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Index > Main > Bug that causes jmp to ecx?

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Tyler



Joined: 19 Nov 2009
Posts: 1216
Location: NC, USA
Tyler
Here's my code. I'm executing it in Qemu and it aborts because my code tries to execute an uninitialized address. I noticed in the reg dump that EIP always equals ECX when it aborts, but I can't find the relation.

Code:
format binary as 'bin'
org 7C00h
use16

   jmp       main
   nop

main:
   mov       ebx,10*18

   call      delay

   hlt


; ebx = seconds to wait / 18.2
delay:
   pusha
   mov       ebp,esp
   sub       esp,4
   label     .lo word at ebp - 2
   label     .ho word at ebp - 4

   mov       eax,0
   int       1ah

   add       edx,ebx

   mov       [.lo],dx
   mov       [.ho],cx

   .longer:
      mov       eax,0
      int       1ah

      cmp       [.ho],cx
      ja        .done

      cmp       [.lo],dx
      ja        .done

      cmp       al,1
      jne       .longer

   .done:

   mov       esp,ebp
   popa

   ret

times 510-($-$$) db 0
dw 0AA55h
    
Post 03 Dec 2010, 03:28
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baldr



Joined: 19 Mar 2008
Posts: 1651
baldr
Tyler,

Under Bochs it works as written. Pun intended: that code looks like weird mix of 16/32-bit instructions (e.g. cx isn't high word of edx as one may suppose looking at it).

Probably QEMU stumbles upon this. I'm using Bochs because it (1) is x86-centric, and (2) supports debugging natively.
Post 03 Dec 2010, 10:50
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Tyler



Joined: 19 Nov 2009
Posts: 1216
Location: NC, USA
Tyler
Okay, so I'll try to change it to the 16bit alternatives. Bochs is okay, but... I prefer Qemu. Not sure why, maybe because it's more Linux-centric.
Post 06 Dec 2010, 21:56
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