IBM XDF has multiple sizes of sectors on each track!
Similarly, the Ensonic Mirage got 880K on a "720K" disk by using five 1024
byte sectors plus a 512 byte one.
The WD 179x type FDC can mix sector sizes, since formatting with it is an
almost RAW write of a track.
It is, of course, "impossible" to mix sector sizes on a track with the NEC
765 type of FDC. Since you specify the sector size and number of sectors,
and then separately tell it what sector headers to write in the sectors.
This brings up the Wile E. Coyote principal of software development.
It is MUCH easier to do the impossible if you are unaware that it is
impossible until after you do it. S'posedly, what IBM did, rataher than
format using a WD FDC, was to tell the 765 that it was thirty-nine 128
byte sectors. Then, when the FDC got to writing them, it over-wrote the
128 byte sectors with sectors consistent with the sector headers that it
was writing.
http://www.os2museum.com/wp/the-xdf-diskette-format/
--
Grumpy Ol' Fred ci...@xenosoft.com
