floppy low level format question

From: Dwight K. Elvey <dwight.elvey_at_amd.com>
Date: Fri Jun 11 11:35:37 2004

Hi Jules
 The formats for MFM and FM are mostly well defined but they
are not the only formats out there. There was a problem
with one of the early controller chips ( I think it was the
1791 ) that had problems with all 0 gaps or something. In
general, FM is FM and MFM is MFM. The clock rates are different
for 8 inch and 5-1/4, single, double and quad.
 There are at least two other common soft sectored formats
that I know of. There is the Aplle II format and there is
M2FM that Intel used.
 Hard sectored formats used all kinds of encoding. Most use
a simple return to zero type clocking. Sector headers had any
number of different combinations.
 Anyway, going directly into the port is always going to be
tough for a PC. PC's are busy with a number of things and don't
make particularly good "real time" processors.
 I'd been thinking of the same things you've been thinking of.
There is another way. A while back there was some modem boards
called "softmodems". These were made by Cardinal and DSI ( later
bought by that sound board company? ). These have DSP chips that
run quite fast enough to bit-bang the data from a floppy.
 One can load code into these boards and run fast enough to
monitor the data stream of a floppy.
 I've hacked the ones used by Cardinal and DSI to use for
some DSP projects. I've been considering doing the same
to capture images of disk. I'd though it would be easiest
to use the disk controller in the PC to deal with select,
track stepping and head loads while using this board to
read the data.
 Once the timing data is captured, one can store or decode
at ones leisure.
Dwight

>From: "Jules Richardson" <julesrichardsonuk_at_yahoo.co.uk>
>
>
>Hmm, my pondering about reading raw data from floppies got me thinking.
>I have some data on low level floppy format, which gives the following
>information:
>
>Each track has an index gap, followed by a gap 1, followed by a number
>of sectors, followed by a termination gap.
>
>Each sector is made up of an ID field, seperator gap, data field, and
>then a trailing gap on all except the last sector on a track.
>
>This is given as the same for both MFM and FM recording.
>
>The information I have gives the makeup of each of the gap types in
>terms of bit patterns, counts, what clock transitions are missing for
>MFM formats etc.
>
>Question is, is this a standard? I mean, for any disk using MFM or FM
>recording are these bit patterns going to be the same? Or is it
>dependant on the controller chip being used?
>
>cheers
>
>Jules
>
>
>
>
Received on Fri Jun 11 2004 - 11:35:37 BST