Organization of the no. 1 ESS stored program

The stored program of the no. 1 ESS must perform switching functions reliably and promptly. Its design must be economical of memory and execution time, and must accommodate office growth easily. The program is organized so that an interrupt system initiates the input-output programs that must be performed to accurate timing tolerances. The data collected by these input-output programs are passed to the call processing programs which decide what course of action the calls shall follow. The program also assigns an appropriate share of central control time for maintenance and administrative functions. The program is generic in the sense that specific quantities which change from office to office are looked up in tables and are not embedded in the program itself. This article describes the basic program structure and illustrates it with some typical programs.