$3.$ Indicate if the following modifications $($A$,$B$,$C$)$ will cause each of the three metrics $($three rightmost

columns$)$ to increase, decrease, or have no effect. Explain your reasoning

Assume the initial machine is pipelined. Also assume that any modification is done in a way that preserves

correctness and maintains efficiency, but that the rest of the machine remains unchanged.

Instructions$/$Program CPI $($Cycles$/$Instruction$)$ Circuit complexity

A

Replace the $2$ operand

ALU with a $3$ operand

one and add $3$ operand

register$-$register

instructions to the ISA

$($for example,

ADD rs$1,$rs$2,$rs$3,$rd

$)$

Decrease

The new instructions

will replace any two$-$

instruction sequence that

accomplished the same,

such as

ADD rs$1,$ rs$2,$ rd

ADD rs$3,$ rd$,$ rd

The same

The ALU will perform

the three$-$way addition in

one cycle. Also

acceptable increase

because more RAW

Increase

Three$-$operand ALU is

more complex than a

two$-$operand one

B

Use the same ALU for

instructions and for

incrementing the PC

by $4$

The same

No difference to

instructions

Increase

All instructions now

use the same ALU

ALU operations now

have to stall

Decrease

Now there is one less

adder$/$ALU cycle

C

Increase the number of

user registers from $32$

to $64$

The same or decrease

If the more registers

enable the Compiler to

avoid loads and stores,

Decrease. Otherwise

the same.

The same

Does not affect the

actions of each

instructions

Increase

More registers

complicate the register

file