In addition, there's also some "up & coming" Rust language projects which fall somewhere along the "framework" to "OS" spectrum (in part, via https://arewertosyet.com):
On the desktop side, I seem to recall in the past, OS such as BeOS & QNX have been presented as a possible future for real time desktop OS that hasn't arrived.
As someone else already mentioned, PREEMPT_RT being merged for Linux is a recent development somewhat in this space which could have impact on both desktop & "embedded" situations but suitability varies dependent on, say, whether you're wanting to use it for audio production versus controlling some 10 tonne robot operating next to humans.
Hope this at least goes some way to answering your question. :)
It's probably reasonable to make a distinction between "Real Time" desktop/server OS (on CPUs) vs "Real Time" embedded hardware OS (on MCUs).
(Even aside from any hard-/soft- real time distinction.)
On the embedded side, in addition to FreeRTOS (upon which Pebble OS is built), I'm aware of others with reasonably high profile such as:
* Zephyr (Linux Foundation, C): https://en.wikipedia.org/wiki/Zephyr_(operating_system)
* NuttX (Apache Software Foundation, C & C++): https://en.wikipedia.org/wiki/NuttX
In addition, there's also some "up & coming" Rust language projects which fall somewhere along the "framework" to "OS" spectrum (in part, via https://arewertosyet.com):
* Tock: https://github.com/tock/tock
* Embassy: https://github.com/embassy-rs/embassy
* Hubris: https://hubris.oxide.computer
On the desktop side, I seem to recall in the past, OS such as BeOS & QNX have been presented as a possible future for real time desktop OS that hasn't arrived.
As someone else already mentioned, PREEMPT_RT being merged for Linux is a recent development somewhat in this space which could have impact on both desktop & "embedded" situations but suitability varies dependent on, say, whether you're wanting to use it for audio production versus controlling some 10 tonne robot operating next to humans.
Hope this at least goes some way to answering your question. :)