Controlling a telephone switching system is a difficult software problem involving many concurrent processes and complex patterns of inter-process communication. This paper describes the functional multiprocessing architecture used in the control of an Experimental Digital Switching office. The software system is partitioned into two subsystems: Peripheral Control and Call Processing. The Peripheral Control Subsystem maintains the status of the switching resources, passes customer inputs to the Call Processing Subsystem, and carries out all the hardware control functions. The Call Processing Subsystem performs all the decision making functions to provide telephone features. By choosing a proper interface between them, the Peripheral Control Subsystem hides the hardware details of the peripherals to provide a high-level abstract switching machine. By isolating feature processing from hardware control, new services and peripherals can be introduced to the system easily. The authors' experience with this system is presented.