Patent US 4,080,648

A micro program control system for use in a data processing system includes a subsidiary control memory for storing the first micro instructions of respective micro programs, and a control memory for storing the second and the remaining micro instructions of the respective micro programs. The subsidinary control memory is coupled to a main memory in which macro instructions of a program for the data processing system are stored. The operation code of the macro instruction includes a code to address the first micro instruction in the subsidiary control memory, so that one of the first micro instructions can be accessed to supply into a control register when the macro instruction is read out from the main memory to an instruction register of the data processing system, whereby control signals for controlling the operation of the data processing system are delivered from the control register according to the contents of the first micro instruction. The second and necessary number of the remaining micro instructions in the control memory are fetched, in turn, into the control register according to the operation code of the macro instruction held in the instruction register and the contents of the preceding micro instruction in the control register, whereby a micro program corresponding to the macro instruction is performed.