Atari File Management Subsystem

Revision as of 05:50, 18 March 2013

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).

(Note to self: Is there a way to hack the system to stick hidden messages in that 720th sector?)

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.

Sector map

This filesystem is pretty narrowly tailored to the particular type of disk used by the Atari 810 drive, with various hard-coded sector positions.

• Sector 1: Boot record
• Sector 2-n: DOS.SYS file (on system disks)
• Sector n+1-359: User file space
• Sector 360: VTOC (Volume Table of Contents)
• Sector 361-368: Directory
• Sector 369-719: User file space
• Sector 720: Unused

Links

• Atari technical manual with lots of tech details