Architectural differences

The number and order of rings depend on the CPU architecture. They are usually numbered with decreasing privilege, making ring 0 the most privileged one. This is true for i386 and x64 architecture that use four rings (from ring 0 to ring 3) but it's not true for ARM, which uses reverse order (from EL3 to EL0). Most operating systems are not using all four levels; they end up using a two level hierarchy—user/application (ring 3) and kernel (ring 0).