Major features

A main design goal of NT was hardware and software portability. Versions of NT were available for a variety of processor architectures, namely Intel IA-32, MIPS, Alpha, PowerPC, SPARC, Intel i860, and Intel i960. Broad software compatibility was achieved with support for several API "personalities", including the primary Win32 API and limited support for POSIX and OS/2 APIs. Partial MS-DOS compatibility was achieved via an integrated DOS Virtual Machine. For secure multiuser server solutions, NT 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 supported Windows network protocols, inheriting the previous OS/2 LAN Manager networking, as well as Unix's TCP/IP networking (for which Microsoft would implement a TCP/IP stack derived from the BSD Unix stack).

Windows NT 3.1 was the first version of Windows to utilize 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, 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.

Notably, in Windows NT 3.x, several I/O driver subsystems, such as video and printing, were user-mode subsystems. In Windows NT 4, the video, server and printer spooler subsystems were integrated into the kernel. Windows NT's first GUI was strongly influenced by (and programmatically compatible with) that from Windows 3.1; Windows NT 4's interface was redesigned to match that of the brand new Windows 95, moving from the Program Manager to the Start Menu/Taskbar design.

NTFS, a journaled, secure file system, was created for NT. NT also allows for other installable file systems, and with versions 3.1 and 3.51, NT could also be installed on DOS's FAT or OS/2's HPFS file systems. Later versions could be installed on a FAT partition gaining speed at the expense of security, but this option is no longer present in Windows Vista.

Development

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. In addition to working on three versions of OS/2, Microsoft continued parallel development of the DOS-based and less resource-demanding Windows environment. When Windows 3.0 was released in May 1990, it 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 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 and RSX-11. The operating system was designed to run on multiple instruction set architectures and multiple hardware platforms within each architecture. The platform dependencies are largely hidden from the rest of the system by a kernel mode module called the HAL (Hardware Abstraction Layer).

Windows NT's kernel mode code further distinguishes between the "kernel", whose primary purpose is to implement processor and architecture dependent functions, and the "executive". This has led some writers to refer to the kernel as a microkernel, but the Windows NT kernel no longer meets many of the criteria of a "microkernel", although this was the original goal of chief architect Cutler. 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; it was this that allowed the late adoption of the Windows API (into the Win32 subsystem). Windows NT was one of the earliest operating systems to use Unicode internally.