21 Nov
2014
21 Nov
'14
8:53 p.m.
On 11/21/2014 07:48 PM, Jerome H. Fine wrote:
(a) As far as I know, (almost?) all soft sectored
8" floppy media
are formatted as either single-density (which is the IBM standard -
also known as the DEC RX01 option) or double-density.(which
is the additional DEC RX02 option).
A couple of nits--the RX02 double-density data fields are not strictly
MFM. In other words, if you'd have a controller that supported standard
MFM recording and that could be switched to MFM mode after using FM for
the ID address header, it still wouldn't work.
DEC in its infinite zeal to be different, realized that certain MFM
patterns could be mistakenly recognized as FM address marks. So there
are some substitutions in the MFM coding to avoid these problem patterns.
Also, IBM-standard formatted "double density" media, i.e. S/36-style
always had the first track as FM, with the remainder MFM. Various
sector sizes were used, from 256 bytes to 1024.
FWIW,
Chuck