OSDev.org

Posted: **Mon Sep 21, 2009 8:56 am**

Hi,

Can someone please show me how to perform call to apm bios? I'm really stucked here. I have checked for APM PMODE 32BIT support, have setup interface connection and have updated my GDT. Then, I tried to far-call the APM entry point with ax=530a (get power status) and bx=1. What i got was GPF and in bochsout was:

BOCHS-DEBUGGER wrote: LOCK prefix unallowed (op1=0x53, attr=0x0, mod=0x0, nnn=0)
LOCK prefix unallowed (op1=0x53, attr=0x0, mod=0x0, nnn=0)
write_virtual_checks(): write beyond limit, r/w

....maybe the values whitch i caught from 5303 int15 is wrong:

BOCHS-DEBUGGER wrote: pm 32bit cs base : 0xf000 (returned in ax)
apm 16bit cs base : 0xf000 (returned in cx)
apm entrypoint : 0xaf0d (returned in ebx)
apm ds base : 0xf000 (returned in dx)
apm 32/16 cs lenght : 0xfff0 (both) (returned in esi)
apm ds lenght : 0xfff0 (returned in di),

Can someone show me how to do this right? It means a lot for me to get this thing working,

Posted: **Mon Sep 21, 2009 12:02 pm**

LOCK prefix unallowed (op1=0x53, attr=0x0, mod=0x0, nnn=0)

Tip of the day: do not try to execute data. Instead jump to the location the bios instructed you to and not somewhere else

Posted: **Tue Sep 22, 2009 5:23 am**

i dont get it

if edx is the entry point and ds is set to apm cs - call [ds:edx] - is this a wrong way how to perform call ?show me what you mean please

Posted: **Tue Sep 22, 2009 5:24 am**

lama wrote:call [ds:edx] - is this a wrong way how to perform call?

I'm 99% sure that it does *not* do what you think

(look it up in the intel manuals)

Posted: **Tue Sep 22, 2009 5:28 am**

wow that was quick

well, it will perform a far call , cs is updated to value in ds, right?

Posted: **Tue Sep 22, 2009 6:38 am**

Hi,

lama wrote:wow that was quick well, it will perform a far call , cs is updated to value in ds, right?

Wrong.

For "call [ds:edx]" the CPU loads EIP from address "ds:edx". What you're trying to do is "call far ds:edx" (e.g. load CS from DS and load EIP from EDX), but the CPU doesn't support that.

What you'd need to do is something like:

Code: Select all

    mov [foo],edx
    mov [foo+4],ds
   call far [foo]

Also note that "call far [foo]" is different to "call [foo]", and they're both entirely different to "call foo".

Cheers,

Brendan

Posted: **Tue Sep 22, 2009 7:06 am**

thanks a lot for the explanation

i know what does [] stands for,but 'call ds:edx' has nasm rejected..so i thought that call [ds:edx] is the same in this case, but cleary it was a complete nonsense , thanks again

now i got different gpf - incorrect cs... so i probably have some junk in gdt ..

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