Windows NT Architecture - User Mode

User Mode

The user mode is made up of subsystems which can pass I/O requests to the appropriate kernel mode drivers via the I/O manager (which exists in kernel mode). Two subsystems make up the user mode layer of Windows NT: the Environment subsystem and the Integral subsystem.

The environment subsystem was designed to run applications written for many different types of operating systems. None of the environment subsystems can directly access hardware, and must request access to memory resources through the Virtual Memory Manager that runs in kernel mode. Also, applications run at a lower priority than kernel mode processes.

There are three main environment subsystems: the Win32 subsystem, an OS/2 subsystem and a POSIX subsystem.

• The Win32 environment subsystem can run 32-bit Windows applications. It contains the console as well as text window support, shutdown and hard-error handling for all other environment subsystems. It also supports Virtual DOS Machines (VDMs), which allow MS-DOS and 16-bit Windows (Win16) applications to run on Windows NT. There is a specific MS-DOS VDM which runs in its own address space and which emulates an Intel 80486 running MS-DOS 5.0. Win16 programs, however, run in a Win16 VDM. Each program, by default, runs in the same process, thus using the same address space, and the Win16 VDM gives each program its own thread to run on. However, Windows NT does allow users to run a Win16 program in a separate Win16 VDM, which allows the program to be preemptively multitasked as Windows NT will pre-empt the whole VDM process, which only contains one running application. The Win32 environment subsystem process (csrss.exe) also includes the window management functionality, sometimes referred to as a "window manager". It handles input events (such as from the keyboard and mouse), then passes messages to the applications that need to receive this input. Each application is responsible for drawing or refreshing its own windows and menus, in response to these messages.
• The OS/2 environment subsystem supports 16-bit character-based OS/2 applications and emulates OS/2 1.x, but not 32-bit or graphical OS/2 applications as used with OS/2 2.x or later, on x86 machines only. To run graphical OS/2 1.x programs, the Windows NT Add-On Subsystem for Presentation Manager must be installed. The last version of Windows NT to have an OS/2 subsystem was Windows 2000; it was removed as of Windows XP.
• The POSIX environment subsystem supports applications that are strictly written to either the POSIX.1 standard or the related ISO/IEC standards. As of Windows XP, POSIX has been replaced by Windows Service for UNIX (SUA).

The integral subsystem looks after operating-system specific functions on behalf of the environment subsystem. It consists of a security subsystem, a workstation service and a server service. The security subsystem deals with security tokens, grants or denies access to user accounts based on resource permissions, handles login requests and initiates login authentication, and determines which system resources need to be audited by Windows NT. It also looks after Active Directory. The workstation service is an API to the network redirector, which provides the computer access to the network. The server service is an API that allows the computer to provide network services.