1. A cache memory system comprising: a cache memory having a plurality of cache locations, each for storing a cache line of data, separately accessed from a main memory, and having a first tag memory, each cache location being indexed by indexes, taken from memory addresses, through first and second hashing functions such that plural memory addresses having a common index access plural memory locations through the first and second hashing functions and different indexes access common memory locations through the first and second hashing functions; hash control storage storing control data comprising hash data associated with each cache location which indicates the hashing function used to store data in the cache location; and a controller coupled to the cache memory responsive to memory addresses in accesses to the main memory for accessing data in the cache memory through the first and second hashing functions and for replacing data in the cache memory from the main memory responsive to the control data and to comparisons between tags of the memory addresses and tags stored in the first tag memory.