Pipeline

Question

Say Machine with 10ns click and takes 4 cycles for AlU and other operations.

When we talk about the time taken by pipelined processor it is = the highest time taken by a stage + overhead

We took the highest time only because we know that there is no stalling so CPI is 1 for pipelined.

When we say Cpi is 1 it means that:-

Suppose if there are n processes then first process will take K*Tp time where K is stage and Tp is highest duration and after that every process will take Tp time so on an average it takes CPI is 1 am i right?

 If not plz explain why then CPI is one in all cases when stalls aren't there.

And plz explain the cpi for non pipelined processor .

Comments

Well mam i saw the lectures they also gave the same reason as i stated above,but my question about: say instruction executing 4 cycles in an ALU is not bit clear they say when pipeline is in steady state instruction is executing per clock on an average so it means that for above case if i say alu takes 4 cycles it means in every cycle it passes through every stage and CPI is 1 isn't it. Mam please can you clear it out?

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An individual instruction takes 4 clock cycles only.
But in case of pipelining when we talk about a set of instructions say 100 or 1000 instructions then their average CPI is 1. The reason behind it is parallel work is done there. For ex. in a 4 stage pipeline, 1^st instruction is in execute stage, at the same time 2^nd instruction is in decode stage, the 3rd one is in fetch stage and so on. Means in a single clock cycle, all this work is going on simultaneously. So when we consider the overall time of all these instruction, Average CPI comes out to be 1 only. 

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Ohk mam got it