Abstract: A processor in a data processing system comprising a plurality of units for performing various functions and at least one data bus interconnecting the units for transferring information therebetween. The processor also includes a control store for storing microprogrammed instructions which are directly coupled to the units of the processor for controlling the operation thereof. Certain ones of the microprogrammed processor instructions, which are utilized in the transfer of information between the units of the processor by means of the data bus, are also stored in a secondary control store whose contents are decoded by logic circuitry within the units of the processor which, in response thereto, couple the desired apparatus within the units to the data bus.