在主存和CPU之间增加Cache的目的是 (17) 。在CPU执行一段程序的过程中，Cache的存取次数为2250次，由内存完成的存取次数为250次。若Cache的存取周期为6ns，内存的存取周期为24ns，则Cache的命中率为 (18) ，CPU的平均访问时间为 (19) ns。
A．9.6
B．7.98
C．7.8
D．6.9
参考答案：C
解析： 以h表示Cache的命中率，t1表示 Cache的存取周期，t2表示内存的存取周期，则CPU的平均访问周期t3=h×t1+(1-h)×t2。本题中t3= 0.9×6+(1-0.9)×24=7.8m。
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