Windows NT is designed to allow different types of applications to run seamlessly on the same graphical desktop. It runs applications written for existing operating systems and APIs such as MS-DOS, OS/2 1.x, and Windows 3.x, POSIX and Win32.
Windows NT supports a variety of applications through the use of environment subsystems, which are Windows NT processes that emulate different operating system environments.
This chapter has described how the Windows NT Executive provides generic services that all environment subsystems can call to perform basic operating system functions. The subsystems build on the Executive's services to produce environments that meet the specific needs of their client applications.
Each subsystem runs as a separate user-mode process, with each subsystem protected from errors in the others: Failure in one won't cause another subsystem or the Executive to be disabled. Applications are also user-mode processes, so they can't hinder the subsystems or Executive.
Windows NT provides the following protected environments subsystems and Virtual DOS Machines (VDMs):
Each environment is optional and is loaded only when its services are needed by a client application.