Question about Intel disk formats

From: Richard Erlacher <>
Date: Thu Jun 21 16:07:10 2001

Tony's right about the Shugart 801's. Drives don't determine the modulation
technique used to record the data. The "density" refers to the number of bits
per linear inch on the track. Since that's really not an adequate measure in
this context, its sensibly applied analog would be bits per revolution. The
head/media combination determines the number of flux changes per inch (fcpi) and
single-density uses two flux changes per bit, while double density uses only one
(not exactly, but close enough). If the drive can manage to swap the flux
direction quickly enough, which the '801 clearly can, then it's a double-density
capable drive, which it is. Confusion arises from the fact that, while the '801
can read/write double density, its board-resident "data separator" can't. Also,
while the older '901 wasn't commonly used for double density, it could manage
MFM if one used 250ns of write precomp. The '801 required only 160-180 ns.


----- Original Message -----
From: "Tony Duell" <>
To: <>
Sent: Thursday, June 21, 2001 1:40 PM
Subject: Re: Question about Intel disk formats

> >
> >
> > I've got some 8" disks that are apparently double density (the
> > manufacturer label says so). I thought they were Intel ISIS formatted
> > disks (which Eric Smith said would make them M2FM and uncompatible with
> > anything other than an Intel MDS development machine) but I'm not so sure,
> > since I can't seem to access any on the two Intel MDS systems I have set
> > up.
> Intel certainly made an M2FM disk controller for the MDS800 (I have one
> here in my MDS). It was couple of Multibus boards with Intel 3000 series
> bit-slice chips on them.
> But I believe they also made a 'normal' single-density disk controller
> board (FM, IBM3740-like format). Maybe that's what was used to record them.
> FWIW, many floppies say double density on the label. This means the media
> is capable of handling double-densisty recording. It doesn't mean that
> they can't also be used for single density. Unless the label written by
> whoever formated and used the disks says 'double density' I wouldn't
> assume that they were.
> > Both machines have Shugart 801 drives in them, and after doing some web
> > research, I've come to find out they are single-density only. This would
> This is news to me (and to all the machines I have with 801 drives that
> quite happily do double density recording). These drives do have an
> internal clock/data separator which is single density only, but most
> people didn't use that (the raw data is available on the interface
> connector as usual). And then you can most certainly use these drives for
> MFM or M2FM (or whatever) recording.
> > probably explain why I am not able to access these disks on these
> > machines.
> >
> > The disks I am trying to access are supposedly CP/M, but the labelling
> > indicates they were perhaps used on an Intel development system (they have
> > filenames on the label with ".HEX" file types; this may not mean
> .HEX probably means an Intel Hex format file. Used on Intel development
> systems, CP/M machines, many EPROM/microcontroller programmers, and a lot
> of other places.
> -tony
Received on Thu Jun 21 2001 - 16:07:10 BST

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