the present invention relates to the operational control of a digital computer system which includes the digital logic circuitry for temporarily storing results internal to an execution unit . an input unit of the execution , which inputs operand words to the execution logic of the execution unit , includes a first stack for holding operand words received from an external memory unit and a second stack for holding the result words of the execution logic . the input unit also includes a switch element for selecting words stored in the first and second stack which are to be utilized as input operand words to the execution logic in response to at least one control signal .