multiple subsystem i / o buses are coupled to one or more system buses of a computer system by interface circuits which perform necessary decoding of memory space and i / o space for allocation of portions of the memory space and the i / o space to each i / o bus . the interface circuits also translate fixed addresses within each i / o bus to permit proper operation of the i / o buses with the computer system . the interface circuits are programmed by the computer system to define the allocated memory spaces and i / o spaces for the corresponding i / o buses . programming of the i / o buses is performed at the time of system configuration by writing appropriate values into configuration registers incorporated into each of the interface circuits .