MFM/RLL data recovery

[Randy McLaughlin]

From: "Scott Stevens" <chenmel at earthlink.net>
Sent: Monday, April 04, 2005 11:00 PM
<snip>
> 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

Exactly, any variation in hardware or firmware causes compatibility 
problems, with an AT 99% of that disappears.

The problem re-emerged on the AT once on board BIOS's and/or different 
hardware was used such as RLL.

On 8 bit PC's trying to recover data when a controller dies is much more of 
an issue than with 16 bit systems.

I am not trying to say it can't be done but it is a problem which is proven 
by the fact that I have seen lots of ideas flying around but have yet to 
hear that any of the data has been recovered.

If it was me and I knew there was info I absolutely had to have I might try 
a 16 bit controller and see if I could physically read any sectors, if so an 
assembly language routine could be written to transfer all readable sectors 
to a file on a second hard drive.  I would bet money that it wouldn't work 
since by definition the hardware is different.  Trying it with the wrong 8 
bit controller would be even harder.

The best chance of recovery is as said in a previous post to keep trying 
different 8 bit controllers and booting with a floppy.

The things working against you:

If it was from a specialty computer like a Tandy they used a different 
incompatible set of controllers.  Some 8 bit computers had their own hard 
disk BIOS's on the system board and required disabling the controllers BIOS 
in that case you must find the same type computer and the same type 
controller.

Finding the right controller.

Does the controller require a real 8 bit machine, yes not all 8 bit 
controllers even worked in 16 bit computers.

Is the drive bad.

Has the drive formatting been messed up.

Is the drive terminated properly.

Is the drive jumpered properly.


To work everything has to work, to fail only one thing has to fail.

Can anyone guess why they charge so much for data recovery?

On IDE all of the truly low level stuff is 100% hidden so the only 
compatibility problems with IDE tended to be drive geometry (early problems 
when the same drive could be addressed by different geometries) .


Good luck,

Randy
www.s100-manuals.com