MadeEasy Test Series 2018: CO & Architecture - Pipelining

Question

The following sequence of instruction is executed in a basic 5 stage pipelined processor (IF, ID, EX, MA and WB). Assume that data dependency present in the program is resolved by operand forwarding techniques. Load instruction output present in 4th stage and ALU instruction output is available in 3rd stage. Assume each stage take 1 cycle.

What is the number of instructions must be inserted to achieve CPI = 1 by using operand forwarding?

Comments

@Ashwin, here they are asking how many of the instructions(given) can be inserted into the pipeline to maintain CPI = 1.

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Shouldn't that be 1,  as you have also explained that split access is required only in case of 3rd instruction?

And the question asks "What is the number of instructions must be inserted to achieve CPI = 1 by using operand forwarding?"

So, we necessarily need only 1 instruction (before 3rd instruction) to facilitate operand forwarding.

5 could be the possible no of instructions among available ones that could do the job.

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One instruction would take 5 cycles since there are 5 pipeline stages. After the first instruction rest, all other instructions would take 1 cycle to complete( after every 1 cycle one instruction gets completed).

Now, this line " number of instructions must be inserted " means that how many of the given instructions can be inserted into the pipeline processor so that CPI is 1.  This is what I know, it might be wrong.

Answer 1

The ans is one. How ?

I1 :	IF	ID	EX	MA	WB						Ro is available after EX
I2:		IF	ID	EX	MA	WB					R4 is available after MA
Extra Instruction			IF	ID	EX	MA	WB
I3:				IF	ID	EX	MA	WB			R5 is available after EX
I4:					IF	ID	EX	MA	WB
I5 :						IF	ID	EX	MA	WB