Re: GCR*MFM=FM

[pausch@saaf.se (Paul Schlyter)]

In article <7c-dnf9g-vXF-fejXTWcrg@comcast.com>,
Joel <joelbuckley54.nospam.@hotmail.com> wrote:
 
> If I remember this at all, old single sided single density disks on the very
> early IBMs and even earlier CP/M machines used FM recording, what might be
> called 4+4, data bits interleaved with clock bits. Apples GCR used a 5+3 and
> later 6+2 reduced clock bit technique (but is capable of 4+4 and even uses
> it for the sector headers and trailers). Then MFM (double density) on the
> IBM effectively eliminated the clock bits, sort of 8+0, in effect. The Apple
> can't read the MFM (8+0) because it needs to see a 1 bit at least every so
> often. But I believe it can both read and write the 4+4 FM patterns. In some
> drawer I still have a raw disk data dump I had done using an Apple drive to
> read a data disk from an Osborne CP/M computer SSSD, maybe 90k disks?
> My memory is that I was able to decode the data from the Osborne disk read
> on an Apple II.
> And while some early drives may have used hard sectoring, there were plenty
> that didn't.
> I believe even later model 5.25 drives for an IBM could still read
> single-sided single density FM disks.
> So by limiting yourself to 35 track single-sided FM (4+4), I think you might
> be able to have a disk readable and writeable on both Apple and IBM 5.25
> drives.
> Just imagine the impact such a program would have had 20 years ago!
 
There are some problems though:
 
The Apple II wouldn't know where the bytes om the FM disk started.
But of course the software could try, and try again, until it got it
right.
 
In addition, the software wouldn't know where the track started: it
starts where the index hole on the disk is, however the Apple II
drives had no sensor for that index hole.
 
Finally, most FM floppies were 8" floppies (8" SSSD -- the only
standard CP/M disk format) .... most CP/M 5.25" floppy formats used
MFM....
 
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