of the digital technologies that define the modern age" and hope that would be sufficient.
But finally I had to admit it: "Code is a book about how computers work."
As I feared, the reactions weren't favorable. "Oh, I have a book like that," some people would say, to
which my immediate response was, "No, no, no, you don't have a book like this one." I still think
that's true. Code is not like other how-computers-work books. It doesn't have big color illustrations of
disk drives with arrows showing how the data sweeps into the computer. Code has no drawings of
trains carrying a cargo of zeros and ones. Metaphors and similes are wonderful literary devices but
they do nothing but obscure the beauty of technology.
The other comment I heard was, "People don't want to know how computers work." And this I'm sure
is true. I personally happen to enjoy learning how things work. But I also like to choose which things I
learn about and which I do not. I'd be hard pressed to explain how my refrigerator works, for
example.
Yet I often hear people ask questions that reveal a need to know something about the inner workings
of personal computers. One such common question is, "What's the difference between storage and
memory?"
That's certainly a critical question. The marketing of personal computers is based on such concepts.
Even novice users are expected to know how many megas of the one thing and gigas of the other thing
will be necessary for their particular applications. Novice users are also expected to master the
concept of the computer "file" and to visualize how files are loaded from storage into memory and
saved from memory back to storage.
The storage-and-memory question is usually answered with an analogy: "Memory is like the surface
of your desk and storage is like the filing cabinet." That's not a bad answer as far as it goes. But I find
it quite unsatisfactory. It makes it sound as if computer architecture were patterned after an office.
The truth is that the distinction between memory and storage is an artificial one and exists solely
because we don't have a single storage medium that is both fast and vast as well as nonvolatile. What
we know today as "von Neumann architecture"—the dominant computer architecture for over 50
years—is a direct result of this technical deficiency.
Here's another question that someone once asked me: "Why can't you run Macintosh programs under
Windows?" My mouth opened to begin an answer when I realized that it involved many more
technical issues than I'm sure my questioner was prepared to deal with in one sitting.
I want Code to be a book that makes you understand these things, not in some abstract way, but with a
depth that just might even rival that of electrical engineers and programmers. I also hope that you
might recognize the computer to be one of the crowning achievements of twentieth century technology
and appreciate it as a beautiful thing in itself without metaphors and similes getting in the way.
Computers are constructed in a hierarchy, from transistors down at the bottom to the information
displayed on our computer screens at the top. Moving up each level in the hierarchy—which is how
Code is structured—is probably not as hard as most people might think. There is certainly a lot going
on inside the modern computer, but it is a lot of very common and simple operations.
Although computers today are more complex than the computers of 25 years or 50 years ago, they are
still fundamentally the same. That's what's so great about studying the history of technology: The
further back in time you go, the simpler the technologies become. Thus it's possible to reach a point
where it all makes relatively easy sense.
In Code, I went as far back as I could go. Astonishingly, I found that I could go back into the
nineteenth century and use early telegraph equipment to show how computers are built. In theory at
least, everything in the first 17 chapters of Code can be built entirely using simple electrical devices
that have been around for over a century.
This use of antique technology gives Code a fairly nostalgic feel, I think. Code is a book that could
never be titled The Faster New Faster Thing or Business @ the Speed of a Digital Nervous System.
The "bit" isn't defined until page 68; "byte" isn't defined until page 180. I don't mention transistors
until page 142, and that's only in passing.
So, while Code goes fairly deep into the workings of the computer (few other books show how
computer processors actually work, for example), the pace is fairly relaxed. Despite the depth, I tried
to make the trip as comfortable as possible.
But without little drawings of trains carrying a cargo of zeros and ones.
Charles Petzold
August 16, 2000
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