1. An information processing system comprising: an instruction register which sequentially receives a plurality of user instruction codes and stores and holds the plurality of user instruction codes while processing of instructions corresponding to the plurality of user instruction codes received is being executed; a state counter which outputs a number indicative of an internal state of the system, which changes in accordance with an incrementation of steps of the processing of said user instruction codes during an execution of processing of the instructions corresponding to the user instruction codes stored and held in said instruction register; a decoder which receives, as input, the plurality of user instruction codes stored and held in said instruction register and the internal state number from said state counter and generates various types of control signals for different portions in the system based on said received user instructions codes and the internal state number; a detection/storage means which detects when a predetermined specific processing operation must be executed in the system and stores a specific one of the plurality of user instruction codes preselected from the plurality of user instruction codes in said instruction register; and a state changing means which cooperates with said state counter and changes the internal state number of the state counter when the detection and storage are performed by said detection/storage means so as to convert the specific one of the user instruction codes to a non-user defined instruction code which cannot be defined by the plurality of the user instruction codes.