For the following questions, assume that for a given program 70% of the executed

Question

For the following questions, assume that for a given program 70% of the executed instructions are arithmetic, 10% are load/store, and 20% arc branch. a. Given tins instruction mix and the assumption that an arithmetic instruction requires two cycles, a load/store instruction takes six cycles, and a branch instruction takes three cycles, find the average CPI. b. For a 25% improvement in performance, how many cycles, on average, may an arithmetic instruction take if load/store and branch instructions are not improved at all? c. For a 50% improvement in performance, how many cycles, on average, may an arithmetic instruction take if load/store and branch instructions are not improved at air?

Explanation / Answer

a)

Average CPI can be calculated as :

( % of Arithmetic inst. )*2 + (% of load inst.)*6 + (% of branch inst. )*3 = 0.7*2 + 0.1*6 + 0.2*3 =       1.4 + 0.6 + 0.6    = 2.6 cycles per instruction on average

i.e. CPI = 2.6

b)

For 25% improvement, CPI should now be 2.6*0.75 ( improved by 25%, left is 75% of actual 2.6 ) i.e. CPI = 1.95

Average CPI = 1.2 + (0.7)*( average cycles arithmetic instructions take ) = 1.95

i.e.    average cycle arithmetic instructions should take = 1.0714

c)

For 50% improvement in CPI,

Average CPI = 1.3 = 1.2 + (0.7)*( average cycles arithmetic instructions take )

i.e.    average cycle arithmetic instructions should take = 0.1428

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