An instruction decoding unit in a microcomputer is disclosed having an
instruction word capable of being selected by a user to conveniently
produce application software while maintaining security. The decoding unit
in a microcomputer includes an instruction register for storing
instructions fetched from a memory, an instruction decoder for decoding
instruction codes of the instructions stored in the instruction register
and for designating micro-instructions to be executed, a micro-ROM for
outputting a series of the micro-instructions designated by the
instruction decoder, and a user instruction selector for selecting or
changing the micro-instructions of the micro-ROM in response to user's
selection so as to change the operation of an instruction word.
