A die expansion bus efficiently couples a supplemental portion of a processing system to an original portion of the processing system on a die. The die expansion bus couples bus subsystems of the supplemental portion of the processing system to the bus subsystems of the original portion of the processing system. The original portion of the processing system is arranged to control the data resources of the supplemental portion of the processing system by accessing memory mapped control registers associated with the bus subsystems of the supplemental portion of the processing system.