Question

I’m having an issue with Visual Studio, it doesn’t recognise “byte ptr” in asm. Does anyone know what I should use instead? I use visual studio because I just wanted to do a little asm next to a c++ code to speed things up a little. So could anyone also check the code, because I’m not sure about the return (this is just a test). And does this work for in also for uint?

__declspec(naked) void bewerken(int letter)
{
    __asm
    {
        push ebp
        mov ebp, esp
        sub esp, 1      // ruimte maken voor 1 variabele van 1 byte
        mov byte ptr [eax], [ebp+8]
        mov esp, ebp
        pop ebp
        ret
    }
}

int _tmain(int argc, _TCHAR* argv[])
{
    char* bYte;
    int letter;
    printf_s("Voer een letter in:\n");
    letter = getchar();

    if (letter < 128)
    {
        __asm 
        {
            push letter
            call bewerken
            add esp, 4
            mov byte ptr bYte, eax
        }
    }
    else
    {
        printf_s("Te hoog.\n");
    }

    getchar();
    return 0;
}

note: “BYTE PTR” doesn’t compile either

Answer 1

There is no valid addressing mode mov whatever [eax], [ebp+8]

One can mov al, [ebp+8] or mov ax,[ebp+8] and mov eax,[ebp+8] and also mov rax,[ebp+0x132220], but also one can move an immediate to an address with immediate offset. That would be the only case, where explicit width attribute is needed: mov byte ptr 0x01,[ebp+8] at least that’s the way it worked in the era of debug.exe…

In every other case the width of the operation should be clear.