PURPOSE: To share a peripheral circuit serving as an integrated circuit in accordance with the switching of software and without deteriorating the processing ability of a microcomputer device.

CONSTITUTION: A peripheral circuit 7 and its control circuit 1 constitute an integrated circuit 10. The circuit 1 consists of the instruction register circuits 4 and 5 which hold the external operating instructions, a selector circuit 6 which transmits the outputs of both circuits 4 and 5, a write selection control circuit 3 which selectively controls the circuits 4, 5 and 6, and a switching instruction register circuit 2 which gives an output to the circuit 3 in response to an external switching instruction.