Comment by cyberax

QNX is hard realtime. At one point, its kernel had O(1) guarantees for message passing and process switching. It could have been rewritten without any loops. I'm not sure if that's still true.

It's also really compact. This used to be a great selling point for underpowered car infotainment systems. Some cars had around 1Mb of RAM for their infotainment, yet they were able to run fairly complex media systems.

QNX is also used for non-UI components, just as a good realtime OS.