INSTRUCTION DECODER CIRCUIT IN MICROPROCESSOR

摘要

PURPOSE:To reduce the chip size of an integrated circuit by constituting an instruction decoder circuit with a device of the same constitution of a ROM or plural combinations of devices of the same constitution as the ROMs and varying an instruction code optimizingly so as to reduce the capacity of the ROM. CONSTITUTION:A program is stored in the ROM 1, the instruction decoder circuit decodes the program read from the ROM 1 to generate a signal controlling other part such as an arithmetic logic unit ALU or the like. Further, the instruction decoder circuit is constituted by a device of the same structure as that of the ROM storing the program or plural combination of the same structure of the ROM. Thus, the instruction code is changed freely and when an instruction not used for the instruction code in the program exists, the redundancy of the instruction code in the lengthwise direction is reduced.