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 and open-source software.

Development began in the late 1980s with early versions available via File Transfer Protocol (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 application programming interface (API) standards including Portable Operating System Interface (POSIX) and μITRON. The API now known as the Classic RTEMS API was originally based on the Real-Time Executive Interface Definition (RTEID) specification. RTEMS includes a port of the FreeBSD Internet protocol suite (TCP/IP stack) and support for various file systems including Network File System (NFS) and File Allocation Table (FAT).

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:


 * ARM
 * AArch64
 * Atmel AVR
 * Blackfin
 * Freescale, now NXP ColdFire
 * Texas Instruments – C3x/C4x DSPs
 * Intel – x86 architecture members 80386, Pentium, and above
 * LatticeMico32
 * Microblaze
 * 68k
 * MIPS
 * Nios II
 * OpenRISC
 * PowerPC
 * Renesas – H8/300, M32C, M32R, SuperH
 * RISC-V RV32, RV64 using QEMU
 * SPARC – ERC32, LEON, V9
 * V850

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 the two paragraph BSD license. This is a permissive license.

RTEMS was originally 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 programming language Ada.