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{{more citations needed|date=July 2008}}
In [[computer science]], '''locality of reference''', also known as the '''principle of locality''',<ref>Not to be confused with the [[principle of locality]] in physics.</ref> is the tendency of a processor to access the same set of memory locations repetitively over a short period of time.<ref>{{Cite book|title=Computer organization and architecture : designing for performance|last=William.|first=Stallings|date=2010|publisher=Prentice Hall|isbn=9780136073734|edition= 8th|___location=Upper Saddle River, NJ|oclc=268788976}}</ref> There are two basic types of reference locality{{snd}} temporal and spatial locality. Temporal locality refers to the reuse of specific data, and/or resources, within a relatively small time duration. Spatial locality (also termed ''data locality''<ref name="NistBig1">"NIST Big Data Interoperability Framework: Volume 1", [https://doi.org/10.6028/NIST.SP.1500-1r2 urn:doi:10.6028/NIST.SP.1500-1r2</ref>) refers to the use of data elements within relatively close storage locations. Sequential locality, a special case of spatial locality, occurs when data elements are arranged and accessed linearly, such as, traversing the elements in a one-dimensional [[Array data structure|array]].
Locality is a type of [[predictability|predictable]] behavior that occurs in computer systems. Systems that exhibit strong ''locality of reference'' are great candidates for performance optimization through the use of techniques such as the [[CPU cache|caching]], [[prefetch instruction|prefetching]] for memory and advanced [[branch predictor]]s at the [[Pipeline (computing)|pipelining]] stage of a processor core.
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* '''Temporal locality''': If at one point a particular memory ___location is referenced, then it is likely that the same ___location will be referenced again in the near future. There is a temporal proximity between the adjacent references to the same memory ___location. In this case it is common to make efforts to store a copy of the referenced data in faster memory storage, to reduce the latency of subsequent references. Temporal locality is a special case of spatial locality (see below), namely when the prospective ___location is identical to the present ___location.
* '''Spatial locality''': If a particular storage ___location is referenced at a particular time, then it is likely that nearby memory locations will be referenced in the near future. In this case it is common to attempt to guess the size and shape of the area around the current reference for which it is worthwhile to prepare faster access for subsequent reference.
** '''Memory locality''' (or ''data locality''<ref name="NistBig1"/>): Spatial locality explicitly relating to [[computer memory|memory]].
* '''[[Branch (computer science)|Branch]] locality''': If there are only a few possible alternatives for the prospective part of the path in the spatial-temporal coordinate space. This is the case when an instruction loop has a simple structure, or the possible outcome of a small system of conditional branching instructions is restricted to a small set of possibilities. Branch locality is typically not a spatial locality since the few possibilities can be located far away from each other.
* '''Equidistant locality''': It is halfway between the spatial locality and the branch locality. Consider a loop accessing locations in an equidistant pattern, i.e., the path in the spatial-temporal coordinate space is a dotted line. In this case, a simple linear function can predict which ___location will be accessed in the near future.
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