Suppose the main memory access time of 200 cycles including all the miss handlin

Question

Suppose the main memory access time of 200 cycles including all the miss handling, the primary cache has 2 cycles access time and the miss rate per instruction of primary cache is 4%

a) What is the average memory access time (AMAT)?

b) The designer wants to add a second level cache to improve the performance. Suppose the second level cache has 20 cycles access time (including all the miss handing due to primary cache miss) and is large enough to reduce the miss rate to main memory to 0.5%, how much faster will be by adding such second level cache?

Explanation / Answer

a)

AMAT = Hit time + Miss rate × Miss penalty

Miss rate=4%=4/100=0.04

Hit Time = 200

Therefore: 2 + 0.04 x (200 x 2)=2 + 16 = 18

b)

Total CPI = Base CPI + main memory-stall cycles/instruction + secondary cache stall cycles/instruction

=> 1+0.5x100 + 0.04*20= 51.8

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