1. A memory system, comprising: a first memory for storing data words having a plurality of bits and capable of being accessed only for one entire data word at a time; a second memory for storing at least some of the data words stored in said first memory; means for accessing a data word from said second memory and including means for changing the value of at least one bit of the accessed data word, said means for changing comprising: a first multiplexer for receiving groups of new bits from a plurality of sources and selecting one of the groups of new bits in response to a control input; and a second multiplexer for receiving the selected group of new bits from said first multiplexer and the data word having the bit to be changed from said second memory and exchanging one bit in the selected group of new bits from said first multiplexer with the bit to be changed in the data word from said second memory; and means for returning the accessed data word, including the at least one changed bit, to both said first memory and said second memory.