Track 18 sector 0 contains the Block Availability Map (BAM) and details about the disk including it's title.
Track 18 Sector 0
| Byte | Content | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | ||
| 00 | 15 | Location of first directory sector | 8 | Header | |||||
| 01 | 7 | 0 | |||||||
| 02 | DOS version type, normally 0x41 "A" | ||||||||
| 03 | |||||||||
| 04 | Free sector count Track 1 | Block Availability Map | |||||||
| 05 | S7 | S6 | S5 | S4 | S3 | S2 | S1 | S0 | |
| 06 | S15 | S14 | S13 | S12 | S11 | S10 | S9 | S8 | |
| 07 | S20 | S19 | S18 | S17 | S16 | ||||
| 08 | Free sector count Track 2 | ||||||||
| 09 | S7 | S6 | S5 | S4 | S3 | S2 | S1 | S0 | |
| 0A | S15 | S14 | S13 | S12 | S11 | S10 | S9 | S8 | |
| 0B | S20 | S19 | S18 | S17 | S16 | ||||
| 44 | Free sector count Track 17 | ||||||||
| 45 | S7 | S6 | S5 | S4 | S3 | S2 | S1 | S0 | |
| 46 | S15 | S14 | S13 | S12 | S11 | S10 | S9 | S8 | |
| 47 | S20 | S19 | S18 | S17 | S16 | ||||
| 48 | Directory Track 18 = 0 Free | ||||||||
| 49 | All allocated so should be set to 0 | ||||||||
| 4A | as entire track is reserved for the directory | ||||||||
| 4B | |||||||||
| 4C | Free sector count Track 19 | ||||||||
| 4D | S7 | S6 | S5 | S4 | S3 | S2 | S1 | S0 | |
| 4E | S15 | S14 | S13 | S12 | S11 | S10 | S9 | S8 | |
| 4F | S18 | S17 | S16 | ||||||
| 60 | Free sector count Track 24 | ||||||||
| 61 | S7 | S6 | S5 | S4 | S3 | S2 | S1 | S0 | |
| 62 | S15 | S14 | S13 | S12 | S11 | S10 | S9 | S8 | |
| 63 | S18 | S17 | S16 | ||||||
| 64 | Free sector count Track 25 | ||||||||
| 65 | S7 | S6 | S5 | S4 | S3 | S2 | S1 | S0 | |
| 66 | S15 | S14 | S13 | S12 | S11 | S10 | S9 | S8 | |
| 67 | S17 | S16 | |||||||
| 78 | Free sector count Track 30 | ||||||||
| 79 | S7 | S6 | S5 | S4 | S3 | S2 | S1 | S0 | |
| 7A | S15 | S14 | S13 | S12 | S11 | S10 | S9 | S8 | |
| 7B | S17 | S16 | |||||||
| 7C | Free sector count Track 31 | ||||||||
| 7D | S7 | S6 | S5 | S4 | S3 | S2 | S1 | S0 | |
| 7E | S15 | S14 | S13 | S12 | S11 | S10 | S9 | S8 | |
| 7F | S16 | ||||||||
| 8C | Free sector count Track 35 | ||||||||
| 8D | S7 | S6 | S5 | S4 | S3 | S2 | S1 | S0 | |
| 8E | S15 | S14 | S13 | S12 | S11 | S10 | S9 | S8 | |
| 8F | S16 | ||||||||
| 90 | Disk Name padded with 0xA0 | Disk title & type | |||||||
| 91 | |||||||||
| 92 | |||||||||
| 93 | |||||||||
| 94 | |||||||||
| 95 | |||||||||
| 96 | |||||||||
| 97 | |||||||||
| 98 | |||||||||
| 99 | |||||||||
| 9A | |||||||||
| 9B | |||||||||
| 9C | |||||||||
| 9D | |||||||||
| 9E | |||||||||
| 9F | |||||||||
| A0 | Unused set to 0xA0A0 | ||||||||
| A1 | |||||||||
| A2 | 7 | Disk ID | 0 | ||||||
| A3 | 15 | 8 | |||||||
| A4 | Unused set to 0xA0 | ||||||||
| A5 | DOS type, usually ASCII "2A" | ||||||||
| A6 | |||||||||
| A7 | Unused set to 0xA0 | ||||||||
| A8 | |||||||||
| A9 | |||||||||
| AA | |||||||||
| AB | Unused, set to 0x00 | Reserved | |||||||
| FF | |||||||||
Offset for each Track BAM entry
| Track | Offset |
|---|---|
| 01 | 04 |
| 02 | 08 |
| 03 | 0C |
| 04 | 10 |
| 05 | 14 |
| 06 | 18 |
| 07 | 1C |
| 08 | 20 |
| 09 | 24 |
| 10 | 28 |
| 11 | 2C |
| 12 | 30 |
| 13 | 34 |
| 14 | 38 |
| 15 | 3C |
| 16 | 40 |
| 17 | 44 |
| 18 | 48 |
| 19 | 4C |
| 20 | 50 |
| 21 | 54 |
| 22 | 58 |
| 23 | 5C |
| 24 | 60 |
| 25 | 64 |
| 26 | 68 |
| 27 | 6C |
| 28 | 70 |
| 29 | 74 |
| 30 | 78 |
| 31 | 7C |
| 32 | 80 |
| 33 | 84 |
| 34 | 88 |
| 35 | 8C |
How BAM entries work
Each track has a BAM entry, which consists of 4 bytes. The first byte is the number of free sectors on that track.
The next three bytes form a bit map of the available sectors in little endian format. A 1 represents a free sector, whilst a 0 represents an allocated sector.
As there are 24 bits available but tracks range from 16 to 21 sectors then the unavailable sectors are marked as allocated, i.e. 0.