The use of caches introduces some new design considerations. Because each
local cache contains an image of a portion of memory, if a word is altered in one
cache, it could conceivably invalidate a word in another cache. To prevent this, the
other processors must be alerted that an update has taken place. This problem is
known as the cache coherence problem and is typically addressed in hardware rather
than by the operating system.We address this issue in Section 17.4.