Abstract: In a unified bus data processing system, a memory expansion unit is connected to the bus to convert bus addresses into addresses in a physical memory which may be many times larger than the maximum size that the bus would have been capable of addressing previously. Thus, the memory expansion unit provides directly addressable memory locations which serve to expand the range of physical addresses allocated to the bus by selectively connecting a plurality of expansion memory units to said bus. The memory expansion unit is programmed to convert groups of bus addresses into the same size group of addresses anywhere in physical memory. Until the memory expansion unit is programmed, physical memory is directly addressable by the central processing unit by applying the appropriate bus address and the normal control signals thereto.