Figure 1. The Alto personal computer, showing a
user at work with the display, mouse, and keyset.
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Alto: A personal
computer
by C. P. Thacker, E. M.
McCreight, B. W. Lampson, R.
F. Sproul!, and D. R. Boggs
CSL-79.11
August 7, 1979; Reprinted February 1984.
©
Copyright 1979 by Xerox Corporation. All rights reserved.
Abstract:
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.
This paper appeared in
Siewiorek, Bell and Newell, Computer Structures: Principles and Examples, second edition, McGraw-Hill, New York, pp.
549-572.
CR categories: 6.21, 8.2
Key words and phrases: personal computer, graphics, local network
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 (summer 1979) nearly a thousand 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:
·
An 875 line raster-scanned
display.
·
A keyboard,
"mouse" pointing device with three buttons, and a five-finger keyset.
·
A 2.5 Mbyte cartridge disk
file.
· An interface to the Ethernet system
("Ethernet"), a 3 Mbit/second communication facility.
§
A
microprogrammed processor that controls input-output devices and supports emulators for a number of instruction sets.
'
64K 16-bit words of
semiconductor memory. expandable to 256K words.
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 (Figure 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 Section 2.
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.
Figure 1. The Alto personal computer, showing a
user at work with the display, mouse, and keyset.
2. The Alto processor
The major
applications envisioned for the Alto were interactive 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 significant numerical processing:
there are extensive user input-output facilities,
but no hardware for arithmetic other than 16-bit integer addition and
subtraction.
The processor is microcoded, which permitted the machine to start out
with rather powerful facilities, and also allows easy expansion as new
capabilities are required. The amount of control store provided has evolved over time as shown in Figure 2.
Initially, the machine contained only 1K words, implemented with PROM. The
most recent version provides 4K words, of which 1K is implemented with PROM, and
3K is RAM.
|
Year |
Main
Memory |
Control
Memory |
Processor Memory |
|||
|
Size |
Technology |
Size |
Technology |
Size |
Technology |
|
|
1973 |
64K Parity |
1K x 1 Dynamic Metal gate PMOS |
1K PROM |
256 x 4 |
32 R registers |
16 x 4 |
|
1 974 |
1K PROM 1K
RAM |
PROM as above |
32 R registers 32 S registers |
16 x 4 |
||
|
1975 |
64K Error Correction |
4K
x 1 Dynamic Si gate NMOS |
||||