ROM telephone exchange programmes modification - uses jump instruction allowing expansion of original programme held in ROM

摘要

The control of central telephone exchange systems is effected by programmes held in read only memory stores. Additional routines held in store may be called up via jump instructions to allow modification or expansion of the original programme. The programme is held in store and is controlled by a central unit and contains the incremental address of the instruction sequence to be modified. The correction or expansion routine is called up and the last instruction provides a jump address. When the first instruction has zero content the control unit provides a return address to the main programme. When the first instruction position has a content the routine is read and return made to the original programme at the end.