Multiplication Problems in MASM32

Question

Hi. I am currently using MASM32 and am having some trouble multiplying things. I read the documentation and it makes no sense on why its not working.

    mov eax, input("X coordinate: ")
    mov ebx, input("Y coordinate: ")
    imul ebx, eax

    mov x, ebx
    print x

It should multiply the contents of ebx and eax and store the result in ebx , but it doesn't. Say you put in a 3 and a 6─all it prints is the 6.

Answer 1

According to Intel documentation, when multiplying two 32-bit registers, the result is stored in the register combination edx:eax , which represents the 64-bit product. This is implemented even in x86-64 processors, for compatibility reasons, because during the time of creation, Intel processors only supported 32-bits.

First of all, in your code, you are printing ebx and not eax , which was expected on your part even if you lack knowledge of Intel documentary. This is the reason why your code is only printing 6, as 6 was the multiplicand in ebx . However, to achieve the correct answer, you still must only print edx:eax .

So, to fix this, you must either shorten the size of the multiplier and multiplicand to 16 bits (recommended way), or, find a way to print the entirety of edx:eax (may prove difficult depending on library imports).

Hope this helps.