Zoë Blade's notebook


TOS is the Atari ST's operating system, chiefly based on GEMDOS.

GEMDOS was originally written for the 8086 by Digital Research. The project was apparently started by an engineer "who had a reputation for being a loose cannon".[1] Before it was even finished,[2] Atari's ST engineers — including those who had come from Commodore — quickly ported it across to the 68000-based ST, just in time for the platform's launch.

The desktop environment GEM, also by Digital Research, runs on top of TOS. GEM provides a friendly graphical interface for users, and another layer of abstraction for programmers in order to easily and consistently use that interface.


The killer was that Digital Research would give me this 8086 code and say "Translate this," and then a week later they'd say "Oh, there were a lot of bugs in that, here's the new one." I never really had a spec. The latest code was the spec.

— Dave Staugas, 1988[2]

We didn't use source control. I'm not sure we even used diff. Mostly we had several directories-full of source files that were compiled by a guy who knew how to do it.

— Landon Dyer, 2008[1]

Technical notes

TOS's API consists of two layers: the lower-level BIOS (Basic Input/Output System, routines that talk directly to common hardware) and XBIOS (eXtended Basic Input/Output System, routines that talk directly to ST-specific hardware), and the higher-level GEMDOS.[3][4] TOS also includes the Line-A routines, masquerading as nonexistent 68000 opcodes, and executed by the exception handler. These are used to quickly draw ST-specific graphics, and are in turn used by GEM's VDI routines.[5]

At the API function calling level, GEMDOS is pretty similar to DOS, which is itself based on CP/M. The ST can read from and write to DOS-formatted 3.5" floppy disks, and in some versions, format them too, as its native filesystem is a minor variation of DOS's.


  1. "The Atari ST, Part 2" Landon Dyer, Apr 2008
  2. "Three Years With the ST" Jeffrey Daniels, STart Magazine, 1988, pp. 22—28
  3. Atari ST Internals K. Gerits, L. Englisch, R. Bruckmann, 1985, ISBN 0-916439-46-1, p. 103
  4. Compute!'s Technical Reference Guide — Atari ST, Volume Three: TOS Sheldon Leemon, 1988, ISBN 0-87455-149-8, p. 4
  5. "The Documentation for TOS: 6.1 The Line-A emulator" Paderborn, Rolf Kotzian, Michael Bernstein

Deep dives

Operating systems: DOS | IRIX | TOS