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1.7 EXCHANGES 37Figure 1.7-4. Exchange with digital switchblock. LC, Line circuit; H, hybrid circuit; DMP, digitalmultiplex port; TC4, analog four-wire trunk circuit; CC, control channel.equipment. The first stored-program controlled (SPC) exchange was placed incommercial operation in 1965 [1].The main elements of a stored-program exchange control system are shown inFig. 1.7-5. A processor (or a group of processors) performs all exchange actions,controlling the switchblock, the line and trunk circuits, and other exchange equipment,by executing instructions in its program. The control channels for theseequipment units are connected to a bus on the processor.Processor memory is divided into semipermanent memory and temporarymemory. The semipermanent memory stores the programs for call processing, charging,and exchange maintenance procedures, and tables with data for digit analysis,call routing and charging, and so on.Temporary memory stores data that are changed frequently: for instance, informationon the status of the subscriber lines and trunks, or records of currentlyexisting calls.Figure 1.7-5. Stored-program exchange control. LC, Line circuits; TC, trunk circuits; CC, controlchannels.