Abstract: A word group priority device for use in a data processing system having a first store and a relatively faster but smaller capacity second store, which device assigns priorities to word groups on a least recently used basis. Upon the read-out or read-in of a word group, (m)- bit priority codes are generated, the first (k) bits thereof serving to identify a group having the lowest priority, the further (m - k) bits serving to code all priority combinations for the groups which do not have the lowest priority. During a read, write, or replacement operation, the device can be used to lower the priority of all non-accessed word groups while assigning the highest priority to the accessed word group, and can also be used to raise the priority of all remaining word groups when one group has been invalidated by assigning to it the lowest priority.