Also known as: RTOS
A real-time operating system (RTOS) is a small operating system that schedules tasks with guaranteed, bounded timing, so a response is delivered within a known deadline.1
Overview
What makes an OS “real-time” is not raw speed but determinism: the worst-case time to react to an event is predictable and small. An RTOS provides a priority-based, usually preemptive scheduler, plus primitives like tasks, queues, semaphores, and timers, all in a footprint of a few kilobytes. On a microcontroller it sits between your application and the hardware, multiplexing several jobs onto one core while honoring deadlines. Popular examples include FreeRTOS, Zephyr, and ThreadX.
Where it fits
Simple firmware often runs bare metal — a single loop plus interrupts — which is enough when there are only a couple of jobs. An RTOS earns its keep once an embedded system juggles many concurrent tasks (reading sensors, driving a radio, servicing a network stack) with competing deadlines. It ports easily across ARM Cortex-M parts, so the same code runs on many chips.
Sources
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Real-time operating system — Wikipedia, on RTOS design and determinism. ↩