Re: Debugging APM - cat /proc/apm produces oops

From: Ondrej Zary
Date: Mon Jul 24 2006 - 17:52:43 EST

On Sunday 23 July 2006 17:06, Stephen Rothwell wrote:
> On Sun, 23 Jul 2006 16:30:53 +0200 Ondrej Zary <linux@xxxxxxxxxxxxxxxxxxxx>

> > printing eip:
> > 00002f9d
> > *pre = 00000000
> > Oops: 0002 [#4]
> > Modules linked in:
> > CPU: 0
> > EIP: 00c0:[<00002f9d>] Not tainted VLI
> ^^^^
> This is the APM BIOS 16 bit code segment.

Looking at BIOS disassembly:
2F97: push bp
2F98: mov bp,sp
2F9A: add sp,-2
2F9D: mov [bp][-2],bx <-- it oopses here

I realized that I can modify the BIOS easily as it's stored in shadow RAM. So
I replaced the offending MOV with three NOPs and tested again. This time it
oopsed at 0x2FAD:
2FAD: cmp w,[bp][-2],1
2FB1: je 2FCB

that jump was taken during my single stepping, so I NOPped out the CMP and
replaced JE with JMPS. Then booted Linux and APM seems to work fine - battery
percentage and remaining time is there as well as AC power status.
There seems to be 4 these operations:
mov [bp][-2],bx
cmp w,[bp][-2],1
cmp w,[bp][-2],8002
cmp w,[bp][-2],8001
but I've hit only the first two of them. I wonder what's that for (especially
when it works without that).

Ondrej Zary
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at