The Unison Operating System is a real-time operating system (RTOS) optimized for system-on-a-chip (SoC), multi-core and digital signal processor (DSP) systems. It is open source. It offers Linux compatibility while being much smaller than Linux. The current version of Unison is completely POSIX compliant, offering application portability and very low cost hardware implementation options primary for embedded microcontrollers, but also for distributed multi-core and multiprocessor systems like autonomous robots and vehicles. It is so tiny that it takes a minimum of 1K byte to run 2 threads.