Greatest Common Divisor (GCD)

[0x4B47]

Hi forum,
I was trying to implement the GCD algorithm in Assembly, but something seems to be going wrong when the execution phase reaches the 'div' instruction. I get an integer overflow error but I dont see why!?! Below is my GCD proc.
;;;;;;;;;;;;;
.data?
x dword ?
y dword ?
;;;;;;;;;;;;;;;;;;
GCD proc uses eax ebx edx
mov eax, 0
mov edx, 0
do:
cmp y, 0
jna finish
mov eax, x
mov ebx, y
div ebx ; this is the troublesome line!!
mov x, ebx
mov y, edx
jmp do
finish:
ret
GCD endp
;;;;;;;;;;;;;;;;;;;;;;;;;;;;
The EDX reg contained 7FFE0304 at the start of the program. just before calling the GCD proc EDX contains 00405014. Using the msdev debugger, when i step through the code it stops at the div instruction with unhandled exception integer overflow error 0xC000095. Just after the error edx contains 00000004. As input i entered 1000 and 12 for x and y respectively.
I'm clueless!
Can somebody plz help me???
Kunal.

 

[AirCon]

I used MASM, and ran your code. It works perfectly.
Variable y is 0, and it always jump to finish.

Are you using inline asm or ... ?
Did you see what is the value of x & y ?

Try to swap your code so you can see what is x and y when you call it

do:
mov eax, x ; eax = ??
mov ebx, y ; ebx = ??
cmp y, 0
jna finish


-- AirCon --

 

[AirCon]

Sorry, I misread your message.

You mentioned that X=1000, Y=12
So I ran again and I see the problem. The problem is EDX must be zero before DIV

GCD proc uses eax ebx edx
do:
cmp y, 0
jna finish
mov eax, x
mov ebx, y
mov edx, 0 ; edx must be 0 before division
div ebx ; now it works
mov x, ebx
mov y, edx
jmp do
finish:
ret
GCD endp


-- AirCon --