A processor is provided with a set of instructions formed in general, of an operation section and an operand section. For a special control instruction, the operand section is transmitted to the operation blocks along a bypass path separate from the normal path in which normal instructions are interpreted. 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.