Interrupt Question

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pal

Joined: 26 Aug 2008
Posts: 227

pal 26 Aug 2008, 16:28

Hey; I'm new to ASM coding and I'm having problems with int 10h. For some reason this code isn't working and I don't know why (especially as examples I've seen have this code too):

Code:

format PE GUI 4.0
entry start

section '.code' code readable writeable
    start:
        mov     bh,0
        mov     dl,0
        mov     dh,0
        int     10h
        ret

I'm trying to set the cursors position (without SetCursorPos). I've tried setting DX and BX by xor'ing them and moving 0 to them but nothing works. The program assembles properly, but it errors upon running it.

26 Aug 2008, 16:28

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20453
Location: In your JS exploiting you and your system

revolution 26 Aug 2008, 17:01

In windows you can't use any INT instructions. Those are for DOS.

I think you misunderstand the situation. Have a look at the Windows examples that come with the fasm download.

26 Aug 2008, 17:01

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 26 Aug 2008, 17:03

Because you are using BIOS interrupts on a Win32 executable.

Code:

org $100

        mov     bh,0
        mov     dl,0
        mov     dh,0
        int     10h 

int 20h

That would work but it wouldn't be a win32 executable anymore but a DOS one (which supports BIOS calls).

26 Aug 2008, 17:03

pal

Joined: 26 Aug 2008
Posts: 227

pal 26 Aug 2008, 17:39

Ahh, I was wondering if it wouldn't work on Windows. Cheers for the quick replies. So if I compile it to be a COM application will it work then? Basically I want to be able to do things APIs can do eg. set the cursor position; but without the APIs.

Also a quick question; if I decompile for example kernel32.dll with a program such as OllyDbg; can I view the code for certain API's such as the actual code which is executed when SetCursorPos API is called (even though SetCursorPos API is in user32.dll, but you get what I mean)?

26 Aug 2008, 17:39

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 26 Aug 2008, 18:05

You'll find a bunch of things that are Windows version specific. If you are writting for the Win32 subsystem then use the APIs of that subsystem (user32.dll, kernel32.dll, gdi32.dll, etc).

BTW, note that SetCursorPos refers to the MOUSE cursor, not the blinking vertical or horizontal bar on the text console.

Follow with OllyDbg the code below but remember that what you'll see is specific to your Windows version, there could be differences even on the same Windows version but with different ServicePack.

Code:

include 'win32a.inc'

format PE GUI 4.0

  push 0
  push 0
  call dword [SetCursorPos]

; Now the mouse pointer should be placed in the upper left corner of the screen

  push 0
  call dword [ExitProcess]

data import
  library kernel32,'KERNEL32.DLL',\
          user32,'USER32.DLL'

  import kernel32,\
                  ExitProcess, 'ExitProcess'
  import user32,\
                  SetCursorPos, 'SetCursorPos'
end data

26 Aug 2008, 18:05

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20453
Location: In your JS exploiting you and your system

revolution 26 Aug 2008, 18:05

You can disassemble any of the system DLLs. But I doubt you will gain much knowledge from them unless you want to spend many many hours in deciphering the code. They are very complex and do many things. Also you need to keep in mind that the DLLs don't always do the work, there is another layer behind there, the heart of the OS, that is not user accessible.

26 Aug 2008, 18:05

pal

Joined: 26 Aug 2008
Posts: 227

pal 26 Aug 2008, 20:35

Yeah I see I see. Alright cool cheers for the help guys.

26 Aug 2008, 20:35

ChrisLeslie

Joined: 04 Jun 2006
Posts: 50
Location: Australia

ChrisLeslie 27 Aug 2008, 05:12

If only we could do those sorts of things without the APIs Evil or Very Mad

27 Aug 2008, 05:12

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