RX01 Floppies

Jerome H. Fine jhfinedp3k at compsys.to
Fri Nov 21 23:21:09 CST 2014

 >Chuck Guzis wrote:

> >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.

Thank you for the extra information.  I have NEVER used any IBM
hardware with 8" floppy media, only DEC hardware almost all the
time.  For one project in the 1980s, I used a Motorola system that
had an 8" SSSD floppy media.  That system could perform a Low
Level Format (LLF) of the floppy - if I remember correctly.

One point to mention (or clarify) is that for DEC, the FORMAT
operation did NOT include writing the file structure.  For the PC,
the FORMAT command does BOTH an LLF as well as writing
an initial root file structure (FAT16??) on the floppy media.  But
for DEC hardware with both the RX01 and the RX02, writing
the file structure made use of the INITIALIZE command (at least
under RT-11 and something similar under RSTS/E and RSX-11).
So with DEC PDP-11 operating systems which used 8" floppy
media, the user understood very clearly the difference between
the FORMAT and INITIALIZE commands and what each one

Jerome Fine

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