A floppy disk surface image format which stores data in FM- or MFM-encoded transitions.
Preliminary specification for v2.20
All offsets are in hexadecimal. This specification is subject to change before its final release.
00000000: Magic 4 bytes ("86BF") 00000004: Minor version (0x14) 00000005: Major version (0x02) 00000006: Disk flags (16-bit) Bit 0 Has surface description data (1 = yes, 0 = no) This data indicates if the corresponding bit on the FM/MFM encoded surface is a normal bit or a special bit (weak bit or hole, depending on the other bit): 0 = The corresponding FM/MFM encoded surface bit is normal 1 = The corresponding FM/MFM encoded surface bit is either a weak bit or a hole: Corresponding FM/MFM encoded bit is 0: Hole (noise on read, not overwritable) Corresponding FM/MFM encoded bit is 1: Weak bit (noise on read, overwritable) Bits 2, 1 Hole (3 = ED + 2000 kbps, 2 = ED, 1 = HD, 0 = DD) Bit 3 Sides (1 = 2 sides, 0 = 1 side) Bit 4 Write protect (1 = yes, 0 = no) Bit 5 Bitcell mode (1 = Extra bitcells count specified after disk flags, 0 = No extra bitcells) The maximum number of extra bitcells is 1024 (which after decoding translates to 64 bytes) Bit 6 Revolutions (0 = one revolution, 1 track has 16-bit number of revolutions) 00000008: Offsets of tracks Note that thick-track (eg. 360k) disks will have (tracks * 2) tracks, with each pair of tracks being identical to each other. Each side of each track is stored as its own track, in order (so, track 0 side 0, track 0 side 1, track 1 side 0, track 1 side 0, etc.). The table of the offsets of tracks is 2048 bytes long, each track offset is an unigned 32-bit integer. An offset of 00000000 indicates the track is not present in the file. As an example, an 86F representing a disk with 80 thin tracks and 2 sides per track, where all the tracks are present in the file, would have the first 160 offsets filled in, same for a disk with 40 thick tracks and 2 sides. Same with only 1 side but only the offsets at 0000000, 0000008, etc. (so every second offset) would be filled in. Track offset + 00000000: Track flags (16-bit) Bits 4, 3 Encoding 00 = FM 01 = MFM 10 = M2FM 11 = GCR Bits 2, 1, 0 Bit rate, if encoding is MFM: 000 = 500 kbps 001 = 300 kbps 010 = 250 kbps 011 = 1000 kbps 101 = 2000 kbps If encoding is FM, the bit rate is half that. The RPM is determined from track length and data rate. Track offset + 00000002: Total bit cells count (32-bit) Track offset + 00000006: Bit cell where index hole is (32-bit) Track offset + 0000000A: FM/MFM/M2FM/GCR-encoded data (track length bytes) Track offset + 0000000A + track length: Surface description data if present (track length bytes) If this is a multi-revolution 86F, then track offset + 00000000 has a 16-bit number of track revolutions, and the track header + data appears for each revolution, while surface description data, if present, can appear any number of times, but only once per encoding + bit rate combination. This needs work to properly make surface data work with flexible multi-revolution support. Track lengths: The total bit cells count is always present. The track is stored as (bit cells >> 8) bytes, with one extra bit cells if the number of bit cells is not divisible by 8.