Question

The code chain below is executed on a data forwarding, 5-stage pipelined processor. Which of the following instructions will encounter a stall? Write the instruction number along with the number of stall cycles that instruction experiences.

(Note the data forwarding)

1. add $4, $5, $6

2. add $7, $4, $5

3. sw $8, 0($7)

4. add $8, $8, $6

5. lw $6, 0($8)

6. add $10, $9, $8

Answer 1

	1	2	3	4	5	6	7	8	9	10
add $4, $5, $6	IF	ID	EX	MEM	WB						$4 is forwarded at the end of 3rd clock cycle
add $7, $4, $5		IF	ID	EX	MEM	WB					$7 is forwarded at the end of 4th clock cycle
sw $8, 0($7)			IF	ID	EX	MEM	WB
add $8, $8, $6				IF	ID	EX	MEM	WB			$8 is forwarded at the end of 6th clock cycle
lw $6, 0($8)					IF	ID	EX	MEM	WB		$6 is available at the end of 8th clock cycle
add $10, $9, $8						IF	ID	EX	MEM	WB

No instruction in the above code experience a stall cycle.