This highlights a specific case where the "read error while reading
drive" message will pop up on boot.
I have an HP Brio 8314 which has this problem. If you don't have this
exact machine, 86Box has a machine with the same problem: Slot 1,
"[i440LX] Micronics Spitfire". It is not a hardware bug, it is a BIOS bug.
For some reason, under some unknown circumstances, certain releases of
PhoenixBIOS 4.0 Release 6.0 (with a copyright extending to 1997) will
incorrectly set DL to 0x00 before jumping into the MBR code. This
results in the MBR loader trying to load a partition boot sector from
the first floppy drive.
The patch I am using right now is roughly equivalent to the following
DEBUG patch (MBR extracted via FDISK /SMBR using the 1.3 Live CD release
and the BOOTNORM loader):
debug boot.mbr
a11d
jmp 1fe0:7d10
a210
mov dl, 80
jmp 122
w
q
However, a proper fix for this should be something like this, executed
before one attempts to use INT 0x13:
;; Work around a bug in 1997 PhoenixBIOS 4.0 Release 6.0,
;; where DL can be erroneously set to 0x00 (first floppy drive)
cmp dl, 0x00
jne nodlbug
mov dl, 0x80
nodlbug:
(I release all code in this email into the public domain.)
I'd need to do further analysis to see how a second hard drive behaves,
but this will cover the most common case.
Two further points of note:
1. In the MS-DOS 4.00 sources, the drive number is loaded from the MBR
in such a way that it gets forced to 0x80. But that makes it effectively
impossible to boot from a different drive number.
2. The PnP BIOS specifications require DL to be set to the boot drive,
and this BIOS does have PnP support. This confirms that the behaviour
from the BIOS is a bug.
Regards,
Ben R
P.S. If you have a later version of FreeDOS FDISK and you used an
earlier version with the /SMBR switch, *DO NOT USE THE /LOADMBR SWITCH!*
You will lose all partitioning information and probably have to wipe
your disk and reinstall everything. Use /LOADIPL in that case instead!
(or the /AMBR alias, which is what you would have been using before.)
Yes, I learnt this from experience, unfortunately._______________________________________________
Freedos-devel mailing list
Freedos-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freedos-devel
