source: bootcd/isolinux/syslinux-6.03/gpxe/src/arch/i386/include/bits/byteswap.h

#ifndef ETHERBOOT_BITS_BYTESWAP_H
#define ETHERBOOT_BITS_BYTESWAP_H

FILE_LICENCE ( GPL2_OR_LATER );

static inline __attribute__ ((always_inline, const)) uint16_t
__bswap_variable_16(uint16_t x)
{
        __asm__("xchgb %b0,%h0\n\t"
                : "=q" (x)
                : "0" (x));
        return x;
}

static inline __attribute__ ((always_inline, const)) uint32_t
__bswap_variable_32(uint32_t x)
{
        __asm__("xchgb %b0,%h0\n\t"
                "rorl $16,%0\n\t"
                "xchgb %b0,%h0"
                : "=q" (x)
                : "0" (x));
        return x;
}

static inline __attribute__ ((always_inline, const)) uint64_t
__bswap_variable_64(uint64_t x)
{
        union {
                uint64_t qword;
                uint32_t dword[2]; 
        } u;

        u.qword = x;
        u.dword[0] = __bswap_variable_32(u.dword[0]);
        u.dword[1] = __bswap_variable_32(u.dword[1]);
        __asm__("xchgl %0,%1"
                : "=r" ( u.dword[0] ), "=r" ( u.dword[1] )
                : "0" ( u.dword[0] ), "1" ( u.dword[1] ) );
        return u.qword;
}

#endif /* ETHERBOOT_BITS_BYTESWAP_H */