A microcomputer capable of on-board programming of dedicated user communication protocols without requiring a serial interface on the mounted board, and that will not destroy the dedicated user communication protocol code even if the system runs out of control. A user boot mat other than a user mat is provided for programming control programs for the user in the on-chip non-volatile memory of the microcomputer. The user boot mat serves as the mat for programming the dedicated user communication protocol and also provides a user boot mode for running the program. The user boot mat cannot program or erase in this user boot mode. By separating the user boot mat and user mat, an interface capable of programming and erasing the user-specified programming can be achieved without having to program a dedicated communication protocol on the user mat.