ONE-CHIP MICROCOMPUTER

PURPOSE:To realize the protection function with a simple circuit by providing a ROM read limiting circuit and permitting read of stored information in a ROM at the time of coincidence between a preliminarily stored key code and an inputted key code. CONSTITUTION:Plural first key codes which are preliminarily and arbitrarily determined and consist of binary patterns are stored in a ROM 9. A CPU has an instruction register and its instruction decoder has a specific instruction word, which limits read of the ROM, and is provided with the decoding function. A second key code consisting of a binary pattern is inputted to ports 2-5 from the external. Then, the CPU compares first and second key codes with each other by a comparing program, and read of stored information of the ROM 9 to the external is permitted only when they coincide with each other. Thus, since the third person who does not know any key code cannot read contents of the ROM 9 though performing the operation, the softwared protection function is realized with the simple circuit using the performance of the CPU as it is.