1. For microcomputer systems which include a program memory and a microprocessor having executing means for executing sets of microinstructions, said executing means including a microinstruction memory for storing sets of microinstructions and an instruction decoder means for converting a control word to a microinstruction memory address, each set of microinstructions being assigned a publicly known OP code, and each set being activated in response to a related control word, the method of securing from unauthorized copying of a program stored in the program memory as a sequence of instructions wherein each instruction includes a control word, said method comprising the steps of defining a set of proprietary control words wherein each control is assigned a distinct publicly known OP code and its set of microinstructions, initially partially fabricating the microprocessors including their microinstruction memories such that each microinstruction set starts at a known address, said partially initially fabricating step, including an incomplete fabrication of the instruction decoder means, finishing the fabricating by completely fabricating the instruction decoder means such that each proprietary control word received by an instruction decoder means is converted to the address of the microinstruction memory which stores the first microinstruction of the set of microinstructions assigned to such received proprietary control word, and recording the program to be protected in the program memories wherein each instruction includes the proprietary control word assigned to a desired set of microinstructions.