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

Fred Cisin cisin at
Mon Jul 26 11:44:22 CDT 2021

On Mon, 26 Jul 2021, Peter Corlett via cctalk wrote:
> 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.

"inter sector gaps", "write splice", etc. not "guard bands"  any "bands" 
(synonym for circle) would be between tracks.

But, VERY good important point that you bring up.
If a disk will ever get a sector re-written, than it needs those write 
splice gaps.
But, if a disk will be written once, and NEVER get any sectors changed, 
then it could get way with very tiny intersector gaps and write splices.

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