[solved] DIV problem

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rc

Joined: 03 Nov 2019
Posts: 55
Location: Germany

rc 16 Mar 2020, 20:46

Hello,

can somebody tell me why this produces an integer overflow when i do the division?

Code:

format PE
entry main
section '.code' code readable executable

main:
        int3
        mov eax, 4
        mov ecx, 2
        div ecx
        push eax

4 is divided by 2, that shouldn't be a problem? Or am i missing something?

16 Mar 2020, 20:46

Feryno

Joined: 23 Mar 2005
Posts: 514
Location: Czech republic, Slovak republic

Feryno 16 Mar 2020, 20:54

but what is the input value in edx ?
if input edx 2 or more the result cannot fit into 32 bit register
try this:

Code:

mov eax,4
xor edx,edx

16 Mar 2020, 20:54

rc

Joined: 03 Nov 2019
Posts: 55
Location: Germany

rc 16 Mar 2020, 21:01

Oh.. i guess i missunderstood div... ?

Code:

mov ecx, 2
div ecx

So "div ecx" in my case divides whatever is in edx by 2?
I thought div ecx divides whatever is in eax by ecx in this case. And the result gets stored back into eax.

[EDIT]

Oh now i understand. div always divides the 64 bits value accross EDX:EAX by a value. Didn't know that. Thank you!

16 Mar 2020, 21:01

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 17 Mar 2020, 17:02

Well, it is definitely unfortunate that I have still not recorded part 10 of my video tutorial, which I planned to be about MUL/DIV. My fault, but I had to postpone continuation of this series for various unrelated reasons.

17 Mar 2020, 17:02

rc

Joined: 03 Nov 2019
Posts: 55
Location: Germany

rc 17 Mar 2020, 18:53

Tomasz Grysztar wrote:

Well, it is definitely unfortunate that I have still not recorded part 10 of my video tutorial, which I planned to be about MUL/DIV. My fault, but I had to postpone continuation of this series for various unrelated reasons.

@Tomasz Grysztar:
Well, we all have stuff to do besides our hobbies Smile

.
I'm following your tutorial series since the beginning and hope you will find the time to continue soon. And i hope this series is going to be a really long series Smile

. They are really helpful. Thanks for taking the time to teach the ones that are new to f/asm.

17 Mar 2020, 18:53

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