Operand forwarding Made Easy Question

Question

Consider 4-stage (IF, ID, EX, WB) pipeline used to execute the following code. All instructions are spending are spending one cycle on all the stages but ALU instructions are spending 3 cycles on  3^rd  stage.

I1: LOAD R0, [2000]

I2: ADD R2, R0, R1

I3: SUB R3, R2, R1

Number of cycles are saved using operand forwarding over without operand forwarding is?

Can someone please explain by drawing the diagram?

 

Answer 1

Without Operand Forwarding:

$\begin{array} {|r|r|}\hline 1 & 2 & 3 & 4 & 5&6&7&8&9&10&11&12&13&14 \\\hline IF & ID & EX & WB \\ \hline & IF& -- & -- & ID & EX & EX & EX & WB &  &  & & &  \\ \hline  & & -- & --& IF& --& -- &-- & --& ID & EX & EX& EX& WB \\ \hline \end{array}$

With Operand forwarding:

Assuming Operand forwarding is from EX to EX stage

$\begin{array} {|r|r|}\hline 1 & 2 & 3 & 4 & 5&6&7&8&9&10\\\hline IF & ID & EX & WB \\ \hline & IF& ID &EX & EX & EX & WB&    \\ \hline  & & IF & ID& --& --& EX &EX& EX& WB  \\ \hline \end{array}$

So cycles saved = 14-10 = 4

Comments

@Abhrajyoti00 and @DebRC in the case ‘without operand forwarding’ how can I3 execute the fetch stage without previous instruction I2 moving to the next stage?

I am saying this based on the solution to the question GATE CSE 1999 

Please correct me if I am wrong

 

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@Chaitanya Kale Yes, your observation is correct. IF should start from Cycle 5 in case of I3. Although the ans won’t change. @DebRC Correction reqd.

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Is this okay now?