说明：  FIFO设计的难点在于怎样判断FIFO的空/满状态。为了保证数据正确的写入或读出，而不发生益处或读空的状态出现，必须保证FIFO在满的情况下，不能进行写操作。在空的状态下不能进行读操作。怎样判断FIFO的满/空就成了FIFO设计的核心问题。
(FIFO design challenge is how to decide the FIFO empty/full. In order to ensure the correct data is written or read, or read the benefits without the risk of the state space, there must be to ensure FIFO in the full circumstances, not for writing. In the empty state can not be read. How to determine the FIFO full/empty FIFO design has become the core issue.)