Question

Show how the following four instructions will be executed within the MIPS pipeline. Also, show the forwarding paths needed. Use the graphical notation showing all stages of MIPS pipeline. Indicate all data dependencies. Which dependencies are data hazards that will be resolved via forwarding? Which dependencies are data hazards that will cause a stall? add $s3, $s4, $s6 sub $s5, $s5, $s2 lw $s7, 100 ($s5) add $s8, $s7, $s2

Answer 1

Solution:

Pipelining fiuure:

		1	2	3	4	5	6	7	8
ADD	S3,S4,S6	IF	ID	EXadd	MEM	WB
SUB	S5, S5,S2		IF	ID	EXsub	MEM	WB
lw	S7,100(s5)			IF	ID	EX­lw	MEM	WB
ADD	S8,S7,S2				IF	ID	EXlw	MEM	WB

The first forwadin is the value of s5 from EX_sub to EX_lw.

The second forwadin is the value of S7 from EX_lw. To EX_add

Data dependencies:

The third LW instruction is depends on second, since S5 is the result of second.

The last ADD instruction is depends on the load instruction for S7.

Data hazrds :

Data hazrds is the data values which are not available during durin decoding.

Both the above mentioned dependencies will resulr in data hazards if no forwardin is there.

Stall:

Both the above dependies will also result in stall, the ID step will not happen till the previous step execution happens,