Re: GCR*MFM=FM

[mjmahon@aol.com (Michael J. Mahon)]

In article <7c-dnf9g-vXF-fejXTWcrg@comcast.com>, "Joel"
<joelbuckley54.nospam.@hotmail.com> writes:

>"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.

Yes, FM is doable--it's MFM that is not.

>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.

I'm not so sure that current PC drives/controllers can deal with
SSSD format--maybe someone here knows?

>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.

You are correct about where the extra capacity comes from.  But Apple
3.5" 800KB disks are still GCR, using essentially the same encoding as
the 5.25" disks.

In the "parent" of this thread, Eric Smith suggested that it might be possible
for an Apple II to read an FM single-density disk by reading each track two
or more times, and correlating the bit-shifted results to determine the
original
content.  I replied with a proposal that might be able to do it in one rotation
per track by sampling the controller's shift register three times per "nibble".
I think that the Apple II could do this with 5.25" disks, but 3.5" disks would
be too fast for it to work.

There would have to be some heuristics to determine correct nibble sync,
but it should be possible.  Of course, a SSSD (90KB) disk is not the most
practical data interchange medium for either a PC or an Apple II.  ;-)

-michael

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