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Programming for shared real-time systems presents problems not normally encountered in conventional computer applications. Interaction among programs of various interrupt levels must be controlled, effects of hardware and software errors must be localized, and processing handling capacity must be maximized through proper task scheduling. The methods used for handling such problems in the Bell System's No. 1 Electronic Switching System (No. 1 ESS) which is such a shared real-time system are the subject of this paper.