DATA PROCESSOR

PURPOSE:To protect the secrecy of data in an EPROM by inputting the read signal of an extension prohibiting bit to a mode control circuit, which forms a control signal, and prohibiting the bus extension mode of a single chip microcomputer when write is executed to the bit. CONSTITUTION:A protecting circuit 8 is provided with the extension prohibiting bit for setting whether the bus extension mode is prohibited or not, and a protect bit for setting whether the data in the EPROM are made secret or not. The respective bits constituting this protecting circuit 8 are constituted so that the extension prohibiting bit is selected in a CPU mode and the protect bit can be selected in a program mode when an EPROM 1 is accessed. A signal read out from the protecting circuit 8 is supplied to a mode control circuit 3 as a protect signal Pr and when write is executed to the bit, a port is controlled so that the bus extension mode of the single chip microcomputer can be prohibited. Thus, the secrecy of the data in the built-in EPROM can be effectively protected.