TOS is kind of like the backend of the Atari ST's operating system, where GEM is the Graphical portion, in the same way that you would have a TTY on *nix with Bash or whatever, and X11 as the graphical interface layer.
Guess that's the best way to explain it. Or AmigaDOS vs Workbench. Or DOS vs Windows Shell.
Anyhow, if you wouldn't mind parting with it for cheap, I'll take it off your hands! I have tons of ST disks just sitting in a room next to mine, and at the moment don't have any ST monitors.
slaapliedje