Question

I don’t understand why this code

#include <iostream>
using namespace std;
int main(){
    int result=0;

    _asm{
        mov eax,3;
        MUL eax,3;
        mov result,eax;
    }

    cout<<result<<endl;
    return 0;
}

shows the following error.

1>c:\users\david\documents\visual studio 2010\projects\assembler_instructions\assembler_instructions.cpp(11): error C2414: illegal number of operands

Everything seems fine, and yet why do I get this compiler error?

Answer 1

Use:

imul eax, 3;

or:

imul eax, eax, 3;

That way you don’t need to worry about edx -register being clobbered. It’s “signed integer multiply”. You seem to have ‘int’ -result so it shouldn’t matter whether you use mul or imul.

Sometimes I’ve gotten errors from not having edx register zeroed when dividing or multiplying. CPU was Intel core2 quad Q9550

There’s numbingly overengineered but correct intel instruction reference manuals you can read. Though intel broke its websites while ago. You could try find same reference manuals from AMD sites though.

Update: I found the manual: http://www.intel.com/design/pentiumii/manuals/243191.htm

I don’t know when they are going to again break their sites, so you really always need to search it up.

Update2: ARGHL! those are from year 1999.. well most details are unfortunately the same.