This hierarchical memory system has two memory units on each level. One of the units called the data store contains all the data at that level of the memory. The other unit called the copy back store contains all the changes that have been made in that data either by addition or modification. While the data store is interfaced with the next higher level in the hierarchical memory system or with the processing units for the data processing system, the second or copy back store can transfer the changes made in the data into the next lower level in the memory hierarchy system if the copy back store is free and the data store in the next lower level is not involved in transferring data up the hierarchy. The data store and the copy back data store in each level are on two different power systems and transfers of the changes to the next lower level are done in the order in which the change entered in the copy back store with the oldest entry being the first to be copied back.
