On Fri, 11 Jun 2004, Jules Richardson wrote:
> Hmm, my pondering about reading raw data from floppies got me thinking.
> . . .
> 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.
your description (quoted only in part) left out the address marks.
Hopefully those ARE included.
> 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?
The simple answer to your question is sort of YES,
they are intended to be compatible.
But,...
You are looking at a format structure that was developed
by IBM (LONG before the PC). "FM" and "MFM" are encoding
names, but don't include the track layout of the format.
The FDC chips support those IBM "3740" formats, with
some quirks. But there do exist FM and MFM formats, not
supported by those chips, that have "standard" FM or MFM
encoding, but do NOT have the same track structure. For
example Amiga is MFM, but has almost no track layout structure,
since the Amiga reads and writes an entire track, and then
parses sectors from that in memory.
Amongst the quirks are differences in gap lengths, and
different address marks. Because of that, for example,
NEC FDC chips (765) sometimes have trouble reading some
formats, such as some TRS80 model III, because the WD 179x
chip that wrote them used an index gap that was shorter
than the NEC chip can handle (can sometimes be solved by
not letting the NEC chip see the index pulse).
And, the TRS80 (1771) TRS-DOS 2.x uses some data address
marks that the 179x can not create. Thus, the model 3
can not create a disk that model 1 TRS-DOS can read.
(that was the seminal factor that led to the creation of LDOS)
And, the WD chips are capable of IGNORING certain fields
in the sector header, but the NEC chip can not ignore them.
Because of that, Kaypro DS (among others) diskettes can not
be read through any manipulation of the BIOS (INT13h), and
require some direct programming of NEC FDC chip to tell it
what INCORRECT value to be looking for in the head number
field of the sector headers of the second side.
So, YES, they are standardized in their structures, making
it possible to write programs such as XenoCopy.
But, NO, they do not always adhere well to the "standard"
--
Fred Cisin cisin_at_xenosoft.com
XenoSoft http://www.xenosoft.com
Received on Fri Jun 11 2004 - 14:14:29 BST