Question

I’ve just started programming in Assembly for my computer organization course, and I keep getting an operand size conflict error whenever I try to compile this asm block within a C program.

The arrayOfLetters[] object is a char array, so shouldn’t each element be one byte? The code works when I do mov eax, arrayOfLetters[1], but I’m not sure why that works, as the eax register is 4 bytes.

#include <stdio.h>
#define SIZE 3

char findMinLetter( char arrayOfLetters[], int arraySize )
{
    char min;

    __asm{
        push eax
        push ebx
        push ecx
        push edx
        mov dl, 0x7f        // initialize DL

        mov al, arrayOfLetters[1] //Problem occurs here

        mov min, dl        // read DL
        pop edx
        pop ecx
        pop ebx
        pop eax
    }

    return min;
}

int main()
{
    char arrayOfLetters[ SIZE ] = {'a','B','c'};

    int i;

    printf("\nThe original array of letters is:\n\n");
    for(i=0; i<SIZE; i++){
        printf("%c ", arrayOfLetters[i]);
    }
    printf("\n\n");

    printf("The smallest (potentially capitalized) letter is: %c\n", findMinLetter( arrayOfLetters, SIZE ));

    return 0;
}

Answer 1

Use mov al, BYTE PTR arrayOfLetters[1].

You can compile the code with MSVC using cl input.c /Faoutput.asm to get an assembly printout – this would show that simply using arrayOfLetters[1] translates to DWORD PTR and you need to explicity state you want a BYTE PTR.