Fundamentals of Quantitative Design and Analysis - Computer Architecture: A Quantitative Approach - page 51

CPI is computed as
This processor figure of merit provides insight into different styles of instruction sets and
implementations, and we will use it extensively in the next four chapters.
By transposing the instruction count in the above formula, clock cycles can be defined as IC
× CPI. This allows us to use CPI in the execution time formula:
Expanding the first formula into the units of measurement shows how the pieces fit togeth-
er:
As this formula demonstrates, processor performance is dependent upon three characterist-
ics: clock cycle (or rate), clock cycles per instruction, and instruction count. Furthermore, CPU
time is equally dependent on these three characteristics; for example, a 10% improvement in
any one of them leads to a 10% improvement in CPU time.
Unfortunately, it is difficult to change one parameter in complete isolation from others be-
cause the basic technologies involved in changing each characteristic are interdependent:
■ Clock cycle time —Hardware technology and organization
■ CPI —Organization and instruction set architecture
■ Instruction count —Instruction set architecture and compiler technology
Luckily, many potential performance improvement techniques primarily improve one com-
ponent of processor performance with small or predictable impacts on the other two.
Sometimes it is useful in designing the processor to calculate the number of total processor
clock cycles as
where IC i represents the number of times instruction i is executed in a program and CPI i
represents the average number of clocks per instruction for instruction i . This form can be used
to express CPU time as