.ST File Format – File system

Where things are

With the parameter block’s numbers: the boot sector is sector 0; the first FAT starts at sector reserved; the second follows it; the root directory follows the last FAT and takes root entries × 32 / 512 sectors; the data area follows. On a 720 KB disk that is the boot sector, 5 sectors for each FAT, 7 sectors of root directory and the data from sector 18, in clusters of 2 sectors (1 KB) numbered from 2.

The allocation table

Each cluster has a 12-bit entry, packed two to three bytes: entry n starts at byte n × 3 / 2; an even entry takes the low 12 bits of the little-endian word there, an odd entry the high 12. Entries 0 and 1 hold the media descriptor and $FFF. An entry of 0 is a free cluster, $FF7 a bad cluster, $FF8 to $FFF the last cluster of a file, and any other value the number of the file’s next cluster. The second FAT is a copy; the explorer notes when it differs.

Directory entries

An entry is 32 bytes. A first byte of 0 ends the directory and $E5 marks a deleted entry.

A subdirectory is a file of entries in its own clusters, starting with the entries . and ... The Atari has no long file names; an entry of the PC long-name kind (attributes $0F) is skipped. The explorer reports a chain that loops or leaves the disk, a file whose size and chain disagree, and a date or time that cannot exist.

Not described

Known, not yet covered

• Reference: the FAT12 packing and the directory entry are the PC’s, taken as they are; no Atari document was read for them, and what the Atari does with the unused bytes at offset 12 is not described. TOS’s own rules (how it allocates clusters, the disk-change detection by serial number) are not described.
• Viewer: only the first FAT is used for chains; a disk with only the second FAT intact is not repaired. Files are not checked against any real Atari disk.

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