Re: GCR <--CP/M --> MS-DOS and vice versa?

[Matthew Montchalin <mmontcha@OregonVOS.net>]

On 6 Mar 2003, Eric Smith wrote:
|Not *quite* total control.  The Disk II controller requires that each
|data pattern (nibble value) that can be read by the CPU have a leading
|one bit.

Ah, so that's how Apple does it!  It doesn't use sync marks like
the Commodore 1541?

|This requirement is the fundamental reason why the 9-bit or 10-bit
|self-sync patterns are used, and why the Disk II controller can't
|read industry-standard FM (single density) disk format (IBM 3740
|format).

Getting rid of that pesky '1' bit after every nybble, must be the
secret to getting the right bytes written on the medium, 8 at a
time?  But no doubt it is more complicated than that.  Can you
suggest a place to read up on the Disk II controller?  What third
party solutions have been offered to the Apple II world, in years
past?  I mean, even at software incompatibility, there must have
been a handful of developers that desired to implement their own
custom formats to restrict software to in-house use only, with
custom in-house hardware?

|The Disk II controller is capable of dealing with FM encoding;
|that's the "4+4" scheme used to encode the volume, track, and
|sector in the address field of a sector.  But it can't sync to
|the IBM 3740-format sync patterns.  It might be possible to
|write IBM 3740 format, but the Apple wouldn't even be able to
|verify the data it had just written.

hmmm

|And in 3740 format a track is supposed to start after the index
|hole, but the Disk II doesn't even have an index sensor, so
|there's no way for the software to know where to start the track.
|
|Actually, I have an idea for a way that it might just barely be
|possible to read 3740 format data, though it would typically take
|multiple disk revolutions to read a sector because it would have
|to count on random chance for synchronization.
|
|But there's *NO* way that the Disk II controller (or its variants)
|is going to read and write MFM at 250 Kbps.  It was a clever design,
|but that wasn't one of its objectives.

Can you suggest a good website that will describe the low level
format of a disk using Disk II?