Dealing with ALU-ALU forwarding

Question

Consider two instruction sequences:

a. SW R16,-100(R6)
    LW R4, 8(R16)
    ADD R5,R4,R4
b. OR R1,R2,R3
    OR R2,R1,R3
    OR R1,R1,R2

Add NOP instructions to this code to eliminate hazards if there is ALU-ALU forwarding only (no forwarding from the MEM to the EX stage).

Comments

Part (a) will need 3 + 3 NOP instructions.

Part (b) will not require any NOP instructions.

Answer 1

a)

Instruction	$I_{1}$	$I_{2}$	$I_{3}$	$I_{4}$	$I_{5}$	$I_{6}$	$I_{7}$	$I_{8}$	$I_{9}$
SW R16,-100(R6)	IF	ID	EX	MEM	WB
LW R4, 8(R16)		IF	ID	EX	MEM	WB
ADD R5,R4,R4			IF	_NOP	_NOP	ID	EX	MEM	WB

 

Here $2$ NOP, for $2$ stalls. These stall are created due to no forwarding from the MEM to the EX stage.

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b)

Instruction	$I_{1}$	$I_{2}$	$I_{3}$	$I_{4}$	$I_{5}$	$I_{6}$	$I_{7}$	$I_{8}$	$I_{9}$
	IF	ID	EX	MEM	WB
		IF	ID	EX	MEM	WB
			IF	ID	EX	MEM	WB

 Due to ALU-ALU forwarding, no stall has been created