说明：  采用LRU替换算法。这种算法选择最久没有被访问的块作为被替换的块。 为了实现LRU算法，要在块表中为每一块设置一个计数器（cnt0，cnt1，cnt2，cnt3，）。计数器的长度为2位。
(using LRU replacement algorithm. This algorithm to choose the most long visit is not being replaced as a block by block. To achieve LRU algorithm, in block form for each one to set up a Counter (cnt0, cnt1. cnt2, cnt3,). To counter the length of two.)