Introduction - Xerox Alto I Hardware Manual

1.0 INTRODUCTION
This document is a description of the Alto, a small personal computing system originally designed at
PARCo
By "personal computer" we mean a non-shared system containing sufficient processing power,
storage, and input-output capability to satisfy the computational needs of a single user.
A basic Alto system is:
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An 875-line television monitor, with a viewing area of about 8%" x 11", oriented with the long
tube dimension vertical. The controller provides a 606 by 808 point display which is refreshed
from main memory at 60 fields (30 frames) per second. It has programmable polarity, a low .
resolution mode which conserves memory space, and a 16 by 16 cursor whose position and
content are under program control.
An unencoded 64-key keyboard.
A mouse (pointing device) and five-finger key set.
Up to two Diablo Model 31 disk drives or a Model 44 disk drive.
An interface to the Ethernet, a 3 Mbps local network that can connect up to 256 Altos and other
computers separated by as much as a mile. Most Ethernets are interconnected by gateways and
leased lines to form a nationwide internet.
A microprogrammed processor which controls the disk, display and Ethernet, and emulates an
instruction set. The standard instruction set for which emulation microcode is supplied in the
microinstruction
ROM
is described in. section 3.0.
64K 16 bit words of 850ns error corrected semiconductor memory, expandable to 256K.
1K microinstruction
RAM
that can be read and written with special microcode to extend the
standard instruction set or to emulate a different instruction set or to drive special
I/O
devices.
The processor, disk, and their power supplies are packaged in a small cabinet. The other
I/O
devices may be a few feet away, and are pleasingly packaged for desk top use.
options:
An expanded microinstruction memory consisting of either 2K of
PROM
or 3K of
RAM.
A Diablo HyType printer.
A Versatec Printer/Plotter.
A controller for CalComp Trident disk drives.
A controller for
MDS
and Kennedy tape drives.
An Orbit, the controller for a vast array of laser-scanned printers.
Communications controllers for BBN-1822,
SDLC,
BiSync and Async.
The remaining sections of this document will discuss the hardware and microcode of the standard
configuration Alto. At present, two slightly different versions of the Alto exist: the Alto I and the Alto
II. Most passages of this document pertain to both machines; those that apply to one only are clearly
marked.
This document does not deal with the numerous non-standard peripheral devices that have been
interfaced to the Alto. Non-standard interfaces and their designers are tabulated in an appendix.