UNIX Time-Sharing System: The MERT Operating System

Abstract

The MERT operating system supports multiple operating system environments. Messages provide the major means of inter-process communication. Shared memory is used where tighter coupling between processes is desired. The file system was designed with real-time response being a major concern. The system has been implemented on the DEC PDP-11/45 and PDP-11/70 computers and supports the UNIX∗ time-sharing system, as well as some real-time processes. The system is structured in four layers. The lowest layer, the kernel, provides basic services such as inter-process communication, process dispatching, and trap and interrupt handling. The second layer comprises privileged processes, such as I/O device handlers, the file manager, memory manager, and system scheduler. At the third layer are the supervisor processes which provide the programming environments for application programs of the fourth layer. To provide an environment favorable to applications with real-time response requirements, the MERT system permits processes to control scheduling parameters. These include scheduling priority and memory residency. A rich set of inter-process communication mechanisms including messages, events (software interrupts), shared memory, inter-process traps, process ports, and files, allow applications to be implemented as several independent, cooperating processes. Some uses of the MERT operating system are discussed.