Abstract: In order to protect key parts of computer programs, the instructions making up those parts are held in an EPROM forming part of a single chip microcomputer. The EPROM is connected to instruction registers by way of an instruction bus. Data used by instructions held in these registers is transferred along a data bus which is separate from the instruction bus and serves a RAM, an ALU and an input/output port. Since the status of the input/output port is ultimately controlled by the instructions and there is no connection between the instruction bus and the data bus the instructions cannot be read either directly or indirectly from the microcomputer.