CPU的特权指令分类

Basic Concept of Protected mode

In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as virtual memory, paging and safe multi-tasking designed to increase an operating system’s control over application software.

Privilege levels

In protected mode, there are four privilege levels or rings, numbered from 0 to 3, with ring 0 being the most privileged and 3 being the least. The use of rings allows for system software to restrict tasks from accessing data, call gates or executing privileged instructions.On most operating systems, Ring 0 is the level with the most privileges and interacts most directly with the physical hardware such as the CPU and memory. In most environments, the operating system and some device drivers run in ring 0 and applications run in ring 3.

REFERENCE
1. Protected mode
2. Protection ring
3. ring0