Using 3.5" HD drives on CP/M systems

From: Dan Lanciani <>
Date: Wed Feb 9 15:31:08 2005

|You mentioned HD media, the thread subject is "HD drives", so I thought
|that was your proposed "standard". But it is more likely that 3.5" DD
|media and drive settings will work for FM/single density/8-inch systems,
|than HD.

No, a DD 3.5" disk uses a 250kb/s data rate just like a 5.25" DD disk. An
HD 3.5" disk uses a 500kb/s data rate just like all 8" disks. FM vs. MFM
is transparent to the drive (except of course that MFM requires twice the
temporal resolution of the flux transitions). An HD 3.5" disk will look
to an 8" controller much like an 8" disk with tracks that are 1.2 times
longer than normal. I expect it should be possible in many cases to use
existing formatting software unchanged to produce 8" formats on 3.5" HD

It was suggested that formatters would fail in attempting to produce 77/26/128
FM formats on 3.5" HD drives, but I'd like to know the details of the failure
modes. All the formatters that I'm familiar with check (if at all) only that
the index hole does not come around again before they finish. They do not
attempt to time the track to make sure that the index hole comes "soon enough."
I suppose it would be possible that some formatters give up on padding too soon
and note some sort of data underrun error. For such systems it might be
easier to use 5.25" HD drives which exactly mirror 8" track timing. It might
also be possible to get very unlucky and have garbage in the unwritten area
of the track look like valid sector headers, but this could probably be avoided
by bulk erasing before formatting.

|I think the problems of a set of "standards" for using 3.5" drives and
|media fall into two categories. First, the technical limits of the
|oldest systems. Already it appears that single density will need its own
|standard - that's one example of technical limits. Reliability is a
|technical issue also: if you use odd formats at odd data rates, you may
|not get reliable results. Like, using HD disks at DD formats: it may
|work, sort of.

Happily, there really are no "odd data rates" involved. Ignoring 2.88M
drives and the special 300kb/s rate used to read 5.25" DD disks in HD
drives without altering the motor speed, the controller technology has
remained largely unchanged. The 250kb/s and 500kb/s rates persist from
the earliest floppy controllers to the present. The drives themselves
have no notion of the higher-level aspects of the format, so track layouts
suggested by drive manufacturers can generally be taken as safe _maximums_
rather than required formats.

|Second, the general issue of cross compatibility. When
|you use new disks on old systems, you'll have "funny" formats that will
|be very unique, to the point that NOBODY BUT YOU can read your disks.
|Backups that can be read by only one computer are not backups, when that
|computer dies.

This is a problem, but it isn't really a function of the new drives. I've
had problems with 8" MFM/77/26/256 formats being incompatible between CP/M
systems even though I had thought that everybody agreed on the extent
parameters for such disks. As far as I can tell, FM/77/26/128 is the only
format for which most everybody agrees on the logical parameters (and I'll
bet there are exceptions even to that). I would think that if you were
going to do an 8" on 3.5" format, FM/77/26/128 would give you the best
compatibility bang for the buck. In the meantime, I'm not about to dump
my 8" drives. :)

                                Dan lanciani
Received on Wed Feb 09 2005 - 15:31:08 GMT

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