Similar to RTOS-based software, a standard OS has all of the libraries and features a developer could ask for. What's missing, however, is a strict focus on meeting timing requirements. Generally speaking, systems implemented with a traditional OS are going to have much less deterministic behavior (and none that can be truly counted on in a safety-critical situation). If there is a lax real-time requirement without catastrophic consequences, if a wishy-washy deadline isn't met on time, a standard OS can be made to work, as long as care is taken in choosing what software stacks are running and their resource use is kept in check. The Linux kernel with PREEMPT_RT patches is a good example of this type of real-time system.

So, now that all of the options for achieving a real-time system have been laid out, it's time to define exactly what we mean when we say RTOS, specifically an MCU-based RTOS.