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72 lines
4.0 KiB
ReStructuredText
72 lines
4.0 KiB
ReStructuredText
86F
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===
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A floppy disk surface image format which stores data in FM- or MFM-encoded transitions.
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Preliminary specification for v2.20
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-----------------------------------
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All offsets are in hexadecimal. This specification is subject to change before its final release.
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.. code-block:: none
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00000000: Magic 4 bytes ("86BF")
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00000004: Minor version (0x14)
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00000005: Major version (0x02)
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00000006: Disk flags (16-bit)
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Bit 0 Has surface description data (1 = yes, 0 = no)
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This data indicates if the corresponding bit on the FM/MFM encoded surface
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is a normal bit or a special bit (weak bit or hole, depending on the other bit):
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0 = The corresponding FM/MFM encoded surface bit is normal
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1 = The corresponding FM/MFM encoded surface bit is either a weak bit or a hole:
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Corresponding FM/MFM encoded bit is 0: Hole (noise on read, not overwritable)
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Corresponding FM/MFM encoded bit is 1: Weak bit (noise on read, overwritable)
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Bits 2, 1 Hole (3 = ED + 2000 kbps, 2 = ED, 1 = HD, 0 = DD)
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Bit 3 Sides (1 = 2 sides, 0 = 1 side)
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Bit 4 Write protect (1 = yes, 0 = no)
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Bit 5 Bitcell mode (1 = Extra bitcells count specified after
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disk flags, 0 = No extra bitcells)
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The maximum number of extra bitcells is 1024 (which
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after decoding translates to 64 bytes)
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Bit 6 Revolutions (0 = one revolution, 1 track has 16-bit number of revolutions)
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00000008: Offsets of tracks
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Note that thick-track (eg. 360k) disks will have (tracks * 2) tracks, with each pair of tracks
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being identical to each other.
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Each side of each track is stored as its own track, in order (so, track 0 side 0, track 0 side 1,
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track 1 side 0, track 1 side 0, etc.).
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The table of the offsets of tracks is 2048 bytes long, each track offset is an unigned 32-bit
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integer. An offset of 00000000 indicates the track is not present in the file.
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As an example, an 86F representing a disk with 80 thin tracks and 2 sides per track, where all
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the tracks are present in the file, would have the first 160 offsets filled in, same for a disk
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with 40 thick tracks and 2 sides. Same with only 1 side but only the offsets at 0000000, 0000008,
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etc. (so every second offset) would be filled in.
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Track offset + 00000000: Track flags (16-bit)
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Bits 4, 3 Encoding
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00 = FM
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01 = MFM
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10 = M2FM
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11 = GCR
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Bits 2, 1, 0 Bit rate, if encoding is MFM:
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000 = 500 kbps
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001 = 300 kbps
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010 = 250 kbps
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011 = 1000 kbps
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101 = 2000 kbps
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If encoding is FM, the bit rate is half that.
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The RPM is determined from track length and data rate.
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Track offset + 00000002: Total bit cells count (32-bit)
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Track offset + 00000006: Bit cell where index hole is (32-bit)
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Track offset + 0000000A: FM/MFM/M2FM/GCR-encoded data (track length bytes)
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Track offset + 0000000A + track length: Surface description data if present (track length bytes)
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If this is a multi-revolution 86F, then track offset + 00000000 has a 16-bit number of track revolutions,
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and the track header + data appears for each revolution, while surface description data, if present,
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can appear any number of times, but only once per encoding + bit rate combination.
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This needs work to properly make surface data work with flexible multi-revolution support.
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Track lengths:
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The total bit cells count is always present.
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The track is stored as (bit cells >> 8) bytes, with one extra bit cells if the number of bit cells
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is not divisible by 8.
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