The CPU Performance Equation (Cont.)

Computing CPU Time (Cont.)

  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



The Big Picture
The following table shows the basic measurements at different levels in the computer and what is being measured in each case. These factors are combined to yield execution time measured in seconds per program: 
   Time = Seconds / Program = ( Instructions / Program ) ×

    ( Clock cycles / Instruction ) × ( Seconds / Clock cycle )
When comparing two computers, you must look at all three components, which combine to form execution time.

Components of Performance Units of Measure
CPU execution time for a program Seconds for the program
Instruction count Instructions executed for the program
Clock cycles per instruction (CPI) Average number of clock cycles per instruction
Clock cycle time Seconds per clock cycle