Consider a cached memory system with tm = 300ns and tc = 50ns. Suppose that, on

Question

Consider a cached memory system with tm = 300ns and tc = 50ns. Suppose that, on the average, an instruction requires 1.3 memory accesses. The typical instruction execution time exclusive of memory accesses is 175ns.

(a) Calculate the average instruction execution time (i.e., execution + memory access) with no cache.

(b) Calculate the average instruction execution time with each of the following cache hit ratios (assume a read through policy, that is, upon a cache miss the data is directly supplied to the CPU as well as the cache) : h = 0.90 h = 0.95 h = 0.98

Explanation / Answer

Solution:

a)

Average instruction execution time= 175+(1.3*300)= 565 ns

b)

when the hit ratio is 0.9 then

Average instruction execution time= 175+1.3*((0.9*50)+(0.1*300))= 272.5 ns

when the hit ratio is 0.95 then

Average instruction execution time= 175+1.3*((0.95*50)+(0.05*300))= 256.25 ns

when the hit ratio is 0.98 then

Average instruction execution time= 175+1.3*((0.98*50)+(0.02*300))= 246.5 ns

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