A system is said to have a "hard real-time requirement" if the system would fail in the event that it missed a deadline.
Open source real-time operating systems (RTOS):
- RTAI (Linux with real-time user-land processes)
- RTLinux (Linux plus real-time kernel-land tasks; see RealTimeLinux)
See links for PIC-specific RTOSes.
Article: "Grab an Open RTOS"