bswap on 64 bit register not working

[krieg]

Okay, so I am learning Assembly and I started with 32bit and bswap seemed to work fine.

Now I have this code

main.c

#include <stdio.h>
#include <stdlib.h>

extern int asm_proc(void);

void print_bits(unsigned long long bits) {

    for (int i = 0; i < sizeof(unsigned long long) * 8; i += 1) {

        printf("%i", (bits & 0x0000000000000001 ? 1 : 0));
        bits >>= 1;
        if((i+1) % 8 == 0) printf("\n");
    }
    printf("\n");
}

int main() {

    print_bits(asm_proc());

    return 0;
}

main.asm

BITS 64

section .text

global asm_proc

asm_proc:
    mov rax, 01h
    cpuid
    mov rax, 0ffffffffffffffffh
    bswap rax
    ret

but the bswap just sets rax to 0x0000000000000000.

Any help would be much appriciated. Thanks :)

[krieg]

Okay. After rewriting my code a few times and examining, I found the problem!

The return type here: extern int asm_proc(void); should have been long long (or just long on most compilers). I'm not sure exactly why this cause the probalems it did, but it is fixed now.

[rubberman]

Yes, on both 32 and 64-bit systems, usually an int is 32-bits, but in your 64-bit system, you need a 64-bit integer, which is a long in the 64-bit system, and long long in a 32-bit one. The long long distinction is currently irrelevant on 64-bit systems, but that may change to a 128 bit value in the future, so caveat (beware) programmer!

[krieg]

My system is a 64bit system. However I am using the MinGWx64 compiler on Windows 8.1 and apparently the sizes are off. I have since decided to use the stdint.h and inttypes.h headers.

Thanks for your reply.