Pielining-Doubt

Question

Consider below sequence of instructions

$1.lw\,s2,0(s1)$

$2.lw\,s1,40(s6)$

$3.sub\,s6,s1,s2$

$4.add\,s6,s2,s2$

$5.or\,s3,s6,0$

$6.sw\,s6,50(s1)$

What is the number of clock cycles taken by above set of instructions when executed on 5-stage MIPS architecture(IF,ID,EX,MA,WB)

(a)Without operand forwarding

(b)With operand forwarding.

My Answers

(a)16 cycles

(b)11 cycles

Are my answers correct?

Source:https://cseweb.ucsd.edu/classes/wi12/cse141-a/solutions/assignment5-soln.pdf

Question 3, part 3 and 4.

Comments

@Ayush Upadhyaya

@Arjun sir said in one of this comments 
"Use split phase unless otherwise mentioned". 
I remember.

what that means?

That means if in question split phase mentioned then u no need to consider it in any case

right?

That means question itself if not mentioned split phase

then both with operand forwarding and without operand forwarding both will do without split phase

right? 

hope u got what I mean

---

@srestha, this is the actual comment of Arjun Sir,

https://gateoverflow.in/1391/gate2005-68?show=95621#c95621

"Use operand forwarding unless otherwise stated.

Use split phase access if data is ready- like between WB/RD and also when operand forwarding happens from EX-ID stages, but not from EX-EX stages. We cannot do split phase access between EX-EX, because there is no guarantee that the instruction execution can be done within a half phase.

Now, tell me any one previous year GATE question where this won't work."

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now plz chk

Answer 1

Without operand forwarding

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

Without operand forwarding (but we can minimize it with split phase)

---

With split phase without operand forwarding 

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

IF

ID

EX

MA

WB

 

Here Split Phase :WB-ID

And stalls occurs due to

1.lw s2,0(s1)

2.lw s1,40(s6)

3.sub s6,s1,s2

4.add s6,s2,s2

5.or s3,s6,0

6.sw s6,50(s1)

So, without operand forwarding=14 clock cycles

 

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With operand forwarding in EX-EX stage

IF	ID	EX	MA	WB
	IF	ID	EX	MA	WB
		IF	ID		EX	MA	WB
			IF		ID	EX	MA	WB
					IF	ID	EX	MA	WB
						IF	ID	EX	MA	WB

With operand forwarding=11 clockcycles

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Ref:https://gateoverflow.in/1391/gate2005-68

https://gateoverflow.in/186403/coa-pipeline

Comments

@Ayush Upadhyaya

You mean without split phase never occur in pipeline?

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chk this https://gateoverflow.in/102565/operand-forwarding-in-pipeline

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thank u very much it was great help