Execute and Writeback Decoupling Allows Program Restart
The queue for results is necessary to resolve issues such as branch mispredictions and exceptions/traps. The results queue allows programs to be restarted after an exception, which requires the instructions to be completed in program order. The queue allows results to be discarded due to mispredictions on older branch instructions and exceptions taken on older instructions.
The ability to issue instructions past branches which have yet to resolve is known as speculative execution.
Read more about this topic: Out-of-order Execution
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