is an operating system that is binary compatible with most Microsoft Windows programs, features, and services. It is mostly licensed under the GNU General Public License, similar to the Linux kernel. ReactOS is built from reverse engineering portions of Windows XP, Windows 95, and Windows Server 2003. Because it is mostly binary compatible with Windows, many users choose it as an alternative to Linux while staying within the realm of free and open source software. Because of the method used to build ReactOS, it's developers claim that their work is entirely original, and that no code is derived from Windows - all developers who work on ReactOS must either write original code, or create documentation based on decompiled binaries of Windows, but not both, in order to comply with American laws on reverse engineering.

The ReactOS kernel is a re-implementation of Windows' ntoskrnl.exe, for Windows Server 2003. It has been combined with a port of the WINE project to provide a userland for the operating system, which is mostly identical to the standard userland available on Windows. As a result, ReactOS is able to run any programs which WINE can run, and WINE has access to what the internals of Windows probably look like, advancing both projects to provide support for Windows programs on UNIX-like systems, such as Microsoft's Xenix, GNU\Linux, FreeBSD, and other operating systems. ReactOS also has limited support for DirectX 9, allowing some games available for Windows XP to run normally under ReactOS without modification.

ReactOS is available for x86 processors, x86-64 processors, ARM, and PowerPC.


ReactOS was first released in 1998 as version 0.07. It was initially a clone of Windows 95, called "FreeWin95". It's name comes from the reaction the developers had to Windows being closed source. It's most recent release is: 0.4.2. It is currently in its alpha stage, and not recommended for use other than evaluation.

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