Question

I’m trying to convert some c code to assmebly, and I need some help.

char encode(char plain){
    __asm{
        mov eax, plain
        add eax, 2      
        ret
    }
    //C code
    /*
    char code;
    code = plain+2;

    return code;*/
}

First problem is that visual studio complains that the register size doesn’t match, i.e. eax is too small/large for char. I was under the impression that they were both DWORDs. Also, if I leave the variable in eax, and ret in assembly, it’ll actually return that variable, right?

Answer 1

Yes, at least with most C compilers on x86 (though it’s not theoretically guaranteed) whatever you put in [[e]a]x will be treated as a 1/2/4 byte return value.

No, char won’t (normally) be a dword — it’ll be a byte.

char encode(char plaintext) { 
    __asm { 
        mov al, plaintext
        add al, 2
        leave
        ret
    }
}