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.

Abstract: A writeable control store for storing a plurality of instructions used to control the operation of a processor in a data processing system. The control store may be utilized in a first mode wherein the instructions are accessed for controlling the operation of the processor and in a second mode wherein data is transferred to or retrieved from the control store. In the second mode of operation, a secondary control means is utilized to control the operation of the data transfers to and from the writeable control store.