Talk:Out-of-order execution

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[edit] Queue

Isn't the point of a queue to have something first in first out? FIFO?. If the instruction is fetched and decoded and stuck in a queue, wouldn't that imply some kind of order of instruction? Or is it more like a priority queue? I'm kind of confused on that point, could someone clarify? Epachamo 03:54, 2 March 2006 (UTC)

Conceptually, the queue holds instructions and (once produced) their results. Instructions are kept in order by the queue. When the result of an instruction is computed, it is written to the proper entry in the queue, next to the instruction. When the instruction commits (reaches head of the queue) its result is read from the queue and written to the programmer visible registers. In summary: it is a FIFO discipline with respect to instructions, but not results. Results are written into the queue out of order, but extracted in program order. Schiermer 19:53, 30 March 2006 (UTC)