Editing Memory protection (section)

=== Protection rings ===
In [[Multics]] and systems derived from it, each segment has a [[protection ring]] for reading, writing and execution; an attempt by a process with a higher ring number than the ring number for the segment causes a fault. There is a mechanism for safely calling procedures that run in a lower ring and returning to the higher ring. There are mechanisms for a routine running with a low ring number to access a parameter with the larger of its own ring and the caller's ring.