MFM/RLL data recovery

[Scott Stevens]

On Mon, 4 Apr 2005 19:04:24 -0500
"Randy McLaughlin" <cctalk at randy482.com> wrote:

> From: "Tom Peters" <tpeters at mixcom.com>
> Sent: Monday, April 04, 2005 6:38 PM
> <snip>
> >>If you are trying to recover files you are probably wasting your
> >time, the >XT controllers used unique formatting and you will never
> >read the data off >of the drives without using controllers indentical
> >with ones used >originally.  It is not good enough to use the same
> >brand or even chipset.
> >
> > I have been able to read many old MFM drives and the occasional RLL
> > drive. When the original controller was jumpered to lie about head
> > or cylinder count or other facts of drive geometry, I have be
> > largely unable to read those. This was several years ago.
> >
> >
> >
> >>Randy
> >>www.s100-manuals.com
> 
> I would be willing to bet you are talking about 16 bit controllers,
> most 16 bit controllers are interchangeable.  Very few 8 bit
> controllers are interchangeable.
> 
> 

I thought it would be worth adding, to clarify this for some people who
aren't familiar with the tech involved, some of the background regarding
AT versus XT HD controllers.  The IBM PC-AT and AT clones have direct
BIOS support for the hard disk and controller built into the
motherboard.  XT and XT-clone motherboards have no direct built-in
support for hard drives.  The Hard Drive BIOS support is included as a
'BIOS extension' on a ROM on the 8-bit controller card.  Said BIOS
extension code can and is widely varied from controller to controller.  

(For further background: the 'BIOS extension' is a feature in the PC
BIOS whereas the built-in BIOS on the motherboard scans a certain
address space looking for a specific signature that indicates a plug-in
card on the I/O channel has inserted BIOS extension ROM at that address,
then loads it as part of the BIOS before proceeding to boot.)

I may not be completely accurate in the above.  Anybody, please add to
or correct me as appropriate.

-Scott