CENTRAL PROCESSING UNIT

First and second circuits store therein source and destination operand addressing information from an instruction code as input information of a main decoder, respectively. A third circuit stores therein information other than operand addressing information from the instruction code as input information of the main decoder. A predecoder analyzes the instruction code and controls states of the first, second and third circuits. A first selector determines which information is input to the main decoder, from the first circuit, the second circuit or the third circuit. A processing selecting circuit causes the first selector to select information for each instruction, in a basic sequence of the first circuit, the second circuit and then the third circuit. The processing selecting circuit controls the first selector so that, if a state of at least one of the first and second circuits indicates that `processing is not required`, the circuit of the state of the indication is not selected and a subsequent circuit is selected instead. The main decoder obtains the information from the first, second and third circuits and controls each component of the central processing unit.