source: bootcd/isolinux/syslinux-6.03/memdisk/memcpy.c

/*
 * memcpy.c
 */

#include <string.h>
#include <stdint.h>

void *memcpy(void *dst, const void *src, size_t n)
{
        const char *p = src;
        char *q = dst;
#if defined(__i386__)
        size_t nl = n >> 2;
        asm volatile ("cld ; rep ; movsl ; movl %3,%0 ; rep ; movsb":"+c" (nl),
                      "+S"(p), "+D"(q)
                      :"r"(n & 3));
#elif defined(__x86_64__)
        size_t nq = n >> 3;
        asm volatile ("cld ; rep ; movsq ; movl %3,%%ecx ; rep ; movsb":"+c"
                      (nq), "+S"(p), "+D"(q)
                      :"r"((uint32_t) (n & 7)));
#else
        while (n--) {
                *q++ = *p++;
        }
#endif

        return dst;
}