Video game conversion

In video gaming parlance, a conversion is the production of a game on one computer or console that was originally written for another system. Over the years, video game conversion has taken form in a number of different ways, both in their style and the method in which they were converted.
History.
The earliest video game conversions were almost exclusively home versions of popular arcade games. The first examples were conversions of Atari's "Pong" in the form of consoles with this one game built-in, as well as consoles that included a number of variations on the game. Atari produced their own "official" conversion of the game for home use, but a number of other imitators such as Sears' Telegames Pong IV were also on the market.
With the beginning of the video game era, Atari released their Atari 2600 console for which they licensed and produced a number of home conversions of popular arcade titles, including "Pac-Man" by "Namco", "Space Invaders" and "Defender." Later, other third-party developers and publishers such as Activision and Coleco produced games like "Donkey Kong" for the Atari 2600.
The mid-1980s and the mid-1990s saw a flurry of conversions of popular arcade games to home computers such as the ZX Spectrum, Commodore 64, Amstrad CPC, Commodore Amiga and Atari ST. These games were mostly developed and published by groups who were not involved with the original developers, but who had bought the rights to create reproductions of these games. Console versions of these arcade games, however, were often produced by the original developers of the arcade title.
Types of conversions.
Direct conversions.
Direct conversions, also referred to as "straight conversions", are conversions in which the source code of the original game is used with relatively few modifications. Direct conversions were fairly rare until the second half of the 1990s. In the case of arcade conversions, this was because arcade systems were usually much more advanced than their contemporary home-based systems, which thus could not accurately recreate the speed, graphics, audio, and in some cases even the gameplay algorithms of arcade games. In the case of personal computer conversions, most games pre-1995 were produced in assembly language, and source-based conversions could not be reproduced on systems with other processors, rendering the original source code useless. Also, while most third-party developers had access to the original graphics and audio, they could not be faithfully reproduced on older home computers such as the ZX Spectrum and developers were forced to recreate the graphics and audio from scratch.
In the early 2000s, source-based conversions of games became more feasible and one-to-one pixel perfect conversions became commonplace.
Imitations/clones.
Imitations of popular arcade games were common, particularly in the early days of video gaming when copyright violations were treated less severely. While the game was fundamentally the same, the title, names, graphics and audio were usually changed to avoid legal challenges.
Developers have created "clones" of their own games. Escape (now Westone) produced a clone of "Wonder Boy" for the NES by the name of "Adventure Island" to circumnavigate a number of legal issues surrounding the "Wonder Boy" name and character.
Remakes.
Developers have remade older video games with modern technology. This was a particular phenomenon during the late 1990s with numerous 3D updates of games such as "Frogger", "Missile Command", "Asteroids" and "Space Invaders".
Retro/Emulation.
Advances in technology and a rising interest in retrogaming have incited a trend whereby collections of "classic" games, usually arcade games, are re-released on modern gaming systems in their original forms. This is usually carried out by means of custom emulators, which reproduce the activity of the original arcade ROMs. In other words, instead of rewriting the code for the game itself onto the new hardware system, the programmers write code which imitates the original hardware system and transfer the game code without alteration.
Nintendo's Game Boy Advance has had hundreds of conversions of older games, including games originally written for the SNES, the hardware of which forms the basis of the Game Boy Advance.