GCR*MFM=FM

["Joel" <joelbuckley54.nospam.@hotmail.com>]

"Michael J. Mahon" <mjmahon@aol.com> wrote in message
20030306134444.09900.00002073@mb-fc.aol.com">news:20030306134444.09900.00002073@mb-fc.aol.com...

>
> >Could a common ground be found, using these formats, to read/write Apple
> >II disks on a PC.  Since both computers can use CP/M, could software be
> >written to convert MFM material to CP/M which is then written to a disk.
> >The disk is then put in a II and read via software into something the II
> >can understand from CP/M.  Material could be written to CP/M on teh II
and
> >put into a PC to translate form CP/M.
> >
> >Let me guess...there is GCR CP/M and MFM CP/M and neither the twain shall
> >meet? :-(
>
> That's right.  Disk encoding is a a lower level of abstraction than file
> system structures.

>The problem of GCR/MFM disk interchange is _not_ solvable by software.


I would hate to disagree with Michael (because I would probably be wrong),
but I'm not sure he's right this time.
I worked on this problem some twenty years ago, and while I never finished
it, I was pretty convinced it could be done.

The answer, as someone hinted at in earlier posts, is FM.

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!

3.5 drives-I don't know. I was never really familiar with the workings of
Apples 3.5 drive. Clearly the IBM can't read Apple's 800k. But could the
Apple read a 720k IBM. I don't know. I thought the increase in data density
on Apple's 3.5 drive was due to putting more blocks per track on the outer
blocks, rather than the encoding format. But I seem to remember that the IWM
was used in the 3.5 drive controller as well, so I'm not really sure.