Out-of-order execution: Difference between revisions

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A larger buffer can, in theory, increase throughput. However, if the processor has a [[branch misprediction]] then the entire buffer may need to be flushed, wasting a lot of [[clock cycle]]s and reducing the effectiveness. Furthermore, larger buffers create more heat and use more [[Die (integrated circuit)|die]] space. For this reason processor designers today favor a [[multi-threaded]] design approach.

Decoupled architectures are generally thought of as not useful for general-purpose computing as they do not handle control-intensive code well.<ref>{{cite journal |author-last1=Kurian |author-first1=L. |author-last2=Hulina |author-first2=P. T. |author-last3=Coraor |author-first3=L. D. |title=Memory latency effects in decoupled architectures |journal=[[IEEE Transactions on Computers]] |volume=43 |issue=10 |date=1994 |pages=1129–1139 |doi=10.1109/12.324539 |s2cid=6913858 |url=https://pdfs.semanticscholar.org/6aa3/18cce633e3c2d86d970d6d50104d818d9407.pdf |archive-url=https://web.archive.org/web/20180612141055/https://pdfs.semanticscholar.org/6aa3/18cce633e3c2d86d970d6d50104d818d9407.pdf |url-status=dead |archive-date=2018-06-12 }}</ref> Control intensive code include such things as nested branches that occur frequently in [[operating system kernel]]s. Decoupled architectures play an important role in scheduling in [[very long instruction word]] (VLIW) architectures.<ref>{{cite journal |author-first1=M. N. |author-last1=Dorojevets |author-first2=V. |author-last2=Oklobdzija |title=Multithreaded decoupled architecture |journal=International Journal of High Speed Computing |volume=7 |issue=3 |pages=465–480 |date=1995 |doi=10.1142/S0129053395000257 |url=https://www.researchgate.net/publication/220171480}}</ref><!--[[User:Kvng/RTH]]-->

== 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, whichand requiresfor 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 that have yet to be resolved is known as [[speculative execution]].<!--[[User:Kvng/RTH]]-->

The ability to issue instructions past branches that are yet to resolve is known as [[speculative execution]].

== Micro-architectural choices ==