CALLING INSTRUCTIONS FOR A DATA PROCESSING SYSTEM

A digital data processing system with a central processor for responding to diverse instructions including instructions for calling subroutines. When the central processor executes a calling instruction, the central processor saves information corresponding to the operating environment for the calling routine and then utilizes corresponding information in the subroutine to establish the operating environment for the subroutine. A common return instruction at the completion of each subroutine causes the central processor to retrieve the saved operating information thereby to reestablish the operating environment for the calling routine.