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2.5 / PERFORMANCE ASSESSMENT 51

quartz
crystal

analog to
digital

From Computer Desktop Encyclopedia, conversion
1998, The Computer Language Co.

Figure 2.14 System Clock

it takes some finite amount of time for the voltage levels to settle down so that an
accurate value (1 or 0) is available. Furthermore, depending on the physical layout
of the processor circuits, some signals may change more rapidly than others. Thus,
operations must be synchronized and paced so that the proper electrical signal
(voltage) values are available for each operation.
The execution of an instruction involves a number of discrete steps, such as
fetching the instruction from memory, decoding the various portions of the instruc-
tion, loading and storing data, and performing arithmetic and logical operations.
Thus, most instructions on most processors require multiple clock cycles to com-
plete. Some instructions may take only a few cycles, while others require dozens. In
addition, when pipelining is used, multiple instructions are being executed simulta-
neously.Thus, a straight comparison of clock speeds on different processors does not
tell the whole story about performance.

INSTRUCTION EXECUTION RATE A processor is driven by a clock with a constant
frequency f or, equivalently, a constant cycle time , where t = 1/ft . Define the in-
struction count, I , for a program as the number of machine instructions executed
c
for that program until it runs to completion or for some defined time interval. Note
that this is the number of instruction executions, not the number of instructions in
the object code of the program. An important parameter is the average cycles per
instruction CPI for a program. If all instructions required the same number of clock
cycles, then CPI would be a constant value for a processor. However, on any give
processor, the number of clock cycles required varies for different types of instruc-
be the number of cycles re-
tions, such as load, store, branch, and so on. Let CPI i
quired for instruction type i. and I be the number of executed instructions of type i
i
for a given program.Then we can calculate an overall CPI as follows:
n (CPI * I )
CPI = ai=1 i i (2.1)
I c