What embedded RTOS have you used? Have you ever written your own from scratch?

An RTOS (real-time operating system) provides deterministic, time-bounded scheduling so tasks meet their deadlines. It typically offers preemptive priority-based task scheduling, inter-task communication and synchronization (queues, semaphores, mutexes, event flags), timers, and interrupt handling—enabling structured concurrent firmware instead of a single bare-metal super-loop.

Common embedded RTOSes include:

• FreeRTOS — very popular, open-source, small footprint.
• Zephyr — open-source, scalable, broad hardware/driver and connectivity support.
• ThreadX (Azure RTOS) — commercial-grade, widely deployed.
• VxWorks — long-established commercial RTOS for high-reliability systems.
• QNX — microkernel RTOS used in automotive and safety-critical systems.
• µC/OS-II/III — well-documented commercial/open RTOS.

Engineers also sometimes write a minimal scheduler/kernel from scratch—implementing a context switch, a tick timer, a ready-list scheduler, and basic synchronization primitives—as a learning exercise or for very constrained targets, though production systems usually favor a mature, tested RTOS.