REXIS (REal-time eXecutive for Intelligent Systems) is a Real-Time Operating System (RTOS) from ImageCraft and a component of the . An RTOS allows a firmware program to be divided into tasks which run (seemingly) concurrently on a single CPU using a feature called multitasking. This simplifies firmware development, as tasks can be written such that each one handles a small problem, and each task can be tested, verified, and code-reviewed independently of other tasks. As MCUs are getting more powerful and firmware requirements are becoming ever more complex, using an RTOS brings significant advantage to on-time firmware development.
REXIS is the third generation RTOS from ImageCraft: in fact, the company was founded in 1994 to commercialize REXIS Version One, which provided subsumption architecture style API for robotic programming. Version Two, with synchronous message passing API, was called eMOS. REXIS (the current version; Version Three) adds support for asynchronous IPC (Inter Process Communication) using mailboxes, in addition to synchronous message passing.
REXIS is ideal for Cortex-M MCUs as it is much smaller than a full-blown OS like Linux, yet it still has all the features you need for embedded firmware development. The complete JumpStart IoT package includes REXIS, the TCP/IP stack lwIP, a TLS module for secured communication, and applications such as MQTT for IoT and http for web server.
NOTE: An individual unit of execution is generally referred as a task in REXIS, although the word process may sometimes be used as a synonym.