# Is pipeline bubbling only used with RAW data hazards?

I am studying computer science at university and we have an exam, which consists of several problems, one of which has to deal with pipelines.

My understanding is that one effective way to solve data hazards is to use pipeline bubbling - adding NOPs in order to stall the pipeline long enough to finish the operation, from which we will fetch the result.

Where my problem lies is conflicting information, regarding when we use bubbling. My professor's lectures state that bubbling is used to solve data hazards, no exceptions, while Wikipedia states that bubbling is used only with the RAW data hazard.

We have had some examples, which have left me very confused.

For example, we have the following operations:

S1: ADD R2,2,R1 ; R1 = R2 + 2

S2: SUB R1,R3,R4 ; R4 = R1 - R3

S3: MULT R5,3,R3 ; R3 = R5 * 3

S4: MULT R3,2,R3 ; R3 = R3 * 2

I have identified the following data hazards:

1. RAW - S1 and S2, S3 and S4
2. WAR - S2 and S3
3. WAW - S3 and S4

I have been told that I need a minimum of two operations between data hazards, which means that I need to add two NOPs between them.

My question is this - do I add NOPs between the RAW data hazards only (between S1 and S2 and S3 and S4) or do I add two between the WAR hazard as well(S2 and S3)?