No. 4 ESS: Software Organization and Basic Call Handling

The No. 4 Electronic Switching System (No. 4 ESS), a stored program switching system, has most of its required control, administrative, and maintenance functions provided via program. The initial generic program for No. 4 ESS is the largest initial program developed to date for Bell System Electronic Switching Systems. It consists of approximately 400,000 instructions and 400,000 words of diagnostic tests requiring in excess of 1.4 million 26-bit words of system storage. In order to meet the overall design objectives for No. 4 ESS, four major objectives were set for the initial program: (i) (ii) (Hi) (iv) Efficiency in real time. Simple man-machine interfaces. Defensive design. Ease of modification. 1113 t Meeting these objectives required a unified software/hardware impact on the system architecture and the introduction of a number of improved software design and implementation concepts. 1 The generic program concept, 2 used in earlier ESSs, will be followed in the introduction of additional features to No. 4 ESS. Each new generic will contain feature additions and will be built on the previous generic's program base.