Windows NT Architectural Modules

Windows NT has a modular architecture, meaning that it comprises separate and distinct components, each having sole responsibility for its functions. This chapter introduces the modules in Windows NT 4.0 and then provides a detailed description of each, from the lowest level, nearest to the hardware, to the highest, nearest to the user applications.

Windows NT 4.0 architecture is divided into two main sections: user mode and kernel mode:

• Kernel mode is a highly privileged mode of operation in which the code has direct access to all hardware and all memory, including the address spaces of all user-mode processes. The part of Windows NT 4.0 running in kernel mode is called the Windows NT Executive. It includes the Hardware Abstraction Layer (HAL), the Microkernel, and the Windows NT Executive Service modules.

• User mode is a less privileged processor mode with no direct access to hardware. Code running in user mode acts directly only in its own address space. It uses well-defined operating system application program interfaces (APIs) to request system services. The environment and integral subsystems run in user mode.

Figure 5.3 shows the new Windows NT 4.0 architecture.

Figure 5.3 Windows NT 4.0 Modular Architecture

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Note

In earlier documentation, the terms kernel and microkernel were used interchangeably. In this document, kernel refers to the entire Windows NT Executive. The Microkernel is a component of the Windows NT Executive. Kernel mode refers to a highly privileged processing mode.