ASM x86 integer overflow

Question

GetLCM PROC
tryAgain:
   mov bx, 0
   inc Multiple
   mov ax, UserInputNum1              ;Move UserInputNum1 to the 16 bit Register
   mov bx, Multiple
   div bx                             ;<-------Error here
   cmp dx,0                           ;If dx is not zero then there is a remainder
   jne tryAgain                       ;If not equal jump
ret
GetLCM ENDP

I keep getting a integer overflow error when trying to div these two numbers. It breaks when i try to use div.

I am trying to find the least common multiple of two numbers entered by the user. I decided to try to divide each number to find prime numbers first then compare each time one is found to the other number the user entered. Of course I would be doing both numbers at the same time, but as of now I am trying to just get past this error.

This is only my fifth week in ASM so I am a little confuse why this is happening. Also I would like to add that I would also like to make this work for 32bit numbers as well. My idea is to use pointers. Is this a good way to do this? Thanks for any help!

Just an algorithmic remark: `lcm(a,b) = a*b/gcd(a,b)`, that's _much_ faster than finding primes dividing either of the numbers. — Daniel Fischer, Nov 15 '12 at 03:39
In order to do assembly, you need to have your architecture's manuals handy, e.g. [Intel's x86 Instruction Set Reference](http://download.intel.com/design/intarch/manuals/24319101.pdf). 16-bit `DIV` works on `DX:AX`. Your `DX` has not been set. To do 32-bit, just reference 32-bit register mnemonics, `EAX`, `ECX`, etc. — A. Webb, Nov 15 '12 at 03:39
Again someone who forgot that `div` is double-width. Apparently it doesn't show up in the suggested questions when asking about it. Can't something be done about that? — harold, Nov 15 '12 at 09:10

score 5 · Answer 1 · answered Nov 15 '12 at 03:37

5

xor dx, dx somewhere before your div

answered Nov 15 '12 at 03:37

Gunner

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Thanks I forgot to initialize dx to 0 – James Leshko Nov 15 '12 at 03:54

score 2 · Accepted Answer · answered Nov 15 '12 at 03:37

2

Read up on the div instruction again. What is divided by what? You are missing to set one of the operands to some sensible value.

answered Nov 15 '12 at 03:37

Jens Björnhager

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ASM x86 integer overflow

2 Answers2

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