Question

A. If precise exception is not a functional requirement, would you expect the out of order execution engine to be easier or more difficult to implement. Justify your answer.

B. Why is precise exception a necessary requirement from a software programmer’s perspective?

C. You are trying to implement a physical register file with register renaming. How does your implementation requirement change if the front of the pipeline does not perform any branch prediction?

Answer 1

A.if the precise exception is not a functional requirement,then the out of order execution engine will be more difficult to implement since precise exceptions aid in software debugging,they enable easy recovery from exceptions (for example:Page faults) and they also enable easy restartable processes.These advantages of precise exception makes it more difficult to implement the out of order execution engine since it is considered that precise exception is not a functional requirement.

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B.Precise exception is a necessary requirement from a software programmer's perspective for the following reasons:

  When the oldest instruction ready-to-be-retired is detected to have caused an exception, the control logic

• Recovers architectural state (register file, IP, and memory)
• Flushes all younger instructions in the pipeline
• Saves IP and registers (as specified by the ISA)
• Redirects the fetch engine to the exception handling routine
• Vectored exceptions.

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C.When we try to implement a physical register file with register renaming. The implementation requirement change if the front of the pipeline does not perform any branch prediction in the following manner:

A branch misprediction resembles an “exception ”

• Except it is not visible to software
• Similar to exception handling except can be initiated before the branch is the oldest instruction
• All three state recovery methods can be used if the front of the pipeline does not perform any branch prediction.

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