RX01 Floppies

[Chuck Guzis]

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