Consider a program with four classes of instructions A B C a

Consider a program with four classes of instructions A, B, C and D. Refer to following table for the data. Can we achieve a 2x overall speedup by only improving the CPI of instruction C? If so. please calculate the new CPI of the instruction C. Show all the steps. 2 Still based on the data shown in the table, can we achieve a 3x overall speedup by only improving the CPI of instruction C? If so, please calculate the new CPI of the instruction C. Show all the steps.

Solution

CPI = .2+.4+5+.4 = 6

1.

speedup = old cpi/new cpi

2 = 6/new cpi

new cpi = 6/2 = 3

1*.2 + 2*.2 + x*.5 + 4*.1 = 3

.2 + .4 + .5x + .4 = 3

.5x = 3-1

x = 2/.5 = 4

so if you want 2x speed up then new CPI of instruction C will be 4 instead 10.

2.

If you want 3x speed up then

3 = 6/new cpi

new cpi = 6/3 = 2

1*.2 + 2*.2 + x*.5 + 4*.1 = 2

.2 + .4 + .5x + .4 = 2

.5x = 2-1

x = 1/.5 = 2

so if you want 3x speed up then new CPI of instruction C will be 2 instead 10.

Instruction	CPI	%	CPI(i)
A	1	20	.2
B	2	20	.4
C	10	50	5
D	4	10	.4