说明：  异步FIFO是一种先进先出的电路，在异步电路中，由于时钟之间周期和相位完全独立，因而数据丢失概率不为零。如何设计一个高可靠性、高速异步的FIFO是一个难点，本代码介绍了一种解决方法。
(Asynchronous FIFO is a kind of advanced first out circuit, in asynchronous circuit, as the clock cycle and phase between full independence, thus data loss probability is not zero. How to design a high reliability, high speed asynchronous FIFO is a difficulty, this code introduced a kind of solution. )