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

... well ...

A real-time operating system tries to be deterministic and meet deadlines, but it all depends on the way you write your application. You can make a RTOS very non real-time if you don't know how to write "proper" code.

Even if you know how to write proper code: It's more about trying to be deterministic than being fast.

When we talk about determinism it's

1) event determinism

For each set of inputs the next states and outputs of a system are known

2) temporal determinism

… also the response time for each set of outputs is known

This means that if you have asynchronous events like interrupts your system is strictly speaking not anymore temporal deterministic. (and most systems use interrupts)

If you really want to be deterministic poll everything.

... but maybe it's not necessary to be 100% deterministic