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Chapter 33

Alto: A Personal Computer1

C. P. Thacker / E. M. McCreight / B. W Lampson / R. F. Sproull / D. R. Boggs

Summary The Alto is a small computer system designed in early 1973 as an experiment in personal computing. Its principal characteristics, some of the design choices that led to the implementation, and some of the applications for which the Alto has been used are discussed.

1. Introduction

During early 1973, the Xerox Palo Alto Research Center designed the Alto computer system ("Alto") as an experiment in personal computing, to study how a small, low-cost machine could be used to replace facilities then provided only by much larger, shared systems. During the succeeding six years, the original Alto underwent several engineering enhancements to increase its memory capacity and reduce its cost, but the basic capabilities of the system have remained essentially unchanged. There are now (early 1979) several hundred Altos in regular use by computer science researchers, engineers, and secretaries.

The primary goal in the design of the Alto was to provide sufficient computing power, local storage, and input-output capability to satisfy the computational needs of a single user. The standard system includes:

All of these components with the exception of the user terminal are packaged in a small cabinet which is an unobtrusive addition to a normal office. The terminal, keyboard, and pointing device are packaged for desktop use (Fig. 1).

The Alto has led to an entirely new computing environment. Many applications devote the entire machine to interacting with a user and satisfying his needs; examples are document production and illustration, interactive programming, animation, simulation, and playing music. These individual applications are supplemented by a large number of services available via communications; examples are printing service, mailbox services for delivering electronic mail, and bulk file storage services. The Ethernet has also given rise to applications that use several Altos concurrently to furnish additional computing power or to allow several people at their machines to interact with one another.

The principal characteristics of the Alto processor are described in Sec. 2 of this chapter. Sections 3 to 6 describe input-output controllers for the display, disk, Ethernet, and printer. Section 7 surveys the environment and applications that grew up for the Alto. Section 8 offers a brief retrospective look at the design.

2. The Alto Processor

The major applications envisioned for the Alto were interaction text editing for document and program preparation, support for the program development process, experimenting with real-time animation and music generation, and operation of a number of experimental office information systems. The hardware design was strongly affected by this view of the applications. The design is biased toward interaction with the user, and away from

1© Copyright 1979 by Xerox Corporation.


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