已知Cache命中率H=0.98，主存比Cache慢4倍，已知主存存取周期为200ns，则系统的平均访问时间是______ns。
- A.50
- B.53
- C.100
- D.125
正确答案及解析
正确答案
解析
本题考查计算机Cache的基础知识。高速缓冲存储器(Cache)最重要的技术指标是命中率。CPU在Cache中找到有用的数据被称为命中，当Cache中没有CPU所需的数据时(这时称为未命中)，CPU才访问内存。为了保证CPU访问时有较高的命中率，Cache中的内容应该按一定的算法替换。一种较常用的算法是“最近最少使用算法”(LRU算法)，它是将最近一段时间内最少被访问过的行淘汰出局。因此需要为每行设置一个计数器，LRU算法是把命中行的计数器清零，其他各行计数器加1。当需要替换时淘汰行计数器计数值最大的数据行出局。这是一种高效、科学的算法，其计数器清零过程可以把一些频繁调用后再不需要的数据淘汰出Cache，提高Cache的利用率。当新的主存块需要调入Cache并且它的可用空间位置又被占满时，需要替换掉Cache的数据，这就产生了替换策略(算法)问题。根据程序局部性规律可知：程序在运行中，总是频繁地使用那些最近被使用过的指令和数据。这就提供了替换策略的理论依据。替换算法目标就是使Cache获得最高的命中率。Cache替换算法是影响代理缓存系统性能的一个重要因素，一个好的Cache替换算法可以产生较高的命中率。主存的存取周期为200ns，主存比Cache慢4倍，则Cache的存取周期为50ns。Cache的命中率为0.98，则系统的平均访问时间是50×0.98+200×0.02=53ns。
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