PURPOSE: To change instruction performance control function without changing the step of a microinstruction for execution of the microinstruction by controlling the valid/invalid states of the instruction performance control based on the data which is designated by the microprogram address of a microinstruction to be executed and read out of a memory.

CONSTITUTION: The microprogram address of a microprogram stored in a microinstruction register 2 is stored in an address register 3 via an address generating circuit 4. The microprogram address of the microinstruction to be executed next is decided by the circuit 4 based on the outputs of both registers 2 and 3. Then a memory 5 is addressed by the output of the register 3 and reads the designated data to a line 500. Then the execution of the microinstruction is kept waiting when the data on the line 500 and a wait timing signal 700 are set at 1.