Fundamentals of Quantitative Design and Analysis - Computer Architecture: A Quantitative Approach - page 52

and overall CPI as
The later form of the CPI calculation uses each individual CPI i and the fraction of occur-
rences of that instruction in a program (i.e., ICi i ÷ Instruction count). CPI i should be measured
and not just calculated from a table in the back of a reference manual since it must include
pipeline effects, cache misses, and any other memory system ineiciencies.
Consider our performance example on page 47, here modified to use measurements of the
frequency of the instructions and of the instruction CPI values, which, in practice, are obtained
by simulation or by hardware instrumentation.
Example
Suppose we have made the following measurements:
Frequency of FP operations = 25%
Average CPI of FP operations = 4.0
Average CPI of other instructions = 1.33
Frequency of FPSQR = 2%
CPI of FPSQR = 20
Assume that the two design alternatives are to decrease the CPI of FPSQR to 2
or to decrease the average CPI of all FP operations to 2.5. Compare these two
design alternatives using the processor performance equation.
Answer
First, observe that only the CPI changes; the clock rate and instruction count re-
main identical. We start by finding the original CPI with neither enhancement: