Using 3.5" HD drives on CP/M systems

[Dan Lanciani]

|Randy suggested in his comp.os.cpm posts that it's basically OK to write 
|in single density FM format on 3.5" high-density (HD) media, as he says 
|the "flux changes in FM and MFM" are at the same rate.

More than that, a correct FM data stream is indistinguishable from a
correct MFM data stream that happens to have no clock bits since every
other "data" bit is "1".

|For the oldest 
|computers, FM is the ONLY format option you may have. I did not respond 
|to Randy's post. But I think the problem of that scheme for use at 3.5" 
|HD will be two-fold. First, the actual data rates of old single density 
|FM 8-inch FDC's are much slower than standard data rates for HD 3.5" 
|media; the same applies for the read/write electronics of the DRIVE.

No, the rates are the same.

|Second, no "modern" computer which uses 3.5" 1.4M drives is likely 
|CAPABLE of reading an FM encoded (single density) data stream, except by 
|accident.

This is certainly a problem, at least for modern PC-class machines.  Not
only do recent integrated Intel floppy controllers fail to support FM mode
but they have never had a read-track command which can sometimes be useful
to read FM data in MFM mode.  When I wanted to back up a large number of
old single-density 5.25" disks that use a non-standard format I had to
employ a machine with a WD1797, reading the FM tracks in MFM mode and
bit-banging the result.  Even that wasn't reliable because there is no
way to guarantee that the controller exposes the correct bit slots without
data marks.  (The 1797 has a 16-bit shift register but passes every other
bit to the host.)  I had to add a circuit to double-up the pulses in the
raw read data stream so that either alignment of the shift register would
be correct.  Of course, if the disks used a standard format I would have
been able to read them in FM mode without all the trouble.

In general, I don't think it is possible to come up with a scheme that is
wire-compatible with old FM controllers yet produces media that can be
read by "modern" PCs with MFM-only controllers that lack a read-track
command.  Given that, I suggest that a SS/SD/77/26/128 format on 3.5" disks
might be the most useful standard for those wishing to replace physical 8"
drives.  Beyond that you might as well use whatever non-standard format you
used on 8" disks.  Keep in mind that tracks/heads/sectors/size is still
not enough information to fully specify a CP/M disk format because vendors
set up the extent mappings differently.  I think I even came across a
DS/DD/77/26/256 8" format that was logically incompatible with my system
(until I DDT'ed the BIOS) even though I had believed that everyone was in
agreement about this "base" double density mode.

				Dan Lanciani
				ddl at danlan.*com