1And in Conclusion¶
One of the costs of pipelining is that it introduces pipeline hazards. Hazards, generally, are issues with something in the CPU’s instruction pipeline that could cause the next instruction to execute incorrectly.
We discuss three types of hazards:
Structural hazards (insufficient hardware)
Data hazards (using outdated values in computation)
Control hazards (executing the wrong instructions).
2Textbook Readings¶
P&H 4.8, 4.10
3Additional References¶
4Exercises¶
4.1Short Exercises¶
Solution to Exercise 1 #
False. The addition of independent ports to the RegFile allows for multiple instructions to access the RegFile at the same time (such as one instruction reading values of two operands, while another instruction is writing to a return register). However, this does not work if two instructions are reading and writing to the same register at the same time. Some solutions to this data hazard could be to stall the latter instruction or to forward the read value from a previous instruction, bypassing the RegFile completely.
Solution to Exercise 2 #
True. The next instruction must wait for the previous instruction to finish EX, MEM, and WB, before it can begin its EX.
Solution to Exercise 3 #
False. Hazards following lw cannot be fully resolved with forwarding because the output is not known until after the MEM stage. We still need a stall in addition to forwarding from the MEM/WB pipeline register to the EX stage.
Solution to Exercise 4 #
False. There are other more advanced techniques such as branch prediction, which predicts which path the branch will take and flushes the pipeline if the prediction is wrong.