 An entire blocks of data is copied after a hit because the principle of locality (局部性原理) tells us that once a byte is accessed, it is likely that a nearby data element will beneeded soon.

 There are three forms of locality:

 Temporal locality (时间局部性) : Recently-accessed data elements tend to be accessed again.

 Spatial locality (空间局部性) : Accesses tend to cluster.

 Sequential locality (顺序局部性) : Instructions tend to be accessed sequentially.

时间局部性：最近访问的数据元素往往会再次被访问。

空间局部性：访问倾向于群集。

顺序局部性：指令往往是按顺序访问的。