Hardware manual

Alto Operating System Reference Manual
OS version 19/16
1. Introduction
This manual describes the operating system for the Alto. The manual will be revised as the system
changes. Parts of the system which are likely to be changed are so indicated; users should try to isolate
their use of these facilities in routines which can easily be modified, or better yet, avoid them entirely, if
possible.
The system and its description can be separated into two parts:
a) User-callable procedures, which are of two kinds:
standard procedures which are always
provided, and
library procedures which must be loaded with the user’s program if they are
desired. This manual describes only standard procedures; the library procedures are documented
in the "Alto Packages Manual."
b) Data structures, such as disk files and directories, which are used by the system but which are also
accessible to user procedures and subsystems.
The system is written almost entirely in Bcpl. Its procedures are invoked with the standard Bcpl calling
sequence, and it expects the subsystems it calls to be in the format produced by the Alto Bcpl loader.
2. Hardware summary
This section provides an overview of the Alto Hardware. Briefly, every Alto has:
a) A memory of 64k words of 16 bits each. The cycle time is 850ns.
b) An emulator for a standard instruction set.
c) Secondary memory, which may consist of one or two Diablo 31 cartridge disk drives, or one
Diablo 44 cartridge disk drive. The properties of these disks are summarized in Table 2.2.
d) An 875 line TV monitor on which a raster of square dots can be displayed, 606 dots wide and 808
dots high. The display is refreshed from Alto memory under control of a list of display control
blocks. Each block describes what to display on a horizontal band of the screen by specifying:
the height of the band, which must be even;
the width, which must be a multiple of 32; the space remaining on the right is filled with
background;
The indentation, which must be a multiple of 16; the space thus reserved on the left is filled
with background;
the color of the background, black or white;
the address of the data (must be even), in which 0 bits specify background. Each bit controls
the color of one dot. The ordering is increasing word addresses and then bit numbers in
memory, top to bottom and then left to right on the screen; and a half-resolution flag
which makes each dot twice as wide and twice as high.
There is also a 16 x 16 cursor which can be positioned anywhere on the screen. If the entire
screen is filled at full resolution, the display takes about 60% of the machine cycles and 30704D
words of memory.
Alto Operating System May 5, 1980 2
For Xerox Internal Use Only -- December 15, 1980