The Computers: Software: Operating Systems: Realtime: (OS-9 Realtime )

From the OS-9 FAQ: "OS-9 is a real-time, multiuser, multitasking operating system developed by Microware Systems Corporation. It provides synchronization and mutual exclusion primitives in the form of events, which are similar to semaphores. It also allows communication between processes in the form of named and unnamed pipes, as well as shared memory in the form of data modules. OS-9 is modular, allowing new devices to be added to the system simply by writing new device drivers, or if a similar device already exists, by simply creating a new device descriptor. All I-O devices can be treated as files, which unifies the I-O system. In addition, the kernel and all user programs are ROMable. OS-9 Realtime Operating Systems Software Computers.

OS-9 is a family of real-time, process-based, multitasking, multi-user operating systems, developed in the 1980s, originally by Microware Systems Corporation for the Motorola 6809 microprocessor. (wikipedia)