Cache memory is intended to provide memory access time approaching that of the fastest memories available and at the same time support a large memory size that has the price of less expensive types of semiconductor memories. There is a relatively large and slow main memory together with a smaller, faster cache memory. The cache contains a copy of a portion of main memory. When the processor attempts to read a byte or word of memory, a check is made to determine if the byte or word is in the cache. If so, the byte or word is delivered to the processor. If not, a block of main memory, consisting of some fixed number of bytes, is read into the cache and then the byte or word is delivered to the processor. Because of the phenomenon of locality of reference, when a block of data is fetched into the cache to satisfy a single memory reference, it is likely that many of the near-future memory references will be to other bytes in the block.