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I study multicore pipelining and the diagrams are not UML sequence diagrams for instance

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Why not remake this diagram like an UML sequence diagram, would not that be more clear so that we can see both how time progresses as well as the data paths since I do not think it is clear distinction between data path and time?

In UML time goes downward and the data path goes right and left.

Can you comment or answer why pipelining generally is not illustrated with UML?

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Your illustration looks very much like a UML sequence diagram. –  Robert Harvey Jul 22 '13 at 3:39
    
@RobertHarvey Ok...But it's still not strictly UMl and I'd like to be clear about what's data and what's time. In this illustration data is just flowing back and forth and propagating, there is no easy way to follow time like an UML sequence diagram has. –  909 Niklas Jul 22 '13 at 7:28
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UML is a horrible tool for illustrating real-world processes. It is not suitable for anything else but making managers look important. As for timing, you must understand that all the pipeline stages are executed in parallel, at the same time, and for this reason your diagram is somewhat misleading. –  SK-logic Jul 22 '13 at 9:26
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I haven't seen a pipeline illustrated with a UML sequence diagram yet. I am not saying it is not possible (I think it is) but it would be a lot more complicated and abstract than your FBD which shows the actual architecture of the simplified processor.

The main problem I see with the sequence diagram is that all the stages happen in parallel and it could become really messy if you would try to illustrate things like forwarding or stalling in it.

Also with the FBD you can demonstrate how to build the processor. You can incrementally add instructions and see how the design evolves. You end up with quite a mess but if you followed from the beginning then it should be much easier to understand it.

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