1. A system for providing hierarchied auxiliary read-only memory in addition to read-only memory on-board a central processing unit module comprising: (a) memory means for providing a plurality of memory categories including permanent read-only memory and alterable programmable read-only memory for data storage in excess or in lieu of on-board read-only memory and for implementation and installation of changes to on-board read-only memory respectively; and (b) decoder means for recognition of addresses for accessing a given memory category of said memory means according to a predetermined hierarchy when the given accessing address falls within more than one or more overlapping subordinated address ranges for the memory categories of said memory means.