TOS combines Digital Research's GEM GUI running on top of the DOS-like GEMDOS.

Features include a flat memory model, DOS-compatible disk format (starting with TOS 1.04), support for MIDI, and a variant of SCSI called ACSI in later versions.

The TOS desktop uses icons to represent files and devices, windows and dialog boxes to display info.

[3] The major differences are the fact that the boot sector does not need to contain the IBM compatible jump sequence at the beginning (typically 0xE9 0x??

0x90), the lack (before TOS 1.04) of an OEM identifier compatible with PC-based systems, and the fact that a checksum is used to mark the boot sector as executable (the PC format uses the signature word 0x55 0xAA instead).

EmuTOS is a replacement for TOS (the operating system of the Atari ST and its successors), released as a free software.