Pipelining

Question 1

What is a pipeline stall?

Answer 1

When a pipelining steps fails and must be repeated, causing a stall in the instructions behind it, and a gap between it and the instruction ahead

Question 2

What causes pipeline stalls?

Answer 2

Control dependencies and data dependencies

Question 3

What are the types of data dependencies?

Answer 3

RAW (true), WAW, WAR

Question 4

What is a WAW?

Answer 4

when two instructions write to the same register, the second one must be written second, otherwise subsequent reads to that address will see the wrong value

Question 5

What is a WAR?

Answer 5

When an instruction writes over an address read by another instruction, the write must happen second

Question 6

What is a hazard? Who causes them?

Answer 6

When a dependency in the pipeline can cause an incorrect execution. This is a property of the processor

Question 7

Are dependencies hazards?

Answer 7

not necessarily, they are a property of the program.

Question 8

What are our options when we detect a hazard?

Answer 8

1. flush dependent instructions (control deps only)
2. stall dependent instructions
3. Fix values read by dependent instructions (“forward”, and doesn’t always work)

Question 9

What is the relationship between number of stages and performance?

Answer 9

More stages == higher CPI (more hazards), but lower cycle time (less work per stage)