RTOS (real-time operating system)

A general-purpose OS optimises for throughput and fairness: every process gets its turn, more or less. An RTOS does the opposite. The scheduler is strictly priority-based and preemptive — a high-priority task that becomes ready takes the CPU immediately, and the worst-case latency can be calculated up front. That number, not the average, is what sells the system.

The kernel is usually tiny: tasks, queues, semaphores, mutexes and timers, linked straight into the firmware with no separate user space. VxWorks (1987) and QNX owned the commercial end for decades; FreeRTOS and the Zephyr Project took over hobbyist and IoT work. The price of predictability is that every task must be written never to block longer than planned — an RTOS only guarantees what the programmer has already guaranteed.