Assembly.... masm... divide overflow.... HELP!!!!!

[Vesayen]

I don't know what causes this... I can not imagine. I get divide by 0 errors all the damn time, I can not for the life of me figure out why. I almost never use the division instruction(which is problematic as you might imagine!) because every time I do I get a divide by zero error.....

Now I don't get it ALL the time. For example if the first instruction in my program is:

mov ax, 1439
mov bx, 16
div bx

it will work

SOMETIMES I can even do:

mov ax, 1439
mov bx, 16
div bx
div bx
dix bx


and the error won't come till the third div bx. If I put this code in a procedure far down in my program, a divide by zero error is often caused by the first division.... for the love of all that is good and holy I can NOT figure this out, any ideas? Is this my computer not having enough memory? That makes no sense... WHAT THE HELL IS CAUSING THIS?

[Kuciwalker]

Divide underflow?

[Vesayen]

Divide OVERFLOW.

For the life of me I can not figure this out.

[Kuciwalker]

I thought you got underflow errors when you divided by a really small number...

[Vesayen]

I am getting a divide overflow error, trying to divide 1439 by 16, repetadly.

When I do that I get AX to be 59h and when I try to divide 59h by 16, I get a divide overflow... this makes no sense.

[Kuciwalker]

Try just dividing 59h by 16.

[Vesayen]

Works fine.

[Vesayen]

Any ideas why succesive divisions will not work? My laptop I am running this on is not great but pretty good,, it runs never winter nights....

[Atahualpa]

what registers does div modify?

[Ari Rahikkala]

"div bx" takes the unsigned int in the register pair dx:ax, divides it by the unsigned int in bx, puts the quotient in ax, and puts the remainder in dx. It raises #de in case the divisor is 0 or the quotient is too big to fit in the target register.

That's the way I understood it, anyway. I've never actually written asm in my life...

[Whoha]

so whats in dx during all this?

[Atahualpa]

Whoha raises the question!

[Whoha]

if dx starts at zero the first 3 divisions should "work" but thats probably not the case. a high enough dx should put greater then 65535 in ax and thats like your problem. any dx greater then 16 will generate this.

[Ari Rahikkala]

I can't see a way dx should become greater than div's source operand, though. If something else uses dx, sure, but as far as I know, the remainder should always be smaller than the divisor in a division

[Urban Ranger]

Run the program through a debugger. Put a breakpoint before the third DIV, and check the various register values.