The Atari TOS debuted with the Atari 520ST in 1985. TOS combines Digital Research's GEM GUI running on top of the DOS-like GEMDOS. Features include a flat memory model, MS-DOS-compatible disk format, support for MIDI, and a variant of SCSI called ACSI in later versions. Atari's TOS is run from ROM chips contained in the computer: Thus, before local hard drives were available in home computers, it was an almost instant-running OS. TOS originally booted off floppy disks, but later ST models came with the latest version of TOS in ROM. TOS consisted of the following: