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Microsoft Windows is a desktop operating system developed by Microsoft since 1983. The latest released version as of June 2021 is Windows 10 May 2021 Update.

The first versions of Windows were an operating environment and graphical shell for MS-DOS. Windows 95 and later used MS-DOS for booting and kernel initialization. NT-based versions of Windows use a redesigned kernel and do not rely on MS-DOS.

16-bit Windows
Microsoft Windows debuted to the world during the Fall COMDEX 1983 computer expo as an operating environment running on top MS-DOS. The final version of the product with the version number of 1.01 was later released on 20 November 1985 and did not gain much popularity. Windows 1.0 was a cooperative multitasking desktop environment with a tiling window manager. The first versions of Windows used the MS-DOS Executive, which was a simple file manager, as a shell, which is generally the first application ran on startup providing the user experience. Other applications included in the first version of Windows included Calculator, Cardfile, Clipboard Viewer, Clock, Control Panel, Notepad, Paint, Reversi, Spooler, Terminal, and Write. Three minor updates were released in the two following years adding support for more hardware.

A major update called Windows 2.0 was released in 1987 adding features such as overlapping windows, which later lead to Apple filing a [https://en.wikipedia.org/wiki/Apple%20Computer,%20Inc.%20v.%20Microsoft%20Corp. lawsuit] for copyright infringement. This version also introduced general support for VGA and PS/2 mouse (later OEM versions of Windows 1.0 already included it). A separate edition called Windows/386 was also introduced that took advantage of the virtual 8086 mode of the i386 processor to multitask MS-DOS applications under Windows; this would be later known as the 386 Enhanced Mode and become the cornerstone of Windows 9x. In later revisions of the Windows 2.0 series, the original edition was renamed Windows/286.

Windows 3.0 was released in 1990 and became the first widely successful version of Windows. The new features included a revamped user experience consisting of the Program Manager, which allowed easy management of installed applications. A new File Manager was also included to replace the former shell, which was now deprecated. The previously separate 286 and 386 editions of Windows were unified into one version with the ability to operate in three different modes according to the hardware configuration:
 * Real mode, which was intended for computers with the original 8088/8086 processor. This was also the only mode which fully supported applications written for earlier versions of Windows.
 * Standard mode, which used the protected mode feature of the 286 and 386 processors in order to gain access to memory beyond the first 640K, although it still switched to real mode to run MS-DOS applications.
 * 386 Enhanced mode, which in addition to the protected mode also used the 386 processor's ability to create and manage virtual machines for real mode MS-DOS applications.

A major update dubbed Windows 3.1 followed in 1992, introducing the new flag logo with colored panes. The user interface was refreshed in this release, including new, more vivid icons. This version of Windows removed support for real mode and the MS-DOS Executive application. It was accompanied by a variant called Windows for Workgroups 3.1 with integrated networking support, which later received a larger update with version number 3.11, introducing 32-bit disk access and also removing the Standard mode. The regular variant of Windows also received the 3.11 update, although it was merely the kernel of Windows for Workgroups 3.11 backported to Windows 3.1.

A 32-bit TCP/IP stack was ported from an early version of Windows 95 and released in 1994 as a downloadable plugin for Windows for Workgroups 3.11, providing early testing for the 16/32-bit compatibility features of the next version of Windows.

Windows NT
In 1988, Microsoft hired Dave Cutler, a former DEC engineer, to create a portable 32-bit version of OS/2, the operating system that it worked on in collaboration with IBM. At the time, OS/2 was a 16-bit operating system very much tied to the x86 platform with considerable DOS heritage. The new operating system intended to be a preemptive multitasking system with multiprocessing support. The design included a portable kernel with executive services layered on top of a hardware abstraction layer, and multiple environmental subsystems running in user mode offering support for multiple APIs. Portable OS/2, or NT OS/2, would have included support for 16-bit and extended 32-bit variants of both OS/2 and Windows APIs, as well as virtual DOS machines and an implementation of POSIX which would allow Microsoft to run for US government contracts. Commercial release was planned for the early 1990s with NT shipping as OS/2 3.0, while IBM was adding 32-bit support to the original OS/2 codebase to create OS/2 2.0.

Initially, NT was developed for the Intel i860 processor, partially to ensure portability by preventing programmers from introducing x86-specific paradigms into code. When the team learned about the disappointing performance of the i860, the operating system was ported to the i386 and MIPS architectures. Following the success of Windows 3.0 in 1990, Microsoft decided to prioritize Windows in its plans, which later lead to the end of the Microsoft-IBM cooperation. As a part of this, the design of NT was slightly changed, with the Windows becoming the main environment providing the system's user interface, while OS/2 support was reduced to character mode OS/2 1.x applications. The change was first announced in January 1991 with the new operating system initially being called Advanced Windows.

DEC filed a lawsuit during NT development, alleging that Microsoft used stolen code from the DEC Mica operating system. The case was settled out of court, with Microsoft agreeing to add support for the DEC Alpha processor to NT.

User interface was added to Windows NT in the first half of 1991, with network support following during summer. The first prototype version was shipped to selected partners in September and the new operating system was publicly demonstrated for the first time at Fall COMDEX 1991 in October. A MIPS build first shipped in a December release. Later builds added crucial components such as the registry, NTFS and the ability to run 16-bit MS-DOS and Windows applications. Windows NT 3.1 was finally released on 27 October 1993 in workstation and server flavors.

An updated version, Windows NT 3.5 was released in 1994 and included performance improvement as well as a new full TCP/IP stack, long filename (LFN) support on the FAT file system and other refinements. Windows NT 3.51 was released in 1995, which ensured compatibility with the upcoming Windows 95 and added support for the PowerPC architecture, the PCMCIA interface, as well as file compression, or replaceable GINA.

Windows 9x
Microsoft first announced a 32-bit version of Windows that would run on top of MS-DOS in 1991. Several plans for a low-end Win32 environment were considered, including a variant of the NT kernel cut down in order to run on an average Windows 3.1 computer. In the end, Microsoft decided to build the new version on top of MS-DOS and Windows in 386 Enhanced mode.

On 24 August 1995, Microsoft released Windows 95, previously known under its codename "Chicago", with a brand new user interface with a Start menu, taskbar, and the desktop, as provided by the new Windows Explorer. Its hybrid 16/32-bit architecture made it possible to make and run 32-bit Windows applications and drivers while keeping a great degree of compatibility with already existing 16-bit ones. Among other improvements in this version was the support for long filenames through an extension to the FAT16 file system.

Windows 95 was the first widespread release of Windows to be packed together with a specific MS-DOS version, however, the old operating system was used only as a bootloader and a compatibility layer for ancient device drivers. Most MS-DOS user applications were either extended with Windows code or entirely replaced with a Windows version, keeping only the ones that were required to run without Windows, e.g. during the OS installation, such as  and.

The new user interface was also ported to the Windows NT line, with several Shell Technology Previews being released for Windows NT 3.51 during the development of Chicago and the Shell Update Release shipping in 1996 as Windows NT 4.0. Alongside the new shell, Windows NT 4.0 also notably moved a part of the GUI stack into kernel mode, which greatly improved performance.

During its lifetime Windows 95 saw several larger updates dubbed the OEM Service Releases (OSR) that were released only to computer manufacturers, specifically OSR 1.0, OSR 2.0, OSR 2.1, and OSR 2.5. A Service Pack was also released that updated an RTM copy of Windows 95 to the OSR 1.0 level. In 1997 a USB Supplement was released for OSR 2.x that added support for the then-new Universal Serial Bus interface.

The classic Windows line received a major update on 25 June 1998 with the release of Windows 98, codenamed "Memphis". It was the first version to integrate Internet Explorer deeply into the operating system's user interface as a part of the Windows Desktop Update. Many parts of the UI started using HTML and Internet Explorer's rendering engine to present a web-like user interface. A feature called Active Desktop made it even possible to set a webpage as the desktop background. Under the hood Windows 98 introduced the new Windows Driver Model, which enabled the use of the same drivers on Windows 9x as well as on the radically different Windows NT based operating systems.

A year later, Windows 98 received an update which was called the Second Edition, which included a new version of Internet Explorer, added Internet Connection Sharing and improved USB support.

In 2000, Windows Me (Millennium Edition), the last release of the classic Windows line was released. It carried over the improvements made to the user interface in its NT-based counterpart, Windows 2000. Windows Me is based on Windows 98, however, access to the real mode MS-DOS was restricted in order to decrease boot time among other changes to the kernel. It was infamously known for its stability problems partially caused by the rushing of its release following the cancelation of the Neptune project. It was replaced by Windows XP in 2001, ending the era of classic Windows.

Windows 2000 and Windows XP
In the late 1990s it became more feasible to discontinue the aging Windows 9x line and release a consumer version based on Windows NT. Microsoft started work on the the next version of Windows NT immediately after the release of Windows NT 4.0, which would improve the user experience of the high-end operating system. Similar to Memphis, the user interface of Windows NT 5.0 initially used Internet Explorer 4.0 with the Windows Desktop Update, being later updated to 5.0 and then 5.5 during development. Management tools, most of which were left unchanged since the first NT release, were rewritten using the new Microsoft Management Console. On servers, the new version brought improvements from the canceled Cairo project, such as Active Directory.

Beside working on the successor of Windows NT 4.0, Microsoft also released several new editions based on the NT 4.0 codebase. An Enterprise edition of Windows NT 4.0 Server was released in 1997, which was the predecessor of the Advanced Server SKU of later versions and was intended for high traffic and high demand workloads. Terminal Server Edition followed in 1998, which introduced the Terminal Services feature, allowing remote access to Windows computers. Windows NT Embedded 4.0 was introduced in 1999, a specialized version of the Workstation edition that allowed embedded vendors to pick only the required OS components for the intended workloads.

As Windows NT 5.0 got further delayed, the consumer NT aspect was moved into a following release codenamed Neptune, which would succeed Windows 98 and later Windows Me. In 1998, Microsoft announced that Windows NT 5.0 would be called Windows 2000, dropping the NT moniker to mark the NT technology becoming mainstream. In the end, it was released on 17 February 2000 with Internet Explorer 5.5 and earned the reputation as one of the most stable versions of Windows.

Work on Neptune begun several months before Windows 2000 was finished. The Neptune team heavily cooperated with the team working on Millennium, with the two projects sharing certain components. Development concentrated on a new task-oriented user interface called Activity Centers, which was also included in contemporary Millennium builds. The project also toyed around with the idea of hybrid startup, later implemented in Windows 8. Neptune was canceled in early 2000 together with its business counterpart codenamed Odyssey and replaced with a new, more conservative project codenamed Whistler that would eventually become Windows XP.

Windows XP was released on 25 October 2001 and was a relatively minor update to Windows 2000, mostly focusing on user experience and better compatibility with Windows 9x. A new theme called Luna was introduced, replacing the classic theme used in previous versions. A new login screen and fast user switching was also added, as well as better support for digital cameras, MS-DOS emulation and wireless networking. Windows XP was originally released in Home Edition and Professional variants, with an embedded version following a few months after. The 64-Bit Edition was also originally launched for the Itanium (IA-64) architecture.

Two additional editions for consumer use were introduced in 2002, both based on the original Professional edition. Windows XP Media Center Edition added the Windows Media Center (and later the Royale theme), while Windows XP Tablet PC Edition added tools and games for use on a tablet PC.

Development of the server counterpart continued after the release of the client version, resulting in Windows Server 2003, which was released in spring 2004 and used a newer kernel. It was also the base for an updated version of the Windows XP 64-Bit Edition. Work on an AMD64 version started late in the development process and finished in 2005 with the release of x64 editions of both Windows Server 2003 and Windows XP Professional (also based on 2003 codebase).

The Windows XP Starter Edition was introduced in 2004 for developing markets, including various Asian and South American countries and Russia. It is based on the original Home Edition, but is noticeably more limited with some features removed and only 3 programs being allowed to run at a time.

Look and feel
Windows allowed customization of its user interface since its first versions. Windows 1.0 and Windows 2.x allowed the user to change the color scheme in their Control Panel, however, there was no selection of pre-made color schemes and the user could only reset to the default scheme by manually editing.

This was improved with Windows 3.0 and its new Colors control panel, which added several color schemes for the user to choose from. Windows NT 3.1 and Windows NT 3.5x also featured the same customization options, although compared to 16-bit Windows where the default preset was provided by the video driver, the defaults were device independent on the new operating system.

Windows 95 introduced a new 3D look inspired by NeXTSTEP and initially only made use of solid colors. This was subsequently refined in Windows 98 and Windows 2000 with the possibility to use 2-color gradients for the titlebar.

Windows XP introduced visual styles, a set of bitmaps that allowed deeper customization of user interface elements. The original release of Windows XP shipped with Luna, while later releases also included Royale. However, users could still switch to Windows Classic if they so chose, which was necessary for accessibility functions such as the High Contrast themes that relied on the classic theme's ability to set individual colors.

Windows Vista introduced the hardware-accelerated Desktop Window Manager, which allowed for advanced effects such as translucent title bars used by the new Windows Aero theme. For users whose hardware couldn't handle Aero, Windows Vista also included the software rendered Windows Basic theme.

The option to disable the DWM was removed in Windows 8, together with the ability to switch to the classic theme. The Aero Lite theme was introduced in Windows 8 to replace the classic theme, which allows the user to customize its colors to a greater extent than regular visual styles, which allowed it to be used for the High Contrast accessibility feature. However, the classic theme implementation as well as support for disabled DWM still remains included with Windows for compatibility purposes.

Supported platforms
Since its initial release, Windows has supported numerous platforms and systems.

Source code
In 2004, incomplete copies of the source code of Windows NT 4.0 and Windows 2000 leaked to the Internet. These leaks were illegal, as the Windows source code is both a trade secret and copyrighted, and as so is protected by law. However, Microsoft has released parts of the source of the Windows Server 2003 kernel for research purposes.

In 2017, The Register and other technology journals reported about a leak of the Windows 10 Shared Source Kits, which are available to qualified customers, enterprises, governments, and partners for debugging and reference purposes, to BetaArchive. Following the controversy, BetaArchive removed all source code content from its archives, which also included the aforementioned incomplete copies of the Windows source code, and adopted a policy of not accepting any more source code material.

In 2018, the source code of the Windows NT File Manager was released on GitHub under the open source MIT license. This was later followed in 2019 by the Windows 10 Calculator application.

In May 2020, a copy of the source code of Windows NT 3.5 build 782.1 leaked to the 4chan /vp/ board. It is not yet known whether the leaked source is complete. This was later followed in September by the source code of Windows XP Service Pack 1 and Windows Server 2003. The leaked copy is mostly complete, but misses activation components, therefore it is likely that the leak originated from a Microsoft partner who had access to the source code rather than Microsoft itself. This code base had been apparently circulating in online circles since at least 2015.