说明：  FIFO的宽度：也就是英文资料里常看到的THE WIDTH，它只的是FIFO一次读写操作的数据位，就像MCU有8位和16位，ARM 32位等等，本程序实现8位的FIFO功能,三位格雷码可表示8位的深度。
(THE WIDTH of THE FIFO: namely information in English often see THE WIDTH, it is only a FIFO data read and write operations, as has 8 bit or 16 bit MCU, ARM 32-bit, etc., THE program achieve THE function of eight FIFO, three gray code can be expressed THE depth of THE eight.)