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ReactOS
ReactOS is a free and open-source operating system for i586/amd64 personal computers that is intended to be binary-compatible with computer programs and device drivers developed for Windows Server 2003 and later versions of Microsoft Windows. ReactOS has been noted as a potential open-source drop-in replacement for Windows and has been of interest for its information on undocumented Windows APIs.
ReactOS has been in development since 1996. As of October 2025[update], it is still considered to be feature-incomplete alpha software. Therefore, it is recommended by the developers to be used only for evaluation and testing purposes. However, many Windows applications are working, such as Adobe Reader 9.3, GIMP 2.6, and LibreOffice 5.4.
ReactOS is primarily written in C, with some elements written in C++, such as the ReactOS File Explorer. The project partially implements Windows API functionality and has been ported to the AMD64 processor architecture. ReactOS is part of the FOSS ecosystem so it re-uses and collaborates with many other FOSS projects, most notably the Wine project that presents a Windows compatibility layer for Unix-like operating systems.
Around 1996, a group of free and open-source software developers started a project called FreeWin95 to implement a clone of Windows 95. The project stalled in discussions on the design of the system.
While FreeWin95 had started out with high expectations, there still had not been any builds released to the public by the end of 1997. As a result, the project members, led by then coordinator Jason Filby, joined together to revive the project. The revived project sought to duplicate the functionality of Windows NT. In creating the new project, a new name, ReactOS, was chosen. The project began development in February 1998 by creating the basis for a new NT kernel and basic drivers. The name ReactOS was coined during an IRC chat. While the term "OS" stood for operating system, the term "react" referred to the group's dissatisfaction inspired reaction to Microsoft's monopolistic position.
In 2002, the ReactOS Foundation was established in Moscow with Maxim Osowski and Aleksey Bragin as executive officers and Vladimir Bragin, Saveliy Tretiakov, and Alexey Ivanov on the board of directors. In 2015, the foundation was liquidated.
In order to avoid copyright prosecution, ReactOS had to be expressly completely distinct and non-derivative from Windows, a goal that needed very careful work. A claim was made on 17 January 2006 by developer Hartmut Birr on the ReactOS developers mailing list (ros-dev) that ReactOS contained code derived from disassembling Microsoft Windows. The code that Birr disputed involved the function BadStack in syscall.S, as well as other unspecified items. Comparing this function to disassembled binaries from Windows XP, Birr argued that the BadStack function was simply copy-pasted from Windows XP, given that they were identical. Alex Ionescu, the author of the code, asserted that while the Windows XP binary in question was indeed disassembled and studied, the code was not merely copy-pasted, but reimplemented; the reason why the functions were identical, Ionescu claimed, was because there was only one possible way to implement the function.
On 27 January 2006, the developers responsible for maintaining the ReactOS code repository disabled access after a meeting was held to discuss the allegations. When approached by NewsForge, Microsoft declined to comment on the incident. Since ReactOS is a free and open-source software development project, the claim triggered a negative reaction from the free software community: in particular, Wine barred several inactive developers from providing contributions[citation needed] and formal high-level cooperation between the two projects remained difficult as of 2006[update].
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ReactOS AI simulator
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ReactOS
ReactOS is a free and open-source operating system for i586/amd64 personal computers that is intended to be binary-compatible with computer programs and device drivers developed for Windows Server 2003 and later versions of Microsoft Windows. ReactOS has been noted as a potential open-source drop-in replacement for Windows and has been of interest for its information on undocumented Windows APIs.
ReactOS has been in development since 1996. As of October 2025[update], it is still considered to be feature-incomplete alpha software. Therefore, it is recommended by the developers to be used only for evaluation and testing purposes. However, many Windows applications are working, such as Adobe Reader 9.3, GIMP 2.6, and LibreOffice 5.4.
ReactOS is primarily written in C, with some elements written in C++, such as the ReactOS File Explorer. The project partially implements Windows API functionality and has been ported to the AMD64 processor architecture. ReactOS is part of the FOSS ecosystem so it re-uses and collaborates with many other FOSS projects, most notably the Wine project that presents a Windows compatibility layer for Unix-like operating systems.
Around 1996, a group of free and open-source software developers started a project called FreeWin95 to implement a clone of Windows 95. The project stalled in discussions on the design of the system.
While FreeWin95 had started out with high expectations, there still had not been any builds released to the public by the end of 1997. As a result, the project members, led by then coordinator Jason Filby, joined together to revive the project. The revived project sought to duplicate the functionality of Windows NT. In creating the new project, a new name, ReactOS, was chosen. The project began development in February 1998 by creating the basis for a new NT kernel and basic drivers. The name ReactOS was coined during an IRC chat. While the term "OS" stood for operating system, the term "react" referred to the group's dissatisfaction inspired reaction to Microsoft's monopolistic position.
In 2002, the ReactOS Foundation was established in Moscow with Maxim Osowski and Aleksey Bragin as executive officers and Vladimir Bragin, Saveliy Tretiakov, and Alexey Ivanov on the board of directors. In 2015, the foundation was liquidated.
In order to avoid copyright prosecution, ReactOS had to be expressly completely distinct and non-derivative from Windows, a goal that needed very careful work. A claim was made on 17 January 2006 by developer Hartmut Birr on the ReactOS developers mailing list (ros-dev) that ReactOS contained code derived from disassembling Microsoft Windows. The code that Birr disputed involved the function BadStack in syscall.S, as well as other unspecified items. Comparing this function to disassembled binaries from Windows XP, Birr argued that the BadStack function was simply copy-pasted from Windows XP, given that they were identical. Alex Ionescu, the author of the code, asserted that while the Windows XP binary in question was indeed disassembled and studied, the code was not merely copy-pasted, but reimplemented; the reason why the functions were identical, Ionescu claimed, was because there was only one possible way to implement the function.
On 27 January 2006, the developers responsible for maintaining the ReactOS code repository disabled access after a meeting was held to discuss the allegations. When approached by NewsForge, Microsoft declined to comment on the incident. Since ReactOS is a free and open-source software development project, the claim triggered a negative reaction from the free software community: in particular, Wine barred several inactive developers from providing contributions[citation needed] and formal high-level cooperation between the two projects remained difficult as of 2006[update].
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