RTEMS

Real-Time Executive for Multiprocessor Systems (RTEMS), formerly "Real-Time Executive for Missile Systems", and then "Real-Time Executive for Military Systems", is a real-time operating system (RTOS) designed for embedded systems. It is free open-source software.
Development began in the late 1980s with early versions available via ftp as early as 1993. OAR Corporation is currently managing the RTEMS project in cooperation with a steering committee which includes user representatives.
Design.
RTEMS is designed for real-time, embedded systems and to support various open API standards including POSIX and µITRON. The API now known as the Classic RTEMS API was originally based upon the Real-Time Executive Interface Definition (RTEID) specification. RTEMS includes a port of the FreeBSD TCP/IP stack as well as support for various filesystems including NFS and the FAT filesystem.
RTEMS provides extensive multi-processing and memory-management services, and even a System-database alongside many other facilities. It has extensive documentation.
Architectures.
RTEMS has been ported to various target processor architectures:
Uses.
RTEMS is used in many application domains. The Experimental Physics and Industrial Control System (EPICS) community includes multiple people who are active RTEMS submitters. RTEMS is also popular for space uses since it supports multiple microprocessors developed for use in space including SPARC, ERC32 and LEON, MIPS Mongoose-V, Coldfire, and PowerPC architectures, which are available in space hardened models. RTEMS is currently orbiting Mars as part of the Electra software radio on NASA's Mars Reconnaissance Orbiter, and the ESA's Trace Gas Orbiter.
License.
RTEMS is distributed under a modified GNU General Public License (GPL), allowing linking RTEMS objects with other files without needing the full executable to be covered by the GPL. This license is based on the GNAT Modified General Public License with the language modified to not be specific to the Ada programming language.