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If our pipeline has 8 stages and looks like this:

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

and we want to compute the needed stalls in case of a conflict with forwarding and register bypassing after an ALU instruction. The formula is p_EX - p_ID - 1. What do I now insert for p_EX? 3 since thats the first EX stage or 6 since that is the last one? What if it has several IF etc. stages? Do I have to take the last or the first stage into account for the formula?

Thanks in advance and best regards
in * TF "Emb. Sys. and Rob." by (1.2k points)

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p_EX is defined as the pipeline stage where ALU results are available. In your case it is stage 6 and not 3, so that your calculation should be

p_EX - p_ID - 1 = 6 - 2 - 1 = 3

The formula also applies if there are further IF stages, and even if there are several ID stages (you then take the last stage for p_ID).
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Thank you very much, now everything is clear :)

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