Doom is one of the most widely ported video games in the first-person shooter genre: starting with the original MS-DOS version (released as shareware on December 10, 1993), it has been released officially for a number of computer operating systems, video game consoles, handheld game consoles, and other devices.
Once Doom's source code was released into the public under the terms of the GNU General Public License in 1997, several unofficial adaptations were created by fans for many others still.
Some of the ports are replications of the DOS version, while others differ considerably, including modifications to creature design and game levels, with some ports even offering levels not included in the original version.
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