The NT Architecture of Windows, a line of operating systems produced and sold by Microsoft, is a layered design that consists of two main components, user mode, and kernel mode. It is a preemptive, reentrant multitasking operating system, which has been designed to work with uniprocessor and symmetrical multiprocessor (SMP)-based computers. To process input/output (I/O) requests, they use packet-driven I/O, which utilizes I/O request packets (IRPs) and asynchronous I/O. Starting with Windows XP, Microsoft began making 64-bit versions of Windows available; before this, there were only 32-bit versions of these operating systems.

windows nt architecture


User mode is made up of various system-defined processes and DLLs.

The interface between user-mode applications and operating system kernel functions is called an “environment subsystem.” Windows NT can have more than one of these, each implementing a different API set. This mechanism was designed to support applications written for many different types of operating systems. None of the environment subsystems can directly access hardware; access to hardware functions is done by calling into kernel mode routines.

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

Win32 Subsystem

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 that runs in its own address space and 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 thread on which to run. However, Windows NT does allow users to run a Win16 program in a separate Win16 VDM, which allows the program to be preemptively multi-tasked, as Windows NT will preempt the whole VDM process, which only contains one running application. The Win32 environment subsystem process (csrss.exe) also includes the window management functionality, sometimes called 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 windows and menus, in response to these messages.

OS/2 environment subsystem

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.

POSIX environment subsystem

The POSIX environment subsystem supports applications that are strictly written to either the POSIX.1 standard or the related ISO/IEC standards. This subsystem has been replaced by Interix, which is a part of Windows Services for UNIX. This was in turn replaced by the Windows Subsystem for Linux.


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 logon requests and initiates logon authentication, and determines which system resources need to be audited by Windows 2000. 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.


Windows 2000 kernel mode has full access to the hardware and system resources of the computer and runs code in a protected memory area. It controls access to scheduling, thread prioritization, memory management, and the interaction with hardware. The kernel-mode stops user mode services and applications from accessing critical areas of the operating system that they should not have access to as user mode processes ask the kernel mode to perform such operations on its behalf.

Kernel-mode consists of executive services, which is itself made up of many modules that do specific tasks, kernel drivers, a kernel, and a Hardware Abstraction Layer, or HAL.


The Executive interfaces with all the user mode subsystems. It deals with I/O, object management, security and process management. It contains various components, including the I/O Manager, the Security Reference Monitor, the Object Manager, the IPC Manager, the Virtual Memory Manager (VMM), a PnP Manager and Power Manager, as well as a Window Manager which works in conjunction with the Windows Graphics Device Interface (GDI). Each of these components exports a kernel-only support routine allows other components to communicate with one another. Grouped together, the components can be called executive services. No executive component has access to the internal routines of any other executive component.

The object manager is a special executive subsystem that all other executive subsystems must pass through to gain access to Windows 2000 resources — essentially making it a resource management infrastructure service. The object manager is used to reduce the duplication of object resource management functionality in other executive subsystems, which could potentially lead to bugs and make the development of Windows 2000 harder. To the object manager, each resource is an object, whether that resource is a physical resource (such as a file system or peripheral) or a logical resource (such as a file). Each object has a structure or object type that the object manager must know about. When another executive subsystem requests the creation of an object, they send that request to the object manager which creates an empty object structure which the requesting executive subsystem then fills in. Object types define the object procedures and any data specific to the object. In this way, the object manager allows Windows 2000 to be an object-oriented operating system, as object types can be thought of as classes that define objects.

Each instance of an object that is created stores its name, parameters that are passed to the object creation function, security attributes and a pointer to its object type. The object also contains an object close procedure and a reference count to tell the object manager how many other objects in the system reference that object and thereby determines whether the object can be destroyed when a close request is sent to it . Every object exists in a hierarchical object namespace.

Further executive subsystems are the following:

  • I/O Manager: allows devices to communicate with user-mode subsystems. It translates user-mode read and writes commands in reading or write IRPs which it passes to device drivers. It accepts file system I/O requests and translates them into device-specific calls, and can incorporate low-level device drivers that directly manipulate the hardware to either read input or write the output. It also includes a cache manager to improve disk performance by caching read requests and write to the disk in the background
  • Security Reference Monitor (SRM): the primary authority for enforcing the security rules of the security integral subsystem. It determines whether an object or resource can be accessed, via the use of access control lists (ACLs), which are themselves made up of access control entries (ACEs). ACEs contain a security identifier (SID) and a list of operations that the ACE gives a select group of trustees — a user account, group account, or logon session — permission (allow, deny, or audit) to that resource.
  • IPC Manager: short for Interprocess Communication Manager, this manages the communication between clients (the environment subsystem) and servers (components of the Executive). It can use two facilities: the Local Procedure Call (LPC) facility (clients and servers on the one computer) and the Remote Procedure Call (RPC) facility (where clients and servers are situated on different computers. Microsoft has had significant security issues with the RPC facility.
  • Virtual Memory Manager: manages virtual memory, allowing Windows 2000 to use the hard disk as a primary storage device (although strictly speaking it is secondary storage). It controls the paging of memory in and out of physical memory to disk storage.
  • Process Manager: handles process and thread creation and termination
  • PnP Manager: handles Plug and Play and supports device detection and installation at boot time. It also has the responsibility to stop and start devices on-demand — sometimes this happens when a bus gains a new device and needs to have a device driver loaded to support that device. Both FireWire and USB are hot-swappable and require the services of the PnP Manager to load, stop and start devices. The PnP manager interfaces with the HAL, the rest of the executive (as necessary), and with device drivers.
  • Power Manager: the power manager deals with power events and generates power IRPs. It coordinates these power events when several devices send a request to be turned off it determines the best way of doing this.
  • The display system has been moved from user mode into the kernel-mode as a device driver contained in the file Win32k.sys. There are two components in this device driver — the Window Manager and the GDI:
    • Window Manager: responsible for drawing windows and menus. It controls the way that output is printed to the screen and handles input events (such as from the keyboard and mouse), then passes messages to the applications that need to receive this input
    • GDI: the Graphics Device Interface is responsible for tasks such as drawing lines and curves, rendering fonts, and handling palettes. Windows 2000 introduced native alpha blending into the GDI.


The kernel sits between the HAL and the Executive and provides multiprocessor synchronization, thread and interrupt scheduling and dispatching, and trap handling, and exception dispatching. The kernel often interfaces with the process manager. The kernel is also responsible for initializing device drivers at bootup that are necessary to get the operating system up and running.

Windows 2000 uses kernel-mode device drivers to enable it to interact with hardware devices. Each of the drivers has well-defined system routines and internal routines that it exports to the rest of the operating system. All devices are seen by user-mode code as a file object in the I/O manager, though to the I/O manager itself the devices are seen as device objects, which it defines as either file, device, or driver objects. Kernel-mode drivers exist in three levels: highest level drivers, intermediate drivers, and low-level drivers. The highest level drivers, such as file system drivers for FAT and NTFS, rely on intermediate drivers. Intermediate drivers consist of function drivers — or main driver for a device — that are optionally sandwiched between lower and higher-level filter drivers. The function driver then relies on a bus driver — or a driver that services a bus controller, adapter, or bridge — which can have an optional bus filter driver that sits between itself and the function driver. Intermediate drivers rely on the lowest level drivers to function. The Windows Driver Model (WDM) exists in the intermediate layer. The lowest level drivers are either legacy Windows NT device drivers that control a device directly or can be a PnP hardware bus. These lower-level drivers directly control hardware and do not rely on any other drivers.


The Windows NT design includes many of the same objectives as Mach, the archetypal microkernel system, one of the most important being its structure as a collection of modules that communicate via well-known interfaces, with a small microkernel limited to core functions such as first-level interrupt handling, thread scheduling, and synchronization primitives. This allows for the possibility of using either direct procedure calls or interprocess communication (IPC) to communicate between modules, and hence for the potential location of modules in different address spaces (for example in either kernel space or server processes). Other design goals shared with Mach included support for diverse architectures, a kernel with abstractions general enough to allow multiple operating system personalities to be implemented on top of it, and an object-oriented organization.

The primary operating system personality on Windows is the Windows API, which is always present. The emulation subsystem which implements the Windows personality is called the Client/Server Runtime Subsystem (csrss.exe). On versions of NT prior to 4.0, this subsystem process also contained the window manager, graphics device interface, and graphics device drivers. For performance reasons, however, in version 4.0 and later, these modules (which are often implemented in user mode even on monolithic systems, especially those designed without internal graphics support) run as a kernel-mode subsystem.

Applications that run on NT are written to one of the OS personalities (usually the Windows API), and not to the native NT API for which documentation is not publicly available (except for routines used in device driver development). An OS personality is implemented via a set of user-mode DLLs (see Dynamic-link library), which are mapped into application processes’ address spaces as required, together with an emulation subsystem server process (as described previously). Applications access system services by calling into the OS personality DLLs mapped into their address spaces, which in turn call into the NT run-time library (ntdll.dll), also mapped into the process address space. The NT run-time library services these requests by trapping into kernel mode to either call kernel-mode Executive routines or make Local Procedure Calls(LPCs) to the appropriate user-mode subsystem server processes, which in turn use the NT API to communicate with application processes, the kernel-mode subsystems and each other.


The Windows 2000 Hardware Abstraction Layer, or HAL, is a layer between the physical hardware of the computer and the rest of the operating system. It was designed to hide differences in hardware and therefore provide a consistent platform on which applications may run. The HAL includes hardware-specific code that controls I/O interfaces, interrupt controllers, and multiple processors.

Windows 2000 was designed to support the 64-bit DEC Alpha. After Compaq announced they would discontinue support of the processor, Microsoft stopped releasing tests build of Windows 2000 for AXP to the public, stopping with beta 3. Development of Windows on the Alpha continued internally to continue to have a 64-bit architecture development model ready until the wider availability of the Intel Itanium IA-64 architecture. The HAL now only supports hardware that is compatible with the Intel x86 architecture.

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