Hands-On System Programming with Linux

Privilege levels or rings on the x86

To understand this important concept better, let's take the popular x86 architecture as a real example. Right from the i386 onward, the Intel processor supports four privilege levels or rings: Ring 0, Ring 1, Ring 2, and Ring 3. On the Intel CPU's, this is how the levels work:

Figure 1: CPU ring levels and privilege

Let's visualize this Figure 1 in the form of a Table 2: x86 privilege or ring levels:

Privilege or ring level	Privilege	Purpose
Ring 0	Highest	OS code runs here
Ring 1	< ring 0	<Unused>
Ring 2	< ring 1	<Unused>
Ring 3	Lowest	Application code runs here (userland)

Table 2: x86 privilege ...