PURPOSE: To seperately diagnose a decoding means and a peripheral circuit by switching the connection between the first to the third terminals of the input/output switching means connected with the output terminal of a decoding means.

CONSTITUTION: This microprocessor is provided with a decoding means 2 translating code data into a microinstruction S1 and outputting it, a storage means 9 which is possible to read and write, a microinstruction exclusive bus 9 connected with the storage means 9 at one terminal and an output switching means 3 where the output terminal of the decoding means 2 is connected with a first terminal 3A, the one terminal of the microinstruction exclusive bus 8 is connected with a second terminal 3C and the input/output terminal of a peripheral circuit is connected with a third terminal 3B. This input/output switching means 3 connects between the first and second terminals 3A and 3C, connects the first and the third terminals 3A and 3B and connects the second and third terminals 3C and 3B, respectively, based on switching signals F1 and F2.