There Is an Instruction that Does Nothing

It's mnemonic is nop

• Other instructions can be turned into nops part way through the pipeline
  • Look at the diagram on page 303
  • Compare it with the diagram on page 316.
    • In particular notice the multiplexor leading into pipeline register ID/EX
    • This multiplexor can be set to put zeroes into all of the control signals stored in the pipeline instead of using the values calculated by the control unit
  • Now we need to consider what lasting effects an instruction can have
    1. it can write into a register
    2. it can write into memory
    3. it can write into the program counter
  • each of these outcomes requires a 1 on a control signal
  • if all the control signals are 0, none of these can happen
  • setting that multiplexor to replace the control signals with 0's will keep that instruction from making any lasting change to the state of the machine
• Why might we ever want to do that?
  • another way to solve the problem that forwarding addresses is to delay an instruction by a tick or two until the register in question can be read correctly
  • sometimes forwarding does not completely solve the problem it is designed to alleviate
  • we can complete the solution by delaying the instruction by a tick
  • the way this happens is to turn the instruction into a nop
  • at the same time the signal to write into the PC is turned off
  • since the program counter does not get changed, the last instruction fetched (that became a nop) will be fetched again
  • on the second try, the instruction completes normally, but one tick later than it otherwise would have
  • the book's metaphor for this is a bubble in the pipeline