Service Programs

The No. 2 Electronic Switching Sj^stem is a stored program control telephone switching system. The minimum program consists of about 75,000 instructions arranged in 22-bit words and written in more than 50 separate sections. It is clearly impractical to write such a program without extensive computer aids. l.i Functions Major steps provided by the No. 2 ESS service programs are shown in Fig. 1. The assembler separately converts each symbolic source program from the language written by the programmer to the binary language of the No. 2 ESS. The loader then combines these separately assembled sections and resolves references between them. The output of the loader may be used by the magnet program to prepare the punched cards used in magnetizing the actual program store, or by the simulator. The simulator, using tables prepared by the assembler, furnishes more powerful program checkout facilities than would be practical on a switching machine and provides these facilities before the laboratory model is available. 2S65