A Microprocessor Operating System for Real-Time Applications

A real-time microprocessor operating system for the 10A RSS (Remote Switching System) is described. An RSS consists of a MAC-8 microprocessor controlled Remote Terminal connected via a transmission medium to a host ESS (Electronic Switching System). The operating system design was influenced by the stack nature of the MAC-8 and the stack usuage conventions of the high-level C language compiler. A memory bank switching scheme was also incorporated to increase the address space of the MAC-8. The operating system approach fit very nicely into the top-down software design methodology used in RSS. Where possible dedicated stacks within process descriptors helped to preserve process origanizaton and structure at code level. Supporting processes in a structured fashion required significant amounts of memory since process descriptors were from 10 to 100 times larger than earlier systems. However, background processes required less memory to support than real-time processes because the number of concurrent background processes could be more easily limited.