Act 1 Computer Abstractions and Technology
Exercise 1
(1) <A4-A6> P55 1.5
Consider three dierent processors P1, P2, and P3 executing the same
instruction set. P1 has a 3GHz clock rate and a CPI of 1.5. P2 has a 2.5GHz
clock rate and a CPI of 1.0. P3 has a 4.0GHz clock rate and has a CPI of
2.2.
1. Which processor has the highest performance expressed in instruc-
tions per second?
2. If the processors each execute a program in 10 seconds, nd the
number of cycles and the number of instructions.
3. We are trying to reduce the execution time by 30%, but this leads
to an increase of 20% in the CPI. What clock rate should we have to get
this time reduction?
answer:
1.
performance of P1 (instructions/sec) = 3 × 10
9
/ 1.5 = 2 × 10
9
performance of P2 (instructions/sec) = 2.5 × 10
9
/ 1.0 = 2.5 × 10
9
performance of P3 (instructions/sec) = 4 × 10
9
/ 2.2 = 1.8 × 10
9
2.
cycles (P1) = 10 × 3 × 10
9
= 30 × 10
9
s
cycles (P2) = 10 × 2.5 × 10
9
= 25 × 10
9
s
cycles (P3) = 10 × 4 × 10
9
= 40 × 10
9
s
3.
No.instructions (P1) = 30 × 10
9
/ 1.5 = 20 × 10
9
No.instructions (P2) = 25 × 10
9
/ 1.0 = 25 × 10
9
No.instructions (P3) = 40 × 10
9
/ 2.2 = 18.18 × 10
9
CP I
new
= CP I
old
× 1.2, then CPI (P1) = 1.8, CPI (P2) = 1.2, CPI (P3)
= 2.6
f = No.instr. × CP I/time, then
f(P 1) = 20 × 10
9
× 1.8 / 7 = 5.14GHz
f(P 2) = 25 × 10
9
× 1.2 / 7 = 4.28GHz
1
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