🚀 go-pugleaf

In article <1991Apr1.212138.18402@umbc3.umbc.edu>, cs106132@umbc5.umbc.edu (cs106132) writes:
>  
> I encountered a strange problem when trying to read the boot sector on a
> 40M hard disk formatted using Zenith DOS 3.3+, the one that supports large
> partitions.  The program I am working on needs to read the boot sector and
> extract some info from the BPB.  I use DOS absolute read interrupt 25h to
> access logical sector 0.  For some reason, this works fine with MS-DOS
> systems, but when I try using Zenith DOS 3.3+, it bombs out.

Since you noted that 3.3+ has support for large partitions I assume that
you are in using one here.  For this case you need to use int 25 with
CX=-1 and DS:BX=far pointer to a parameter packet, a la the MS-DOS 4.0 way.
Normally, your application should try the int 25 as you have it now, and if
you get an error (carry set and AX=0207) retry the int 25 with the CX and
DS:BX changes.

In case you're wondering, this approach is also compatible with generic
MS-DOS 4.0 as well as the soon-to-be-available MS-DOS 5.0.  In fact, this is
one of the few ways of reliably determining that you have a large partition.

Just in case you don't have the appropriate reference manual...
the DS:BX parameter packet referred to above is:
   starting sector number   dd  ?
   number of sectors        dw  ?
   transfer address         dd  ?

> Regards,
> Tarkan

Best of luck...
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