Emulator

An emulator, in the most general sense, duplicates (provide an emulation of) the functionality of one system on a different system by translating calls designed for the target hardware into calls that the host system's software can understand and interpret correctly into an output, so that the software appears to behave identical to the target system. Unlike a simulation, it does not attempt to precisely model the state of the device being emulated; it only attempts to reproduce its behaviour. This behaviour can have differing degrees of accuracy between emulators.

A popular use of emulators is to run software and games, often referred to as ROMs, written for hardware that is no longer sold or readily available, such as the Commodore 64 or early Amiga models. Emulating these on modern desktop computers is usually less cumbersome than relying on the original machine, which may be inoperable. However, software licensing issues may require emulator authors to write original software that duplicates the functionality of the original computer's bootstrap ROM and/or BIOS, often through high-level emulation.

Legality of emulators

Most emulators are perfectly legal under United States and international law, protected by laws that cover reverse engineering. However, emulators can be illegal if they use copyrighted code from the original console, computer, or program.

Legality of ROM images

ROM images are copyrighted code and are protected by international law. The only legal images are homebrew games of original content, created by programmers, images released into public domain, or images downloaded with the permission of the copyright holder.^{[citation needed]}

Common legality myths

24-hour rule: Many emulation download sites like to claim that you can keep unlawfully downloaded ROMs for 24 hours and then delete them, and still remain legal. This rule has no basis in law.^{[citation needed]}
Owning the game: Owning the game in its original form does not make downloading a ROM image of it legal, according to the letter of the law. The only fully legal way is to make a backup image yourself using special hardware. However, this hardware can be expensive and/or complex to create (and illegal sometimes, as is the case of Nintendo ROM image extracting hardware), and many gamers consider it ethically correct to download an image as long as you own the original game.^{[citation needed]}