1. For use in a data processing system: two logically distinct memory arrays, containing identical data in corresponding locations thereof; selection means selectively and mutually exclusively logically connecting the outputs of said logically distinct memory arrays to other parts of said data processing system so the output of a first one of said logically distinct memory arrays is logically connected to other parts of said data processing system; detection means for detecting an error in data read from said logically distinct memory arrays; and reversing means responsive to the detection of a first error by said detection means to cause reversal of said selection means thereby logically connecting the output of the second of said logically distinct memory arrays to other parts of said data processing system; said reversing means being further responsive to the detection of another error by said detection means to again cause reversal of said selection means to logically reconnect the output of said first one of said logically distinct memory arrays to other parts of said data processing system.