RTOS = real-time operating system?
See the wiki!: https://wiki.freepascal.org/Operating_Systems_written_in_FPC But that is only less than half of it: there are many proprietary Pascal compilers for RTOS systems.
http://www.on-time.com/rtos-32.htmThis is not a recommendation!I do not know if it is good and if it is usable with FPC.It is usable with Delphi.This seems to be new:https://www.acontis.com/en/And of course there is RTLinux.