A data processing system having separate read-only memory and read-write memory integrated circuits coupled to a central processing unit via the same interface system. The data processing system is comprised of bus means having either command, address, or data signals present and conducted thereon. In the preferred embodiment, the bus means is comprised of a four binary digit bidirectional conductor bus coupling between the central processing circuit and the memory circuits. In the preferred embodiment, the data processing system is further comprised of processor means within said central processing unit coupled to the bus means for selectively transmitting selected ones of said command, data and address signals onto the bus means, said processor means further including means for receiving certain other ones of said data and address signals from the bus means; and memory means including, in the preferred embodiment, read-only memory circuits and read write memory circuits, coupled to the bus means (and to the processor means via the bus means), including means for selectively transferring data with the processor means in response to said received command and address signals being within a unique subset of commands and addresses. In the preferred embodiment, each separate individual circuit comprising the memory means includes the means for responding to the command signal received via the bus means and for storing or outputting address signals or data signals responsive to decoding said received command signal.