Write modulo result

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TmX

Joined: 02 Mar 2006
Posts: 841
Location: Jakarta, Indonesia

TmX 05 Sep 2012, 15:48

I tried to calculate the result of modulo operation, let's say 1234 mod 10

Code:

format PE console 4.0
entry main

include 'win32a.inc'

section '.data' data readable writeable
num1 dd 1234
num2 dd 10
fmt db "%d", 13, 10, 0

section '.code' code readable executable
main:
        xor edx, edx
        mov ebx, num1
       mov ecx, num2
       div ecx
     cinvoke printf, fmt, edx
    invoke ExitProcess, 0
       
section '.idata' import data readable
library kernel32,'kernel32.dll', msvcrt,'msvcrt.dll'
import kernel32, ExitProcess,'ExitProcess'
import msvcrt, printf, 'printf'

Why is the output of the code 4101638, not 4?

05 Sep 2012, 15:48

AsmGuru62

Joined: 28 Jan 2004
Posts: 1626
Location: Toronto, Canada

AsmGuru62 05 Sep 2012, 16:11

MOVE EAX,NUM1

05 Sep 2012, 16:11

TmX

Joined: 02 Mar 2006
Posts: 841
Location: Jakarta, Indonesia

TmX 06 Sep 2012, 06:08

Still yields the wrong result... :/

06 Sep 2012, 06:08

sinsi

Joined: 10 Aug 2007
Posts: 789
Location: Adelaide

sinsi 06 Sep 2012, 06:26

Brackets.

06 Sep 2012, 06:26

TmX

Joined: 02 Mar 2006
Posts: 841
Location: Jakarta, Indonesia

TmX 06 Sep 2012, 08:02

You mean like this?

Code:

xor edx, edx
mov eax, num1
mov ecx, num2
div ecx
cinvoke printf, fmt, [edx]

The output is 1234.
The reminder of a DIV operation is stored in EDX, right?
http://css.csail.mit.edu/6.858/2011/readings/i386/DIV.htm

06 Sep 2012, 08:02

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20341
Location: In your JS exploiting you and your system

revolution 06 Sep 2012, 08:05

num1 is a pointer. But we assume you actually want the number stored at num1, is that correct? If so then:

Code:

mov eax,[num1]

06 Sep 2012, 08:05

AsmGuru62

Joined: 28 Jan 2004
Posts: 1626
Location: Toronto, Canada

AsmGuru62 06 Sep 2012, 10:24

And brackets on num2 also.

06 Sep 2012, 10:24

TmX

Joined: 02 Mar 2006
Posts: 841
Location: Jakarta, Indonesia

TmX 06 Sep 2012, 15:13

Code:

xor edx, edx
mov eax, [num1]
mov ecx, [num2]
div ecx
cinvoke printf, fmt, edx
invoke ExitProcess, 0

works as expected. thanks Very Happy

06 Sep 2012, 15:13

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