A computer’s operating system is an orga
nized collection of small built-in programs that enables the
computer to communicate with external devices, such as the
keyboard, display screen, and disk drive, and to perform
fundamental tasks like loading and running an application
program. While most people regard GEM as the ST's operat
ing system, it's just a friendly user interface on top of a more
conventional operating system to make the computer easier
to program and to operate. While GEM provides higher level
functions like support for drop-down menus, dialogs, and
icons, it still must rely on a set of low-level operating system
routines for tasks such as reading a file from the disk drive.
It is this set of low-level routines that shall be referred to as
TOS.
TOS Organization
The ST operating system is contained in a set of TOS ROM
(Read-Only Memory) chips that contain a total of 192 kilob
ytes of program code and data. The name TOS may have at
one time stood for The Operating System, but is now more
commonly thought of as an acronym for Tramiel Operating
System, named after the Tramiel family that now owns Atari.
The TOS ROMs contain all of the ST's system software. This
includes:
GEM This software provides ST applications
with a consistent user interface, fea
turing drop-down menus, multiple
windows, icons, and dialog boxes.
GEM is divided into the VDI (which
provides low-level graphics calls), AES
(which provides user services like
menus), and the GEM Desktop pro-
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