An emulator in computer sciences duplicates (provides an emulation of) the functions of one system using a different system, so that the second system behaves like (and appears to be) the first system. This focus on exact reproduction of external behavior is in contrast to some other forms of computer simulation, which can concern an abstract model of the system being simulated.
Emulation refers to the ability of a computer program or electronic device to imitate another program or device. Many printers, for example, are designed to emulate Hewlett-Packard LaserJet printers because so much software is written for HP printers. If a non-HP printer emulates an HP printer, any software written for a real HP printer will also run in the non-HP printer emulation and produce equivalent printing. A hardware emulator is an emulator which takes the form of a hardware device. Examples include the DOS-compatible card installed in some old-world Macintoshes like Centris 610 or Performa 630 that allowed them to run PC programs and FPGA-based hardware emulators. In a theoretical sense, the Church-Turing thesis implies that any operating environment can be emulated within any other. However, in practice, it can be quite difficult, particularly when the exact behavior of the system to be emulated is not documented and has to be deduced through reverse engineering. It also says nothing about timing constraints; if the emulator does not perform as quickly as the original hardware, the emulated software may run much more slowly than it would have on the original hardware, possibly triggering time interrupts to alter performance.
A video game console emulator is a program that allows a computer or modern console (cross-console emulation) to emulate a different video game console's behavior. Emulators are most often used to play older video games on personal computers and modern video game consoles, but they are also used to play games translated into other languages or modify (or hack) existing games. Emulators are also a useful tool in the development process of homebrewed demos and new games for older systems.
One advantage to ROM images is the potential for ROM hacking. Amateur programmers and gaming enthusiasts have produced translations of foreign games, rewritten dialogue within a game, applied fixes to bugs that were present in the original game, as well as updating old sports games with modern rosters. Software that emulates a console can be improved with additional capabilities that the original system did not have, such as anti-aliasing, running in High Definition video resolutions, anisotropic filtering (texture sharpening), audio interpolation, save states, online multiplayer options or the incorporation of cheat cartridge functionality. Not all emulation is of a questionable nature though. Consoles have legally used the technology to allow the playing of previous generation games.
Due to differences in hardware, the Xbox 360 is not natively backwards-compatible with original Xbox games. However, Microsoft achieved backwards-compatibility with popular titles through an emulator. The PlayStation 3 uses physical PlayStation hardware to play original PlayStation titles. In US 60gb models, original PS2 graphics and CPU hardware was also present to run PS2 titles, however the PAL and later US models removed the PS2 CPU, replacing it with software emulation working alongside the video hardware to achieve partial hardware/software emulation. In later releases, backwards compatibility with PS2 titles was completely removed when the PS2 graphics chip was removed and could not be emulated through software alone.
Commercial developers have also turned to emulation as a means to repackage and reissue their older games on new consoles. Square Enix has re-released several Final Fantasy titles on the PlayStation, Game Boy Advance, and DS; while Sega has created collections of Sonic the Hedgehog games. Likely the most notable example of commercial emulation is Nintendo's Virtual Console, which comes packaged with their seventh-generation system, the Wii. Virtual Console emulates various titles on the NES, SNES, Nintendo 64, Sega Master System, Sega Mega Drive (Genesis in the US), NEC's TurboGrafx-16 (PC Engine in Japan) and Turbo CD, SNK's Neo Geo, and Exclusive ones to some regions; Commodore 64 (In Europe and America) and MSX (In Japan), as well as select arcade games.
Source:WikiPedia
