Re: GS/OS for xgs emulator?

[nparker@cie-2.uoregon.edu (Neil Parker)]

In article <01bc2f0d$ff493780$de7192cf@system1> "Dan Yertzell"
<Dan-Yertzell@worldnet.att.net> writes:
>[...]
>But PC formats followed the already established standard of *digital*
>recording.  Apple's is analog, and *that* is the incompatibility.  There
>was a standard (though not a "PC" standard) and Apple ignored it.

As hesitant as I am to leap into a discussion that looks like it's becoming
vociferous enough to turn into a flame war, I couldn't let this one pass by
without comment.

Apple disks are, in fact, no more or less digital than PC disks.  Both use
the presence of a reversal of the polarity of the recording medium to
represent a "1" bit, and no reversal (within a certain time interval) to
represent a "0" bit.

The incompatibiltiy between the disks is, at heart, a timing issue.  In
order to correctly decode an MFM data stream, it is necessary to
distingiush a 4-microsecond interval between polarity reversals from a
6-microsecond interval.  The Apple disk controller is incapable of this
distinction--it can identify or create a 4-microsecond interval and an
8-microsecond interval, but not a 6-microsecond interval.


When Apple produced its first disk dive, there was good reason to go
against the "standard," such as it was (and at the time, there really
wasn't much of a standard in the 5.25-inch disk world).  Those early
5.25-inch disk systems generally used FM encoding or something comperable,
which could only fit about ten 256-byte sectors onto a track.  By using
their own custom encoding scheme, Apple was able to fit 13, and later 16,
sectors on a track.  Multiply that by 35 tracks per disk, and you discover
that Apple's scheme significantly increased disk capacity.

True, the industry finally settled on MFM encoding, which can fit ~18-20
256-byte sectors on a track, but by the time that format became standard,
Apple was already locked into its own private format.

               - Neil Parker
-- 
Neil Parker, nparker@{cie-2,cie}.uoregon.edu, http://cie-2.uoregon.edu/~nparker
     "Evolution is vastly overrated."  -- Ambassador Delenn, _Babylon_5_

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