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by Tom DeMarco

Adapted from Strategies for Real-Time System Specification. Reprinted by permission. All rights reserved. See below for copyright notice.

"Most systems people use the term real-time rather loosely," the young manager said. We were seated over dinner with three members of her staff and some other managers who took part in the day's seminar. "They say they've got a real-time constraint when they're worried about impatient insurance brokers or bankers sitting in front of their terminals. A real-time system, in their minds, is just one that needs to be 'quick as a bunny.' If they fail to meet that constraint, their users might be inconvenienced or even annoyed. When we use the term, it means something rather different."

Her co-workers began to smile, knowing what was coming. "We build systems that reside in a small telemetry computer, equipped with all kinds of sensors to measure electromagnetic fields and changes in temperature, sound, and physical disturbance. We analyze these signals and transmit the results back to a remote computer over a wide-band channel. Our computer is at one end of a one-meter long bar and at the other end is a nuclear device. We drop them together down a big hole in the ground and when the device detonates, our computer collects data on the leading edge of the blast. The first two-and-a-quarter milliseconds after detonation are the most interesting. Of course, long before millisecond three, things have gone down hill badly for our little computer. We think of that as a real-time constraint."

I had been lecturing that day on the use of data flow modeling techniques for system specification. There had been the odd question about the use of such techniques for real-time systems, and my (typically glib) answer was that the techniques were applicable, though perhaps not totally sufficient. After all, my own earliest work with data flow modeling had been at Bell Labs and at La CEGOS Informatique, in both cases working on real-time systems. Or were those real-time systems? Maybe they were really just systems that needed to be 'quick as a bunny'? The young manager's graphic example had left me in some doubt.

It has always been clear that something more than the basic tools of structured analysis are needed to specify timing and synchronization requirements. In the simplest case, that "something" could be as trivial as a set of textual annotations, perhaps directly on the data flow diagrams. But for even slightly more complicated cases, there is the possibility of linked timing and ordering constraints that may involve a dozen or more system components. There has been a need for a systematic way to deal with such constraints.

Over the last few years, I began to hear favorable reports of some real-time modeling techniques called the Hatley/Pirbhai Extensions. This multi-perspective approach combined data flow decomposition with model components constructed in control- and information-space. The result appealed to me as a specification technique that was not only applicable, but for most cases sufficient for real-time systems. I contacted the developers and started to try out their extensions.

The act of writing a Foreword is a kind of endorsement. It says, if nothing else, "Read this book, it couldn't hurt." In this case, I can be considerably more positive than that. I can tell you that I learned valuable new techniques from Hatley and Pirbhai, and that I apply them regularly on real-world real-time applications.

October 1987
Camden, Maine

Tom DeMarco
The Atlantic Systems Guild

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COPYRIGHT NOTICE: This excerpt from Strategies for Real-Time System Specification [ISBN:0-932633-11-0] appears by permission of Dorset House Publishing. Copyright © 1988 by Derek J. Hatley. All rights reserved. See http://www.dorsethouse.com/books/strat.html. The material contained in this file may be shared for noncommercial purposes only, nonexclusively, provided that this Copyright Notice always appears with it. This material may not be combined with advertisements, online or in print, without explicit permission from Dorset House Publishing. For copies of the printed book or for permissions, contact Dorset House Publishing, 1-800-342-6657, 212-620-4053, http://www.dorsethouse.com, info@dorsethouse.com, New: 3143 Broadway, Suite 2B, New York, NY 10027 USA. Additional rights limitations apply, as presented in the Legal Disclaimer posted at http://www.dorsethouse.com/legal.html.



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