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Index > DOS > Debugging ROM BIOS / register offsets

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Paul6253



Joined: 19 Oct 2003
Posts: 31
Location: NY
Paul6253
Any Ideas on how I could play with the BIOS setup util?
I'd like to map the code to a buffer and mod it. Any ideas?

Also, how do seg:offset work in debug?
I wanted to do something like
mov byte al,[f000:0]
thats an error -- complains about closed bracket
As an alternative I tried putting f000:0000 into eax
like
xor eax,eax
mov ax,f000
rol eax,16
;eax = f0000000
Thats not a seg:offset either!

Thanks for help

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Post 21 Feb 2004, 06:55
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vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia
vid
mov byte al,[f000:0]
"f000" is not a number, it starts with letter so it is symbol. use 0F000h or 0xF000 or $F000.

you want to grab BIOS's code and play with it?
Post 21 Feb 2004, 11:39
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Bitdog



Joined: 18 Jan 2004
Posts: 97
Bitdog
MOV AX, BYTE PTR [F000]

seg over rides consist of the seg before the instruction
But you can't put a seg reg infront.
Seg regs CS, DS, ES, etc have a value, and that value is placed before
DB 66
MOV AX, BYTE PTR [F000]
or something like that,
you'll want the value of DS seg reg,
so assemble a blank.asm to .com that has two lines of code in it.
MOV BYTE [100h],5
MOV BYTE [DS:100h],5
then run it through a debugger and you'll see the machine code value for
DS or any other seg reg.
Then use that in debug.exe as a pre instruction seg over ride.
Debug.exe works as a disassembler, and from it's output you can learn
how to use it as an assembler. I have a debug disassembler for you....
http://bitdog.home.att.net/files/fasmenv.zip
look for the DISBUG dir
DISBUG MY.COM 45
disassembles a 45 byte .com file named MY.COM
it makes assemble batch files and two source .asm files. .BUG & .DAT ?
It comes with its Fasm .asm source code.

I hope it helps somehow
Bitdog
Post 21 Feb 2004, 17:36
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vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia
vid
Embarassed , yes, sorry. It so long from when i was working with segments last time...
but you must use *h/0x*/$* anyway.

so it would be somethink like
Code:
mov ax,0F000h
mov dx,ax
mov al,[ds:1]
    
Post 21 Feb 2004, 19:10
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