Abstract: A first in - first out auxiliary memory array for storing binary data wherein each member (word) of the array includes a special bit which is used in combination with the special bits of the other members comprising the same member set to form the address of the next member whose data is to be replaced. Each member comprises an identifier field, a data field and the aforementioned special bit. When a member set is addressed, each member of the set is read to determine whether there is a match on the respective identifier field. If there is a match, the data field of the same member is utilized. If there is no match on the identifier field of any member of the addressed set, the main memory is accessed for the necessary replacement information which is to be written into the member which contains the oldest data. The address of the last-named member is determined by the exclusive ORing of the special bits of all the members comprising the given set.