Windows Evolution and Architecture

Windows NT Architecture

Windows NT architecture uses two processor access modes: user mode and kernel mode .

User mode includes application processes (typically Microsoft Win32 programs) and a set of protected subsystems. These subsystems are referred to as protected because each one is a separate process with its own protected virtual address space. The most important subsystem is the Win32 subsystem, which supplies much of the Win32 functionality to 32-bit Windows-based applications. The Windows subsystems, including the Win32 subsystem, are discussed in greater detail later in this chapter.

Another important subsystem, particularly with respect to UNIX applications, is the POSIX subsystem. POSIX stands for Portable Operating System Interface for computing environments, and consists of a set of international standards for implementing UNIX-like interfaces. POSIX began as an effort by the IEEE community to promote the portability of applications across different versions of UNIX. However, POSIX is not limited to the UNIX environment and has been implemented on a number of non-UNIX operating systems, including Windows NT. The POSIX subsystem implements these standards-based interfaces, and allows applications developers to more easily port their applications to Windows from another operating system.

Kernel mode is a highly privileged mode of operation in which program code has direct access to all memory, including the address spaces of all user mode processes and applications, and to hardware. Kernel mode is also known as supervisor mode , protected mode , or Ring 0 . The kernel mode of Windows NT contains the executive as well as the system kernel . The executive exports generic services that protected subsystems call to obtain basic operating system services, such as file operations, input/output (I/O), and synchronization services. Partitioning of the protected subsystems and the executive simplifies the base operating system design and makes it possible to extend the features of an individual protected subsystem without affecting the kernel. The kernel controls how the operating system uses the processors, and performs such operations as scheduling, multiprocessor synchronization, and providing objects that the executive can use or export to applications.

Figure 2.2 on the next page is a high-level illustration of Windows NT architecture.

Figure 2.2: Windows NT architecture