Slide 3.6: The classic CPU performance equation

The Classic CPU Performance Equation

The basic performance equation can therefore be written in terms of

instruction count (the number of instructions executed by the program),
CPI (clock cycles per instruction), and
clock cycle time.


   CPU times = Instruction count × CPI × Clock cycle time

or, since the clock rate is the inverse of clock cycle time:

   CPU times = ( Instruction count × CPI ) / Clock rate

Computing CPU Time
Consider two different implementations, P1 and P2, of the same instruction set architecture. There are three classes of instructions, A, B, and C. The clock rate and CPI of each implementation are given in the following table:

	Clock Rate	CPI Class A	CPI Class B	CPI Class C
P1	2.0 GHz	1	2	3
P2	1.5 GHz	2	2	2

Given a program with 10⁴ instructions divided into classes as follows: 30% class A, 30% class B, and 40% class C, which implementation is faster?