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CPI and the instruction count are considered in assessing the merits of a given
computer or equivalently in comparing the performance of two machines.
A different performance measure that has been given a lot of attention in recent
years is MIPS (million instructions-per-second (the rate of instruction execution
per unit time)), which is defined as
Instruction count
Execution time
Clock rate
CPI
MIPS
¼
10
6
¼
10
6
Example
Suppose that the same set of benchmark programs considered above
were executed on another machine, call it machine B, for which the following
measures were recorded.
Instruction
category
Percentage of
occurrence
No. of cycles
per instruction
ALU
35
1
Load & store
30
2
Branch
15
3
Others
20
5
What is the MIPS rating for the machine considered in the previous example
(machine A) and machine B assuming a clock rate of 200 MHz?
P
i
¼
1
CPI
i
I
i
38
1
þ
15
3
þ
42
4
þ
5
5
CPI
a
¼
Instruction count
¼
¼
2
:
76
100
10
6
Clock rate
CPI
a
200
MIPS
a
¼
10
6
¼
10
6
¼
70
:
24
2
:
76
P
i
¼
1
CPI
i
I
i
35
1
þ
30
2
þ
20
5
þ
15
3
CPI
b
¼
Instruction count
¼
¼
2
:
4
100
10
6
Clock rate
CPI
a
200
MIPS
b
¼
10
6
¼
10
6
¼
83
:
67
2
:
4
Thus MIPS
b
.
MIPS
a
.
It is interesting to note here that although MIPS has been used as a performance
measure for machines, one has to be careful in using it to compare machines
having different instruction sets. This is because MIPS does not track execution
time. Consider, for example, the following measurement made on two different
machines running a given set of benchmark programs.
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