DOSBox

DOSBox is a free and open-source emulator of an Intel x86 personal computer designed for the purpose of running software created for disk operating systems on IBM PC compatibles, primarily DOS video games. It was first released in 2002, when DOS technology was becoming obsolete. It is notable for its widespread use for running DOS games, as well as being used in commercial re-releases of those games.
Development.
Before Windows XP, consumer-oriented versions of Windows were based on MS-DOS. Windows 3.0 and its updates were operating environments that ran on top of MS-DOS, and the Windows 9x series consisted of operating systems that were still based on MS-DOS. These versions of Windows could run DOS applications. Conversely, the Windows NT operating systems were not based on DOS. A member of the series is Windows XP, which debuted on October 25, 2001, to become the first consumer-oriented version of Windows to not use DOS. Although Windows XP could emulate DOS, it could not run many of its applications, as those applications ran only in real mode to directly access the computer's hardware, and Windows XP's protected mode prevented such direct access for security reasons. MS-DOS continued to receive support until the end of 2001, and all support for any DOS-based Windows operating system ended on July 11, 2006.
The development of DOSBox began around the launch of Windows 2000—a Windows NT system—when its creators, two Dutch programmers Peter Veenstra and Sjoerd van der Berg, discovered that the operating system had dropped much of its support for DOS software. The two knew of solutions at the time, but they could not run the applications in windowed mode or scale the graphics. The project was first uploaded to SourceForge and released for beta testing on July 22, 2002.
Features.
DOSBox is a command-line program, configured either by a set of command-line arguments or by editing a plain text configuration file. For ease of use, several graphical front ends have been developed by the user community.
The DOSBox project aims to be fully compatible with all DOS programs, and tries to replicate the experience as accurately as possible. In the vanilla version, long filenames are not supported; because DOS does not support them, filenames must follow the 8.3 naming convention, with a maximum of 8 characters before the full stop, followed by up to 3 characters for the file extension. Otherwise, they will be aliased to follow the convention.
There are versions available on the DOSBox website that support long filenames, at the cost of possible compatibility with some older programs.
The focus of the vanilla version is on gaming, and features such as support for Ctrl-Break may be missing. Some of the alternative versions support features not present in the vanilla version such as APM power off, direct parallel port passthrough for printing, and support for East Asian characters. 
Because DOSBox accesses the host computer's file system, there thus is a risk of DOS malware exploiting the emulator's security vulnerabilities and causing damage to the host machine, although these vulnerabilities continue to be patched with new DOSBox updates.
Users can also capture screenshots and record videos of DOS sessions, although a codec is required to play the videos. It is also possible to record OPL sound card and MIDI commands, as well as save sound output on a WAV file. Keyboard keys and the buttons of a game controller can be mapped to other keys and combinations thereof.
OS emulation.
DOSBox is a full-system emulator that provides BIOS interrupts and contains its own internal DOS-like shell. This means that it can be used without owning a license to any real DOS operating system. Most commands that are found in COMMAND.COM are supported, but many of the more advanced commands found in the latest MS-DOS versions are not. In addition to its internal shell, it also supports running image files of games and software originally intended to start without any operating system. Besides emulating DOS, users can also run Windows 3.0 and applications designed for it, as well as versions of Windows within the Windows 9x family.
When the DOSBox application is opened, it automatically mounts to a virtual, permanent codice_1 drive that stores DOSBox commands and utilities. The reasons for the virtual drive are related to security, but the user can mount a different drive letter in the emulator to a directory, image file, floppy disk drive, or CD-ROM drive on the host to access its data. A configuration file and its AUTOEXEC section can be used to respectively configure DOSBox settings and execute DOS commands at startup.
Hardware emulation.
DOSBox is capable of running DOS programs that require the CPU to be in real mode or protected mode. Since DOSBox can emulate its CPU by interpretation, the environment it emulates is completely independent of the host CPU. On systems which provide the x86, ARM, or RISC instruction sets, however, DOSBox can use dynamic instruction translation to accelerate execution. The emulated CPU speed of DOSBox is also manually adjustable by the user to accommodate the speed of the systems for which DOS programs were originally written.
DOSBox uses the Simple DirectMedia Layer external library to not only build new versions of DOSBox from source, but also handle graphics, audio, and input devices. Graphically, it can use the DirectDraw or OpenGL APIs, and can also use bilinear interpolation and scale graphics for computers with modern displays. Graphical emulation includes text mode, Hercules, CGA, EGA, VGA, VESA, S3 Trio 64, and Tandy. Sound emulation includes the PC speaker, AdLib, Gravis Ultrasound, Sound Blaster, Disney Sound Source, Tandy, and MPU-401. However, because DOSBox does not come packaged with Gravis Ultrasound drivers, they need to be installed separately for full support.
DOSBox can simulate serial null modems using the TCP/IP protocol and IPX network tunneling, which allows for DOS multiplayer games using one of them to be played over local area networks or the Internet. It can also simulate the PC joystick port, with limited options being to emulate one joystick with 4 axes and 4 buttons; one gamepad with 2 axes and 6 buttons; two joysticks each with 2 axes and 2 buttons; a Thrustmaster Flight Control System joystick that has 3 axes, 4 buttons, and a hat switch; and a CH Flightstick with 4 axes, 6 buttons that can be pressed only one at a time, and a hat switch. Newer joysticks and gamepads will need to use one of these configurations to function.
Reception.
DOSBox has become the "de facto" standard for running DOS games. "Rock, Paper, Shotgun" positively remarked on the project's continual reception of updates, its influence on PC gaming, and some front ends designed to facilitate using it. Freelance writer Michael Reed lauded the quality of scaled graphics and the project's overall focus on compatibility and accurate emulation, but criticized the lack of both save states and user-friendly control over the emulator during runtime, even with the front ends available at the time of his review. DOSBox was named SourceForge's Project of the Month in May 2009 and again in January 2013, making it the first project in the website's history to receive two Project of the Month awards. On the SourceForge website, it reached 10 million downloads on July 21, 2008, and was downloaded more than 25 million times as of October 2015.
Usage.
Since January 2011, the developers of the Wine compatibility layer have integrated DOSBox into Wine to facilitate running DOS programs that are not supported natively by the Wine Virtual DOS machine.
Since January 2015, the Internet Archive has added thousands of DOS games to its software library. Its DOSBox fork, Em-DOSBox, uses Emscripten to convert the emulator's C++ code to JavaScript, making the games playable on a web browser. The collection is provided for "scholarship and research purposes only". , the DOS library contained 6,934 games.
Commercial.
DOSBox has also been both the most used DOS emulator and, because of the straightforward process of making the games work on modern computers, the most popular emulation software for developers re-releasing legacy versions of their games. id Software has used DOSBox to re-release vintage games such as "Wolfenstein 3D" and "Commander Keen" on Valve's Steam. In the process, it was reported they violated the program's license, the GNU GPL; the breach, which was reported as an oversight, was promptly resolved. Activision Blizzard has also used it to re-release Sierra Entertainment's DOS games. LucasArts used it to rerelease ' and ' for modern machines on Steam and GOG.com. 2K Games producer Jason Bergman stated the company used DOSBox for Steam rereleases of certain installments of the "XCOM" series. Bethesda Softworks has recommended DOSBox and provided a link to the DOSBox website on the downloads page for ' and '. It also included DOSBox with both games in "The Elder Scrolls Anthology" release.
Electronic Arts' Origin client uses DOSBox for the platform's DOS games, including Electronic Arts titles such as "Syndicate" and "SimCity 2000".