A processor is provided with a set of instructions formed, in general, of an operation section and an operand section. For at least one of the instructions, the operand section represents operation control signals of the processor. In this way, an extension of the set of instructions can be achieved for tailoring the set of instructions to the user's own requirements. Consequently, the processor control unit should be capable of coupling its outputs to its inputs upon receiving one such instruction, thereby to transfer such internal operation control signals without interpretation.