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Updated Apple GCR disk encoding (markdown)

Thomas Harte 2018-05-11 13:04:50 -04:00
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Apple-GCR-disk-encoding.md

@ -62,7 +62,7 @@ A complete sector header is formed on disk as:
`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. `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. 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 '5 and 3' encoding was the first to be deployed; '5 and 3' was phased out in favour of '6 and 2' in 1980.
# '6 and 2' Encoding # '6 and 2' Encoding
Apple's second deployed sector data encoding fits six data bits into every on-disk byte. Apple's second deployed sector data encoding fits six data bits into every on-disk byte.