An I/O control system (19) for a general-purpose computer (13) having multiple bus branches (21) separated by routing circuitry (43, 44) prepares a digital map (48, 53) of I/O device locations on the bus branches, and copies versions of the maps to registers in the routing circuitry (213) on startup and reset (211). The routing maps provide immediate routing information for I/O requests issued by one or more CPUs (40, 41, 42) in a system, allowing therouting circuitry between bus branches to immediately route requests to the proper device with a minimum of wait states. In one aspect I/O devices are polled (205) for location at startup and reset (201), and in another aspect, a universal map protocol (207) is a part of the BIOS or storage accessible by the BIOS, making a system self-configuring and providing the necessary information for the mapping for the routing circuitry.