Posted: Nov. 17, 2003

Building on the foundation of the Windows NT architecture, Longhorn introduces new subsystems and APIs that will change how developers write applications. At the lowest levels, the OS remains largely unchanged, with the Hardware Abstraction Layer (HAL), device drivers, and kernel essentially the same. Other subsystems, such as the NTFS file system; the Graphics Device Interface (GDI), which works with graphic hardware; and "User," which provides common controls such as windows and buttons, remain intact for backward compatibility, ensuring that legacy Windows applications will run on Longhorn.

The old collection of APIs that developers used, such as the Win32 APIs, are augmented by newer class libraries and the Common Language Runtime (CLR), making Longhorn a platform designed primarily for managed code, although with backward compatibility support for native code.

Three new subsystems and their associated class libraries will improve the way users and applications interact with the file system (WinFS), how graphics are drawn and displayed (Avalon), and how programs pass information between each other, including communicating using Internet protocols (Indigo).

The new user interface and shell that define how users interact with the OS (Aero) is an element of the Avalon subsystem and is likely to undergo the most change between now and the final release of Longhorn.