Re: Reading apple disks on PC

[pausch@saaf.se (Paul Schlyter)]

In article <XAFQa.1178$OC4.956@nwrddc01.gnilink.net>,
Michael Pender <mpender@hotmail.com> wrote:
 
> Paul Schlyter <pausch@saaf.se> wrote in message
> news:betttj$1b9n$1@merope.saaf.se...
>> In article <yLtQa.13001$D%1.10374@nwrddc01.gnilink.net>,
>> Michael Pender <mpender@hotmail.com> wrote:
>>
>> The incompatibility in disk format which makes it impossible to read
>> Apple disks in standard PC hardware is at the lowest-level encoding:
>> the Apple II used GCR whihe the PC uses MFM.  Early Macs used GCR
>> too; later MFM disks were used by Mac but a lot of Macs have floppy
>> drives capable of reading GCR disks too.  PC floppy drives have
>> never been able to read GCR disks and will never be.
> 
> Is a reference available that outlines the differences between GCR and MFM?
> If that is the only difference, then it should be possible to read/write
> PC-format disks on an Apple II, even if the opposite is not true.
 
I've never seen any reference comparing MFM and GCR.
 
However, GCR is described in "Beneath Apple ProDos".  There also was
a description in "Beneath Apple DOS" which was inaccurate; the
corrected description appears in "Beneath Apple ProDos".
 
Both GCR and MFM are based on FM, where every other pulse is a clock
pulse (always "1") and every other pulse a data pulse.  GCR is just
FM with all the clock pulses removed.  MFM is another way to compress
the data, by writing some of the pulses at "half-pulse" positions.
 
 
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