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

[Ethan Dicks]

On Mon, Jul 26, 2021 at 6:02 AM Peter Corlett via cctalk
<cctalk at classiccmp.org> wrote:
> 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.

In particular, the Amiga trackdisk.device driver would start by
spinning up the disk motor, then writing out about 10% of a track of
gap bytes, then emit all 11/22 sectors in a tight line,
(intentionally) overwriting some of the just-written gap bytes.  The
little transition right at the end where the write stopped was
irrelevant since it would just be skipped over by the
find-the-first-sector scheme.

It also meant that you didn't have to wait for part of a rotation for
your sector to come around.  As soon as the disk was up to speed, you
could begin writing immediately.

The "disk controller" was really a giant shift register that read or
wrote media-ready bits.  The encoding/decoding happened in RAM, using
part of the graphics subsystem to transform unencoded-binary blocks
to/from MFM-encoded data.

Screwy but it was pretty flexible.  Reading/writing "DOS floppies" was
a simpler process, and there was a different diskette driver for that
(mfm.device) and a filesystem handler that knew about the FAT
filesystems.

-ethan