说明：  FIFO的概念　FIFO： 　　一、先入先出队列(First Input First Output，FIFO)这是一种传统的按序执行方法，先进入的指令先完成并引退，跟着才执行第二条指令。
(FIFO, the concept of FIFO: a first FIFO (First Input First Output, FIFO) which is a traditional sequential method of implementation, first access to complete and retire the first instruction, followed by implementation of the second instruction only.)