Ring 0

Ring 0 is one of the privilege levels in which an x86 processor can operate at any time. The others are rings 1, 2 and 3. Of these, most x86 operating systems use only two levels, namely Ring 0 and Ring 3. Ring 0 is the highest privilege level and is the level at which the operating system kernel runs (kernel mode). Ring 3 is the lowest privilege level and is the level at which user applications are run (user mode).

Ring 0 is also the title of a Japanese film directed by Tsuruta Norio. See Ringu 0.