Re: GCR*MFM=FM

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

"Exegete" <millers@noneofyourbusiness.com> wrote in message .
>
> Well, to be a real nit picker, if you have the older IBM SSDD drives....
> BTW, IBM never had SSSD 5.25 drives.
> The minimum was 160K - SSDD 8 sectors per, as opposed to the 180K SSDD 9
> sectors per.
> Of course, there was also the 320K disks DSDD 8 sectors....
>
> Roy

You are right that IBM supported single sided 8 sector per track, but not
single density.

I have a vague recollection that there were programs that enabled IBM
computers to read single-sided single density disks that were used on many
CP/M computers at that time. If that's true, it might be possible to write
in an Apple readable single-density (FM) format.

Alternately, if you can't get the IBM to write in single density format, I
had come up with this fallback idea.

MFM (double density) is like an 8+0 format, vs the Apple's 6+2, 5+3, and 4+4
formats.
Apple can't read 8+0. So restrict the IBM to only write data using only the
subset of bytes that the Apple can read. Reencode the data before writing to
the IBM disk just like Apple's DOS does. 256 8 bit bytes become 342 6(+2)
bytes.
Similarly, the Apple should be able to write 64 out of the 256 byte patterns
that the IBM can recognize. You just rencode your data before writing.
You may have a not very large capacity disk (maybe only 35-50k).
It may require pre and post-processing to encode and decode it.
But it would have provided a way without nul-modem cables and terminal
programs to transfer data between the Apple II and IBM.

Joel