Install Floppies (Was: Compaq Deskpro boards/hard drives from

Peter Corlett abuse at
Mon Jul 26 05:02:31 CDT 2021

On Sun, Jul 25, 2021 at 11:46:17AM -0600, Grant Taylor via cctalk wrote:
> On 7/24/21 10:26 PM, Chuck Guzis via cctalk wrote:
>> My recollection of the DMF Microsoft period was that if you purchased a
>> retail MS product using the DMF format and couldn't get it read on your
>> system, a call to MS would result in a standard format copy being shipped.

> It's my understanding that The DMF disks that Microsoft (and comparable from
> IBM with PC-DOS) used a different non-FAT file system which took up less space
> on the disk, thus yielding more storage for data. But that they both fit on
> the same /standard/ ""1.44 MB disks.

> I also seem to recall that Macintosh's could get 1.7 MB on the same ""1.44 MB
> disks.

HD disks can hold "up to" 2MB (12,500 bytes per track, times two sides, times 80
tracks), as printed on some of the more misleadingly-labelled brands. However,
splitting that into sectors and adding guard bands reduces the usable space.
Similarly, DD disks are "up to" 1MB.

When writing, PC-style disk controllers scan for the appropriate sector header
then switch to write mode to overwrite the old sector data. This requires guard
bands between sectors and sector headers. The PC's standard of 1,440kiB seems
have particularly generous guard bands, possibly to account for really shoddy
old systems which may be slow at switching modes and/or whose drives are
spinning a bit fast.

The Amiga could get 880kiB on a DD disk, and 1760kiB on a HD disk if you have
one of those hen's teeth drives which spin at 150RPM. It does this by doing a
read-modify-reformat of the entire track of 11 or 22 sectors, which allows
omitting all of the guard bands except for the one between the start and end of
the track. The hardware could do the mode-switch thing, but I'm not sure that it
saw much use, if any.

There was a third-party device driver for the Amiga which took out some of the
unused label areas in Amiga disk sector headers, and squeezed 12 or 24 sectors
per track. It could also optionally go up to track 83, giving 1,032,192 or
2,064,384 bytes per disk, although that's kind of risky.

The DMF format presumably also takes the approach that if the disk isn't
intended to be written to by random drives, they can tighten the guard bands
somewhat. I'm surprised that they went with 21 sectors per track when 22 is
clearly possible. Perhaps it was a hedge against people writing to them anyway,
or machines being unable to read them.

These figures assume MFM encoding. Halve it for FM encoding, with a hard upper
limit for double for fancier schemes. My back-of-beermat suggests 2,560kiB is
plausible for HD disks on the Amiga or similarly-flexible third-party controller
for the PC.

> But I could be completely wrong.

Apple had tighter control over their platform, so could tweak the timings to
increase the available space for data. I don't know whether they did: the only
time I've used a floppy on a Mac is to interoperate with a PC so it had to use
the lowest common denominator.

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