An Introduction to Amoeba

Andrew S. Tanenbaum

Dept. of Mathematics and Computer Science
Vrije Universiteit
De Boelelaan 1081
1081 HV Amsterdam, The Netherlands
Internet: ast@cs.vu.nl

Sape J. Mullender

Centrum voor Wiskunde en Informatica
Kruislaan 413
1098 SJ Amsterdam, The Netherlands
Internet: sape@cwi.nl

1. BACKGROUND

As the 1990s take hold, it is increasingly clear that computer operating systems designed for single processor systems back in the 1970s and 1980s will no longer be appropriate for the needs of the new decade. UNIX? is now almost 20 years old. Although it has gotten much bigger over the years, the basic ideas have not really changed since it was created in the early 1970s. MS-DOS, although not quite as old, is in many ways even less appropriate than UNIX for the powerful computer systems of the 1990s. Perhaps it is time to start over fresh with something new. In this collection of papers we describe the Amoeba distributed operating system, which has been designed and implemented with the technology of the 1990s in mind.
What are the key characteristics of computing now and in the future? We are convinced that two factors will dominate the next decade:

d The need for physically distributed hardware
d The need for logically centralized software

Let us now discuss these in turn.

First, computers are becoming cheaper at an enormous rate. In the 1970s, it was normal for many people to share a single mainframe or minicomputer by running a timesharing system on it. Each user had a terminal with which to access the computer. The ratio of computers to people was very low, often 20 or 50 or even 100 people per machine.

In the 1980s, the personal computer and personal workstation became popular. By the end of the decade, many universities and companies operated using a model in which each person had his or her own machine, all connected by a local area network. The ratio of computers to people became approximately 1 to 1, as many machines as people.

In the 1990s, hardware prices will continue will continue to drop dramatically. We will soon come to a situation in which it is economically feasible to have 20 or 50 or even 100 computers per person. Clearly the current model of giving each person a personal computer or workstation breaks down under these conditions. Nevertheless, the availability of large numbers of powerful single-chip processors is a given. Any system for the

? UNIX is a Registered Trademark of AT&T Bell Laboratories.