For a similar embedded RTOS there is RTEMS https://www.rtems.org/ https://en.wikipedia.org/wiki/RTEMS

An interesting example would be GRiSP where Erlang's BEAM was ported to an Arm Cortex M7 microcontroller with 64 MB of external RAM. https://www.grisp.org/

Checkout https://osdev.org

And actually, x86 started life as an embedded processor :)

Ah, much simpler than I anticipated. Thank you for sharing