说明：  LRU置换算法是选择最近最久未使用的页面予以置换。该算法赋予每个页面一个访问字段，用来记录一个页面自上次被访问以来经历的时间T，当须淘汰一个页面时，选择现有页面中T值最大的，即最近最久没有访问的页面。这是一个比较合理的置换算法。
(LRU replacement algorithm is to select the page from lack of use the most recent to be replaced. The algorithm gives an access field in each page to record a page being accessed has experienced since the last time T, when required to phase out a page, select an existing page, the maximum value of T, that is not a recent visit to the oldest page. This is a reasonable replacement algorithm.)