The volume table of contents is track 17 sector 0 (DOS logical sector 0). Numbers of two bytes are low byte first. The values in the right column are those of the real DOS 3.3 System Master disk read for this site.
Fields
| Offset | Size | Meaning | System Master |
|---|---|---|---|
$00 | 1 | Not used (the byte DOS reads as the first of the sector) | $04 |
$01 | 1 | Track of the first catalogue sector | 17 |
$02 | 1 | Sector of the first catalogue sector | 15 |
$03 | 1 | DOS release: 3 for DOS 3.3 (2 for DOS 3.2) | 3 |
$04–$05 | 2 | Not used | 0 |
$06 | 1 | Disk volume number, 1 to 254 | 1 |
$07–$26 | 32 | Not used | 0 |
$27 | 1 | Track/sector pairs a track/sector list sector holds | 122 ($7A) |
$28–$2F | 8 | Not used | 0 |
$30 | 1 | The last track where DOS allocated sectors | 11 on the System Master |
$31 | 1 | Direction of allocation: $01 towards track 34, $FF towards track 0 | $FF |
$32–$33 | 2 | Not used | 0 |
$34 | 1 | Tracks on the disk | 35 |
$35 | 1 | Sectors a track | 16 (13 for DOS 3.2) |
$36–$37 | 2 | Bytes a sector | 256 |
$38… | 4 × tracks | The free-sector bitmap, four bytes a track | 35 tracks, to $C3 |
The explorer treats a sector as a VTOC when the track count is 1 to 50, the sectors a track agree with the image (16, or 13 for a .D13), the sector size is 256 and the catalogue starts inside the disk. It then
adds to its score for a sane release, 122 pairs a list, 35 tracks, and for a catalogue that reads without problems and whose files add up to the used sectors.
Free-sector bitmap
Four bytes for each track, from $38. The first two say which sectors are free: byte 0 holds sectors 15 down to 8 (bit 7 is sector 15), byte 1 sectors 7 down to 0 (bit 0 is sector 0). A set bit
is a free sector. The last two bytes of each group are not used. A disk that is full has zeros; tracks 0–2 and 17 of a bootable disk are all zero.
On the System Master, track 0 reads 00 00 00 00 (nothing free) and 283 sectors are free in all, which agrees with a count of the set bits made by hand. The sectors the files claim (213: every file's sectors including its track/sector lists) plus tracks 0–2 (48) and track 17 (16) account for all 277 sectors in use.