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Adds information on '4 and 4' encoding.

Thomas Harte 2018-05-10 10:35:21 -04:00
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Apple-GCR-disk-encoding.md

@ -35,4 +35,31 @@ As above, sync words lie in the gaps between sectors and between the header and
A sync word is simply an `ff` encoded byte followed by as many zeroes as the content rules will allow: a single zero for '5 and 3', or two zeroes for '6 and 2'.
Given the top-bit-set rule, a series of sync words has the effect of bringing a CPU polling loop as above into phase with the start of each sync word.
Given the top-bit-set rule, a series of sync words has the effect of bringing a CPU polling loop as above into phase with the start of each sync word.
# '4 and 4' Encoding
Sector header content is encoded in '4 and 4' form regardless of the encoding in use for sector contents.
'4 and 4' encoding encodes the source byte b, with bits b7, b6, b5 ... b0 as the two on-disk bytes:
1 b7 1 b5 1 b3 1 b1
1 b6 1 b4 1 b2 1 b0
Which is equivalent to FM encoding other than in bit order. The bits are ordered different to allow for efficient decoding:
(((1 b7 1 b5 1 b3 1 b1) << 1) | 1) & (1 b6 1 b4 1 b2 1 b0) = original byte
A complete sector header is formed on disk as:
three bytes prologue: 0xd5, 0xaa, 0x96
two bytes: '4 and 4' encoded volume
two bytes: '4 and 4' encoded track
two bytes: '4 and 4' encoded sector
two bytes: '4 and 4' encoded check value — the exclusive OR of (volume, track, sector)
three bytes epilogue: 0xde, 0xaa, 0xeb
`track` counts upwards from zero for the outermost track. `sector` counts upward from zero for the first sector on a track.
`volume` has at least two context-dependent meanings. In both of Apple's operating systems it defaults to `254` for Disk II-compatible media and in Pro DOS is used to confirm the volume *type*. Some software prefers to use it as volume *number*, for distinguishing different disks or sides of a disk. It should be written as `254` unless there is a reason to do otherwise.
In principle the entire disk contents could have been encoded in '4 and 4' form, to give exactly the same data density as FM encoding. In practice the more-efficient '5 and 3' encoding was the first to be deployed.