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Index > Windows > Another Question: Regarding The Output of Math Operations

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Misha06



Joined: 18 May 2009
Posts: 8
Misha06 19 May 2009, 05:38
Code:
Include '%fasminc%/win32ax.inc'
.data

Three   db 3
Four     db 4

xor eax,eax

ToPrint dd ?

.code 
        start:

               add al,[Three]
               add al,[Four]
               add al,30h 
               mov [ToPrint],eax

                invoke  MessageBox, NULL,[ToPrint],"Math", MB_OK
                invoke  ExitProcess,0
.end start 
    


I remember from Java that it was necessary to do some arithmetic- add a displacement value- before it was possible to output as a String. I am having some trouble doing this in FASM because I am unsure of the offset on UTF16 and I am generally unsure if this is the correct approach.
Post 19 May 2009, 05:38
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20487
Location: In your JS exploiting you and your system
revolution 19 May 2009, 05:43
You need to initialise your register first:
Code:
...
start:
mov eax,0 ;<--- start with a known value (can use 'xor eax,eax' if you want)
add al,[Three]
...    
There are some advanced instructions you can use also:
Code:
...
start:
movzx eax,[Three] ;<--- zero the upper part of EAX
...    
Post 19 May 2009, 05:43
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Misha06



Joined: 18 May 2009
Posts: 8
Misha06 19 May 2009, 05:56
That was what I originally tried to do, but every time I run the file Windows tells me the program has crashed.

Code:
 start:
              mov eax,0
              mov al,[Three]
              ...
    
Post 19 May 2009, 05:56
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Madis731



Joined: 25 Sep 2003
Posts: 2139
Location: Estonia
Madis731 19 May 2009, 06:00
I will help you out - the working code:
EDIT: Moving eax-zeroing to the correct place
Code:
include 'win32ax.inc'
.data

Three   db 3
Four     db 4

ToPrint dd ?

.code 
        start:
               xor eax,eax
               add al,[Three]
               add al,[Four]
               add al,30h 
               mov [ToPrint],eax

                invoke  MessageBox, NULL,ToPrint,"Math", MB_OK
                invoke  ExitProcess,0
.end start
    


Last edited by Madis731 on 19 May 2009, 07:06; edited 1 time in total
Post 19 May 2009, 06:00
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Misha06



Joined: 18 May 2009
Posts: 8
Misha06 19 May 2009, 06:05
Thank you for your time, but I believe thats what I originally had, and to my chagrin it gives me the same results Sad

Here, is a print screen of the crash that happens when I try to zero out eax register:
http://yfrog.com/e9crashj
Post 19 May 2009, 06:05
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20487
Location: In your JS exploiting you and your system
revolution 19 May 2009, 06:07
Oh, this also needs to be:
Code:
 invoke  MessageBox, NULL,ToPrint,"Math", MB_OK    
Note the removal of the square brackets.
Post 19 May 2009, 06:07
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Misha06



Joined: 18 May 2009
Posts: 8
Misha06 19 May 2009, 06:09
Oh thank you but, the results are still odd - I get something like "^?xv"
Post 19 May 2009, 06:09
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20487
Location: In your JS exploiting you and your system
revolution 19 May 2009, 06:10
The string in ToPrint needs to be null terminated. So setting the high part of EAX to zero gives you this.
Post 19 May 2009, 06:10
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Misha06



Joined: 18 May 2009
Posts: 8
Misha06 19 May 2009, 06:11
Okay, thank you now everything is working perfectly.
Post 19 May 2009, 06:11
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madmatt



Joined: 07 Oct 2003
Posts: 1045
Location: Michigan, USA
madmatt 19 May 2009, 06:25
One other little thing, the 'xor eax,eax' will never get executed in your data section, you should put that just below the 'start:' label.
Post 19 May 2009, 06:25
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Madis731



Joined: 25 Sep 2003
Posts: 2139
Location: Estonia
Madis731 19 May 2009, 06:56
EDIT: a little too late Smile
Post 19 May 2009, 06:56
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