A Facility for Real-Time Program Development

Abstract

As part of a multi-access system, a facility was developed to assist in the preparation of real-time programs for a specialist application. The multi-access system (GPO, 1970) is located within the Electronic Switching Group GPO Research Station, Dollis Hill. This group is presently engaged on the development of a computer-controlled telephone exchange. As can be imagined, a considerable software effort is involved even in the initial stage, which consists initially of a four-line exchange controlled by a small computer via a data switch. Plans are in hand to test the system simulating much larger traffic. This software comprises the exhange system executive, a large number of operating routines, of the order of 100, and a complex test program used in the development of the data switch and the other pieces of hardware which make up the exchange. Powerful debug facilities were available and it was desired to extend their use to real-time programs. Because of the programming effort required to implement the system for the exchange and also because the system was to be implemented on a small computer, it was necessary to produce initially an operating system that would provide both multiaccess capabilities and comprehensive debug facilities. A further advantage was that it would permit hardware and software development to proceed simultaneously. The first provision is an obvious one since all computing is done in a 'hands-on' situation, i.e. each programmer is allotted time, during which he may assemble, edit or debug his program. Particularly during the last stage, the computer will be idle for appreciable periods during which the programmer cogitates. Multi-access, therefore, is an obvious method of increasing the efficiency of use of the machine, The comprehensive debug facility ensures that the programmer uses his time as efficiently as possible. The small machine concerned was a Honeywell DDP 516 with 32K words of storage. A high speed punch and reader are connected together with two Teletype ASR 33 and one KSR 35. Optional features which are incorporated in the system are memory lock-out and standard and priority interrupt lines. The high speed reader and punch are connected to the standard interrupt line, while the teletype, real-time clocks, data switch and other hardware links are connected to the priority lines. In the Honeywell interrupt system, when an interrupt occurs, it causes the contents of the program counter, which contains the address of the current instruction to be executed, to be automatically changed, thereby changing the sequence of instruction execution. Interrupts have unique memory locations dedicated to them, whose contents are interpreted as an indirect address. The action of an interrupt causes the program to branch to the location whose address is stored in the dedicated location. These dedicated locations are held in the base sector.