Interesting to mention the Raspberry Pi. I saw an article just a day or two ago that claimed that the RpiOS was stated by and ran on top of RTOS. That's particularly interesting because at one time years ago, I saw suggestions that Linux could run as a task on an RTOS. Things that required hard real time deadlines could run on the RTOS and not be subject to the delays that a virtual memory system could entail.

I don't recall if this was just an idea or was actually implemented. I also have seen only the one mention of RpiOS on an RTOS so I'm curious about that.