fig2 is an address mapper 28 which allows the user to select the address range within which the address decoders of selected resources will be enabled in the illustrated form an address decoder 30 will respond to a particular address on address bus 16 by enabling a map register 32 to load a pair of resource map addresses provided simultaneously by the processor 12 on data bus 14 thereafter a comparator 34 will continuously compare the first of the resource map addresses in map register 32 to a corresponding number of for example the high order bits on address bus 16 and will enable a gate 36 when this set of address bits is identical to the first resource map address gate 36 in turn will then enable an address decoder 38 of a particular resource say timer 22 or interface 24 or both to respond to an appropriate low order set of address bits on address bus 16 provided that the balance of the address bits if any are all zero thus the decoder 38 will be enabled only when the address on address bus 16 is within the address range defined by the user as the first of the resource map addresses in the map.