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majidkamali1370



Joined: 31 Oct 2010
Posts: 50
Location: Iran
majidkamali1370
Hi.
I recently found a C program that draws pixels on the screen.
I wrote (converted) that program with FASM but it has some errors and causes CPU exceptions but I don't know what is the problem. please check.
Thanks Smile


Description: Parameters are pushed to stack left to right.
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Filename: pixel.asm
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Post 29 Dec 2011, 14:17
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17476
Location: In your JS exploiting you and your system
revolution
When you use call the CPU will push the return address on the stack so you must account for that when trying to read the parameters back off the stack.
Code:
             push %                  ; X = current loop counter
          push %                  ; Y = current loop counter
          push %                  ; color = current loop counter
              call plot_pixel_slow    ; call Draw function --->(CPU pushed the return address)
;...
plot_pixel_slow:
 pop ax                  ; Move color to al --->(pops the return address)
 mov ah, WRITE_DOT       ; Move 0x00 to ah
   pop dx                  ; Y position of pixel --->(pops the color)
       pop cx                  ; X position of pixel --->(pops the Y value)
     int VIDEO_INT           ; Call interrupt function
   ret ;--->ret returns to the X value!    
Post 29 Dec 2011, 14:31
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majidkamali1370



Joined: 31 Oct 2010
Posts: 50
Location: Iran
majidkamali1370
I didn't understand. You mean I use two pop instructions to get function parameters, because first is waste?
Code:
push %                  ; color = current loop counter
                call plot_pixel_slow    ; call Draw function --->(CPU pushed the return address)
;...
plot_pixel_slow:
        pop ax                  ; Move color to al --->(pops the return address)
        pop ax                  ; <-- You mean this?
        mov ah, WRITE_DOT       ; Move 0x00 to ah
        pop dx                  ; Y position of pixel --->(pops the color)     
Post 29 Dec 2011, 16:45
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17476
Location: In your JS exploiting you and your system
revolution
You have no choice, the CPU will push the return address so you must find a way to skip that to access your parameters. Usually the BP register can be used with the ret n instruction.
Code:
     push    %               ; X = current loop counter
  push    %               ; Y = current loop counter
  push    %               ; color = current loop counter
      call    plot_pixel_slow ; call Draw function --->(CPU pushes the return address)
;...
plot_pixel_slow:
 push    bp              ; save bp
   mov     bp,sp
       mov     ax,[bp+4]       ; Move color to al
  mov     ah, WRITE_DOT
       mov     dx,[bp+6]       ; Y position of pixel --->(pops the color)
       mov     cx,[bp+8]       ; X position of pixel --->(pops the Y value)
     int     VIDEO_INT
   leave
       ret     6               ; pop all parameters off the stack and return    
Post 29 Dec 2011, 17:17
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majidkamali1370



Joined: 31 Oct 2010
Posts: 50
Location: Iran
majidkamali1370
Thank you, but would you explain your code please? why you add 4 with bp for the first parameter?
BTW, the screen closes fast. How can I stop it in a period of time or wait for key press?
Thanks Very Happy
Post 29 Dec 2011, 19:14
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17476
Location: In your JS exploiting you and your system
revolution
majidkamali1370 wrote:
why you add 4 with bp for the first parameter?
Because there are two other things on the stack 1) the return address and 2) the saved BP register value.
Post 29 Dec 2011, 19:25
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majidkamali1370



Joined: 31 Oct 2010
Posts: 50
Location: Iran
majidkamali1370
Thank you. Laughing
Post 29 Dec 2011, 19:37
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