Windows NT is a proprietary graphical operating system produced by Microsoft as part of its Windows product line, originally released for the workstation, office, and server markets as Windows NT 3.1 on July 27, 1993, and later available to consumers since the release of Windows XP in 2001. The underlying technology of Windows NT continues to exist to this day with incremental changes and improvements, with the latest version of Windows based on Windows NT being Windows Server 2025, which was announced in 2024.
The name "Windows NT" originally denoted the major technological advancements that it had introduced to the Windows product line, including eliminating the 16-bit memory access limitations of earlier Windows releases such as Windows 3.1. Each Windows release built on this technology is considered to be based on, if not a revision of Windows NT, even though the Windows NT name itself has not been used in any other Windows releases since Windows NT 4.0 in 1996.
Windows NT provides more features than other Windows releases, among them being support for multiprocessing, multi-user systems, a 32-bit kernel without legacy 16-bit considerations, 32-bit memory addressing, portability between different computer architectures, and other system services such as Active Directory and more. Newer versions of Windows NT support 64-bit computing, with a 64-bit kernel and 64-bit memory addressing while retaining compatibility with 32-bit software.
Product line
Windows NT is a group or family of products—like Windows is a group or family. Windows NT is a sub-grouping of Windows.
The first version of Windows NT, 3.1, was produced for workstation and server computers. It was commercially focused—and intended to complement consumer versions of Windows that were based on MS-DOS (including Windows 1.0 through Windows 3.1x). In 1996, Windows NT 4.0 was released, including the new shell from Windows 95.
Eventually, Microsoft incorporated the Windows NT technology into the Windows product line for personal computing and deprecated the Windows 9x family. Starting with Windows 2000, "NT" was removed from the product name yet is still in several low-level places in the system—including for a while as part of the product version.
Installing Versions of Windows NT are installed using Windows Setup, which, starting with Windows Vista, uses the Windows Preinstallation Environment, which is a lightweight version of Windows NT made for deployment of the operating system.
Since Windows Vista, the Windows installation files, as well as the preinstallation environment used to install Windows, are stored in the Windows Imaging Format. It is possible to use the Deployment Image Servicing and Management (DISM) tool to install Windows from the command line and skip the GUI installer.
Naming It has been suggested that Dave Cutler intended the initialism "WNT" as a play on VMS, incrementing each letter by one. However, the project was originally intended as a follow-on to OS/2 and was referred to as "NT OS/2" before receiving the Windows brand. Two of the original NT developers, Mark Lucovsky and Dave W. Plummer, state that the name was taken from the original target processor—the Intel i860, code-named N10 ("N-Ten"). A 1991 video featuring Bill Gates and Microsoft products specifically says that "Windows NT stands for 'New Technology. Seven years later in 1998, during a question-and-answer (Q&A) session, he declared that the letters no longer carried any specific meaning. The letters were dropped from the names of releases from Windows 2000 and later, though Microsoft described that product as being "Built on NT Technology".
"NT" was a trademark of Northern Telecom (later Nortel), which Microsoft was forced to acknowledge on the product packaging.
Major features One of the main purposes of NT is hardware and software portability. Various versions of NT family operating systems have been released for a variety of processor architectures, initially IA-32, MIPS, and DEC Alpha, with PowerPC, Itanium, x86-64 and ARM supported in later releases. An initial idea was to have a common code base with a custom Hardware Abstraction Layer (HAL) for each platform. However, support for MIPS, Alpha, and PowerPC was later dropped in Windows 2000. Broad software compatibility was initially achieved with support for several API "personalities", including Windows API, POSIX, and OS/2 APIs—the latter two were phased out starting with Windows XP. Partial MS-DOS and Windows 16-bit compatibility is achieved on IA-32 via an integrated DOS Virtual Machine—although this feature is not available on other architectures.
NT has supported per-object (file, function, and role) access control lists allowing a rich set of security permissions to be applied to systems and services. NT has also supported Windows network protocols, inheriting the previous OS/2 LAN Manager networking, as well as TCP/IP networking (for which Microsoft used to implement a TCP/IP stack derived at first from a STREAMS-based stack from Spider Systems, then later rewritten in-house).
Windows NT 3.1 was the first version of Windows to use 32-bit flat virtual memory addressing on 32-bit processors. Its companion product, Windows 3.1, used segmented addressing and switches from 16-bit to 32-bit addressing in pages.
Windows NT 3.1 featured a core kernel providing a system API, running in supervisor mode (ring 0 in x86; referred to in Windows NT as "kernel mode" on all platforms), and a set of user-space environments with their own APIs which included the new Win32 environment, an OS/2 1.3 text-mode environment and a POSIX environment. The full preemptive multitasking kernel could interrupt running tasks to schedule other tasks, without relying on user programs to voluntarily give up control of the CPU, as in Windows 3.1 applications (although MS-DOS applications were preemptively multitasked in Windows starting with Windows/386).
Notably, in Windows NT 3.x, several I/O driver subsystems, such as video and printing, were user-mode subsystems. In Windows NT 4.0, the video, server, and printer spooler subsystems were moved into kernel mode. Windows NT's first GUI was strongly influenced by (and programmatically compatible with) that from Windows 3.1; Windows NT 4.0's interface was redesigned to match that of the brand-new Windows 95, moving from the Program Manager to the Windows shell design.
NTFS, a journaled, secure file system, is a major feature of NT. Windows NT also allows for other installable file systems; NT can also be installed on FAT file systems, and versions 3.1, 3.5, and 3.51 could be installed on HPFS file systems.
Windows NT introduced its own driver model, the Windows NT driver model, and is incompatible with older driver frameworks. With Windows 2000, the Windows NT driver model was enhanced to become the Windows Driver Model, which was first introduced with Windows 98, but was based on the NT driver model. Windows Vista added native support for the Windows Driver Foundation, which is also available for Windows XP, Windows Server 2003 and to an extent, Windows 2000.
Development
Microsoft decided to create a portable operating system, compatible with OS/2 and POSIX and supporting multiprocessing, in October 1988.. When development started in November 1989, Windows NT was to be known as OS/2 3.0, the third version of the operating system developed jointly by Microsoft and IBM. To ensure portability, initial development was targeted at the Intel i860XR RISC processor, switching to the MIPS R3000 in late 1989, and then the Intel i386 in 1990. Microsoft also continued parallel development of the DOS-based and less resource-demanding Windows environment, resulting in the release of Windows 3.0 in May 1990.
Windows 3.0 was eventually so successful that Microsoft decided to change the primary application programming interface for the still unreleased NT OS/2 (as it was then known) from an extended OS/2 API to an extended Windows API. This decision caused tension between Microsoft and IBM and the collaboration ultimately fell apart.
IBM continued OS/2 development alone while Microsoft continued work on the newly renamed Windows NT. Though neither operating system would immediately be as popular as Microsoft's MS-DOS or Windows products, Windows NT would eventually be far more successful than OS/2.
Microsoft hired a group of developers from Digital Equipment Corporation led by Dave Cutler to build Windows NT, and many elements of the design reflect earlier DEC experience with Cutler's VMS, VAXELN and RSX-11, but also an unreleased object-based operating system developed by Cutler at Digital codenamed MICA. The team was joined by selected members of the disbanded OS/2 team, including Moshe Dunie.
Although NT was not an exact clone of Cutler's previous operating systems, DEC engineers almost immediately noticed the internal similarities. Parts of VAX/VMS Internals and Data Structures, published by Digital Press, accurately describe Windows NT internals using VMS terms. Furthermore, parts of the NT codebase's directory structure and filenames matched that of the MICA codebase. Instead of a lawsuit, Microsoft agreed to pay DEC $65–100 million, help market VMS, train Digital personnel on Windows NT, and continue Windows NT support for the DEC Alpha.
Windows NT and VMS memory management, processes, and scheduling are very similar. Windows NT's process management differs by implementing threading, which DEC did not implement until VMS 7.0 in 1995.
Like VMS, Windows NT's kernel mode code distinguishes between the "kernel", whose primary purpose is to implement processor- and architecture-dependent functions, and the "executive". This was designed as a modified microkernel, as the Windows NT kernel was influenced by the Mach microkernel developed by Richard Rashid at Carnegie Mellon University, but does not meet all of the criteria of a pure microkernel. Both the kernel and the executive are linked together into the single loaded module ntoskrnl.exe; from outside this module, there is little distinction between the kernel and the executive. Routines from each are directly accessible, as for example from kernel-mode device drivers.
API sets in the Windows NT family are implemented as subsystems atop the publicly undocumented "native" API; this allowed the late adoption of the Windows API (into the Win32 subsystem). Windows NT was one of the earliest operating systems to use UCS-2 and UTF-16 internally.
Architecture
Windows NT uses a layered design architecture that consists of two main components, user mode and kernel mode. Programs and subsystems in user mode are limited in terms of what system resources they have access to, while the kernel mode has unrestricted access to the system memory and external devices. Kernel mode in Windows NT has full access to the hardware and system resources of the computer. The Windows NT kernel is a hybrid kernel; the architecture comprises a simple kernel, hardware abstraction layer (HAL), drivers, and a range of services (collectively named Executive), which all exist in kernel mode.
The booting process of Windows NT begins with NTLDR in versions before Vista and the Windows Boot Manager in Vista and later. The boot loader is responsible for accessing the file system on the boot drive, starting the kernel, and loading boot-time device drivers into memory. Once all the boot and system drivers have been loaded, the kernel starts the Session Manager Subsystem. This process launches winlogon, which allows the user to log in. Once the user is logged in File Explorer is started, loading the graphical user interface of Windows NT.
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