If the particular address is found in the cache, the block of data is sent to the CPU, and the CPU goes about its operation until it requires something else from memory. When the CPU finds what it needs in the cache, a hit has occurred. When the address requested by the CPU is not in the cache, a miss has occurred and the required address along with its block of data is brought into the cache according to how it is mapped. Cache processing in some computers is divided into two sections: main cache and eavesdrop cache.  Main cache is initiated by the CPU within. Eavesdrop is done when a write to memory is performed by another requestor (other CPU or IOC). Eavesdrop searches have no impact on CPU performances. CACHE MAPPING TECHNIQUES.— Cache mapping is the method by which the contents of main memory are brought into the cache and referenced by the  CPU.  The  mapping  method  used  directly  affects  the performance of the entire computer system. . Direct  mapping —Main memory locations can only be copied into one location in the cache. This is accomplished  by  dividing  main  memory  into  pages  that correspond in size with the cache (fig. 5- 10). l Fully  associative  mapping  —Fully  associative cache mapping is the most complex, but it is most flexible with regards to where data can reside. A newly read block of main memory can be placed anywhere in a  fully  associative  cache.  If  the  cache  is  full,  a Figure  5-10.—Example  of  direct  mapping  used  in  cache memory. Figure  5-11.—Example  of  fully  associated  mapping  used  in cache  memory. replacement algorithm is used to determine which block in the cache gets replaced by the new data (fig. 5-11). l Set associative mapping —Set associative cache mapping combines the best of direct and associative cache mapping techniques. As with a direct mapped cache, blocks of main memory data will still map into as specific set, but they can now be in any N-cache block frames within each set (fig. 5-12). CACHE READ.—  The  two  primary  methods  used to read data from cache and main memory are as follows: Figure  5-12.—Example  of  set  association  mapping  used  in cache  memory. 5-16


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