Atari File Management Subsystem

The Atari File Management Subsystem (FMS) is the filesystem on an Atari 810 floppy disk.

The 720 sectors of the disk were numbered from 1 to 720, but (perhaps due to poor communication between the different development teams at Atari) the FMS filesystem was designed to support sector addresses in the range from 0 to 719, which meant that sector 720 was not addressable but a nonexistent sector 0 was. This resulted in the filesystem only using the 719 sectors in the overlap between what is addressable and what actually exists, so sector 720 is unused (a waste of a perfectly good 128 bytes).

The bytes within a sector are numbered from 0 to 127, because at least the development teams managed to agree on zero-based numbering there.

Links

 * Atari technical manual with lots of tech details