memcpy类复制粘贴可改为8字节为单位的复制粘贴，效率高点

eric2013

两次4字节操作。

__PACKED_STRUCT T_UINT32_WRITE { uint32_t v; };
#define __UNALIGNED_UINT32_WRITE(addr, val)    (void)((((struct T_UINT32_WRITE *)(void *)(addr))->v) = (val))


__PACKED_STRUCT T_UINT32_READ { uint32_t v; };
#define __UNALIGNED_UINT32_READ(addr)          (((const struct T_UINT32_READ *)(const void *)(addr))->v)


/* Fast-copy data fragments to ETH-DMA buffer */
for ( ; len > 7U; dst += 8, frame += 8, len -= 8U) {
    __UNALIGNED_UINT32_WRITE(&dst[0], __UNALIGNED_UINT32_READ(&frame[0]));
    __UNALIGNED_UINT32_WRITE(&dst[4], __UNALIGNED_UINT32_READ(&frame[4]));
}

/* Copy remaining 7 bytes */
for ( ; len > 1U; dst += 2, frame += 2, len -= 2U) {
    __UNALIGNED_UINT16_WRITE(&dst[0], __UNALIGNED_UINT16_READ(&frame[0]));
}