PURPOSE: To save the microprogram memory, by enabling the common use of microroutine, through the provision of function which makes ineffective the microcommand in the microinstruction.

CONSTITUTION: The data from the execution sequence control means 6 of microinstruction is given to the address register 7, the instruction is fetched to the instruction register 2 from the microprogram memory 1, and microinstruction is executed. By providing the microcommand control means 4 possible for setting or resetting from the microinstruction, a part of the microcommand from the control circuit 3 producing the microcommand from the field A of the microinstruction stored in the microinstruction register 2 is made in effective with the status of the microcommand control means 4. By making ineffective a part of the instructions in the microroutine, the routine can be used in common.