How do Real Time Operating Systems work?

Question

I mean how and why are realtime OSes able to meet deadlines without ever missing them? Or is this just a myth (that they do not miss deadlines)? How are they different from any

Answer 1

Your RTOS is designed in such a way that it can guarantee timings for important events, like hardware interrupt handling and waking up sleeping processes exactly when they need to be.

This exact timing allows the programmer to be sure that his (say) pacemaker is going to output a pulse exactly when it needs to, not a few tens of milliseconds later because the OS was busy with another inefficient task.

It's usually a much simpler OS than a fully-fledged Linux or Windows, simply because it's easier to analyse and predict the behaviour of simple code. There is nothing stopping a fully-fledged OS like Linux being used in a RTOS environment, and it has RTOS extensions. Because of the complexity of the code base it will not be able to guarantee its timings down to as small-a scale as a smaller OS.

The RTOS scheduler is also more strict than a general purpose scheduler. It's important to know the scheduler isn't going to change your task priority because you've been running a long time and don't have any interactive users. Most OS would reduce internal the priority of this type of process to favour short-term interactive programs where the interface should not be seen to lag.