2.2. Understanding Major Operating System Components

In the following sections, you briefly examine how applications actually run on Windows computers. For a more complete description of how applications are supported, see Book VI, Chapter 2.

2.2.1. Getting into the architecture

Windows 2000 was a complete rewrite of the OS, and, as such, Microsoft was able to do things that it couldn't do with Windows 9x. When Windows boots, it immediately enters a 32-bit protected-mode or a 64-bit long-mode state. With a 32-bit processor, the entire OS operates from this 32-bit state, and the kernel is loaded into Ring 0, which Microsoft refers to as kernel mode (see Figure 2-3). All processes running in kernel mode are protected from any processes running in Ring 3, or user mode.


The Intel processor design divides operation of the code into four separate execution levels, called rings. Ring 0 is at the middle of this arrangement, and Ring 3 is at the outside. These are the only two rings that Microsoft implemented during the development of 32-bit Windows OSes.

The kernel is the core part of the OS that controls everything else that happens on the computer. The kernel is responsible for keeping user-mode processes separated. Each application is started in its own area. One application is not directly allowed to interact with other applications and must pass such requests through Executive Services, which operate in kernel mode. This isolation of the applications from each other and from ...

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