  Modern control systems applications are often built on top of a real time
operating system (RTOS) which provides the necessary hardware abstraction as
well as scheduling, networking and other services. Several open source RTOS
solutions are publicly available, which is very attractive, both from an
economic (no licensing fees) as well as from a technical (control over the
source code) point of view. This contribution gives an overview of the RTLinux
and RTEMS systems (architecture, development environment, API etc.). Both
systems feature most popular CPUs, several APIs (including Posix), networking,
portability and optional commercial support. Some performance figures are
presented, focusing on interrupt latency and context switching delay.
