Find the average access time of a computer system for read a

Find the average access time of a computer system for read and write requests. A write through procedure is used in the system, where 25 percent are memory write requests, access time for cache memory is 5ns and that of main memory is 8ns, and the hit ratio for read access is reported to be 0.8

Solution

Average memory access time = Time spend for read + Time spend for write

= Read time when cache hit + Read time when cache miss
+Write time when cache hit + Write time when cache miss

= 0.25 0.8 5 + 0.25 0.2 (8+5) (assuming hierarchical read from memory and cache as only simultaneous write is mentioned in question)
+ 0.2 0.8 8 + 0.2 0.2 8 (simultaneous write mentioned in question)

= 1 + 0.65 + 1.28 + 0.32 = 3.25 ns