Hints for Computer System Design

Butler W. Lampson
Computer Science Laboratory
Xerox Palo Alto Research Center
Palo Alto, CA 94304
Abstract
Studying the design and implementation of a number of computer has led to some general hints
for system design. They are described here and illustrated by many examples, ranging from
hardware such as the Alto and the Dorado to application programs such as Bravo and Star.
1. Introduction
Designing a computer system is very different from designing an algorithm:
The external interface (that is, the requirement) is less precisely defined, more complex, and
more subject to change.
The system has much more internal structure, and hence many internal interfaces.
The measure of success is much less clear.
The designer usually finds himself floundering in a sea of possibilities, unclear about how one
choice will limit his freedom to make other choices, or affect the size and performance of the
entire system. There probably isn’t a ‘best’ way to build the system, or even any major part of it;
much more important is to avoid choosing a terrible way, and to have clear division of
responsibilities among the parts.
I have designed and built a number of computer systems, some that worked and some that didn’t.
I have also used and studied many other systems, both successful and unsuccessful. From this
experience come some general hints for designing successful systems. I claim no originality for
them; most are part of the folk wisdom of experienced designers. Nonetheless, even the expert
often forgets, and after the second system [6] comes the fourth one.
Disclaimer: These are not
novel (with a few exceptions),
foolproof recipes,
laws of system design or operation,
precisely formulated,
consistent,
always appropriate,
approved by all the leading experts, or
guaranteed to work.
1 This paper was originally presented at the. 9th ACM Symposium on Operating Systems Principles and appeared in
Operating Systems Review 15, 5, Oct. 1983, p 33-48. The present version is slightly revised..