Mastering Assembly Programming

The rings

Almost all modern platforms, except a few embedded ones, utilize the same security principle --the division of execution environments by security levels and privileges; in this case, this means the ability to access certain resources. On Intel-based platforms, there are four security levels known as protection rings. These rings are numbered 0 through 3, where the greater the number, the less privileged the ring. Obviously, the code running at a less privileged level cannot access memory with a higher privilege level directly. We will shortly see how data is transferred between different privilege levels.

The following figure illustrates the concept of protection rings:

Here's a description of the different privileges of protection ...