Rings

• Intel-compatible X86 operating systems run programs in a number of protection levels that are known as rings. These rings define what programs running under that ring are permitted to do. The highest level, Ring 0, allows the program to use the full resources of the system, while lower rings, going down to Ring 3, add more and more restrictions to what programs are allowed to do on the system.
  
  

Operating Systems vs. Applications

• The core of the operating system, the kernel, nearly always runs in Ring 0, while user applications typically run in Ring 3.
  
  

Virtual Machine Guests

• Unless the computer is designed to be a server with dedicated support for virtualization, a virtual computer system must be run within Ring 1, not giving it enough control to create its own virtual machines.