MERT is a multi-environment real-time operating system for the Digital Equipment PDP-11/45 and 11/70 computers. It is a structured operating system built on top of a kernel which provides the basic services such as memory management, process scheduling, and trap handling needed to build various operating system environments. Real-time response to processes is achieved by means of preemptive priority scheduling. The file system structure is optimized for real-time response. Processes are built as modular entities with data structures that are independent of all other processes. Interprocess communication is achieved by means of messages, event flags, shared segments, and shared files. Process ports are used for communication between unrelated processes.