Guru's Guide to SQL Server Architecture and Internals, The

Subsystems

Windows ships with three environment subsystems: Win32, OS/2, and POSIX. Each of these provides a different environment or personality for the OS. Of these, Win32 is preeminent because it's not optional (it must always be running, regardless of the environmental subsystem chosen and regardless of whether anyone is logged in) and because it provides the most direct and most complete access to Windows itself. The other environment subsystems aren't used much and don't provide the same level of functionality as the Win32 subsystem. Applications that run on Windows are compiled and linked to run under a particular environment subsystem. Obviously, most of these, including SQL Server, are Win32 applications.

The Win32 environment subsystem ...