Pipelining

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Pipelining

Attempting to simultaneously do multiple instructions at once. The usual example is having many loads of laundry. Instead of Washing them Drying them Folding them, and repeating: we can speed up by immediately starting another load the moment we can begin another step:

Cycles	1	2	3	4	5
	W	D	F
		W	D	F
			W	D	F
				…	…

Issues:

Hazards:

• Structural - Resource conflict
• Data - 3 types
• Control - change in control flow

These issues arise when pipelined code can’t easily work one after another. Take the simple RAW example below:

Data Hazard

1. RAW (Read After Write) - Attempting to read from a register after writing to it. You need to wait for the write to finish.
   1. ADD R1, R2, R3
   2. SUB R5, R1, R2
      We are using R1 in the calculation for R5, so we cannot do them both simultaneously. The data won’t be ready until after the WRITE stage of instruction 1.
2. WAW (Write After Write) - The concern of an earlier write executing after a later write, leaving the wrong value in your register
   1. lw R1, R2, R3
   2. AND R1, R5, R6
      If AND executes much faster than lw, we might have to lw value in R1 after everything
3. WAR (Write After Read) - Rarely comes up in hardware, but can arise in code scheduling when moving things around.