compaq portable plus problems

Michael B. Brutman mbbrutman-cctalk at
Sun Feb 3 13:21:15 CST 2008

Another thing to try is a BIOS call.  On the XT class controllers there 
is a BIOS interrupt to do a low-level format.  According to my Phoenix 
BIOS ref, it is Int 13h, function 7.  Parameters are:

AH = 07h
AL = Interleave
CH = Start Cylinder
DL = Drive number (80h or 81h)

Outputs are:

AH = 07h if bad drive number in DL
    = 00h no error
    = xxh More error codes .. see table

CF = 0 No error
    = 1 Error (see AH)


Marvin Johnston wrote:
> When I am using debug to format a HD on an unknown controller, I'll do an
> unassemble at C800:5 and C800:6. The correct address will have a jmp
> instruction.
> Besides the HD formatting program on the utilities disk, going *way* back, I
> think Western Digital put out a formatting program WDFMT (?) that would allow
> the formatting of MFM HDs. I think there also might have been some PD/shareware
> programs to do the same thing. Assuming the Simtel archives are still around, I
> would expect to be able to find these programs there.
>> From: Roger Pugh <rogpugh at>
>> does anyone know the correct incantation to low level format an MFM 
>> drive, a rodime 351, driven by a Compaq controller board.
>> I,ve tried with debug G=C800:5 to no avail.  there is no format program 
>> there!.

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