memcpy64.asm

global memcpy64, _memcpy64
section .text

memcpy64:
_memcpy64:
    push edi
    push esi
    call 0x33:.heavensgate
    pop esi
    pop edi
    ret

BITS 64
.heavensgate:
    mov rdi, [esp + 20]
    mov rsi, [esp + 28]
    mov rcx, [esp + 36]
    mov eax, edi
    mov rdx, rdi
    shr rdx, 32
    rep movsb
    retf

memcpy64.h

#include <stdint.h>

#ifdef __cplusplus
extern "C"
#endif
uint64_t memcpy64(uint64_t dest, uint64_t src, uint64_t n);