Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially in telecommunications and computing.
Modifying a system in a way that does not allow backward compatibility is sometimes called "breaking" backward compatibility.
A related term from programming jargon is hysterical reasons or hysterical raisins (near-homophones for "historical reasons"), as the purpose of some software features may be solely to support older hardware or software versions.
A simple example of both backward and forward compatibility is the introduction of FM radio in stereo. FM radio was initially mono, with only one audio channel represented by one signal. With the introduction of two-channel stereo FM radio, many listeners had only mono FM receivers. Forward compatibility for mono receivers with stereo signals was achieved by sending the sum of both left and right audio channels in one signal and the difference in another signal. That allows mono FM receivers to receive and decode the sum signal while ignoring the difference signal, which is necessary only for separating the audio channels. Stereo FM receivers can receive a mono signal and decode it without the need for a second signal, and they can separate a sum signal to left and right channels if both sum and difference signals are received. Without the requirement for backward compatibility, a simpler method could have been chosen.
Full backward compatibility is particularly important in computer instruction set architectures, one of the most successful being the x86 family of microprocessors. Their full backward compatibility spans back to the 16-bit Intel 8086/8088 processors introduced in 1978. (The 8086/8088, in turn, were designed with easy machine-translatability of programs written for its predecessor in mind, although they were not instruction-set compatible with the 8-bit Intel 8080 processor as of 1974. The Zilog Z80, however, was fully backward compatible with the Intel 8080.) Fully backward compatible processors can process the same binary executable software instructions as their predecessors, allowing the use of a newer processor without having to acquire new applications or operating systems. Similarly, the success of the Wi-Fi digital communication standard is attributed to its broad forward and backward compatibility; it became more popular than other standards that were not backward compatible.
A data format is said to be backward compatible with its predecessor if every message or file that is valid under the old format is still valid, retaining its meaning, under the new format.
The earliest cases of backward compatibility in video games came through console add-ons. The Atari 2600's library is playable on its direct successor, the Atari 5200, with a cartridge adapter, as well as competitors Intellivision and ColecoVision in such a manner. The Japanese version of the Master System and its predecessor, the Sega Mark III, were compatible with most software and peripherals designed for the SC-3000 and SG-1000 series of platforms, Sega's earliest gaming platforms. Likewise, the Mega Drive/Genesis can play Master System cartridges and cards via a peripheral known as the Master System Converter in Europe and the Power Base Converter in North America.
The first console in North America to widely support backward compatibility without additional hardware is the third-generation Atari 7800, which could play most 2600 games. Most Nintendo handhelds since the Game Boy Advance (which could play original Game Boy and Game Boy Color game cartridges) have backwards compatibility with their immediate predecessor, with some exceptions such as the Game Boy Micro and Nintendo DSi, while the Neo-Geo Pocket and Wonderswan would receive "Color" refreshes. The PlayStation 2 was compatible with original PlayStation software, as well as most peripherals due to employing the same controller ports and memory card slots. Early models of the PlayStation 3 console came equipped with the Emotion Engine, allowing it to play both, original PlayStation and PlayStation 2 discs, but this component would be removed in later models, leaving only compatibility with original PlayStation discs through software emulation. The original Xbox's first two sequential successors, the Xbox 360 and the Xbox One, can support a fraction of games released for their predecessors via emulation, although some supported Xbox games may not function properly on the Xbox 360. The Wii features full compatibility with GameCube software and peripherals thanks to inclusion of four GameCube controller ports and two memory card slots, but this features was excised from later revised models as a cost-reducing measurement. Its successor, the Wii U, has a legacy mode for full compatibility with original Wii software, including digital WiiWare and Virtual Console titles. The PlayStation 5 and Xbox Series X/S can play almost all games designed for their respective, immediate predecessors, the PlayStation 4 and Xbox One, and can even optimize their performance.
As Sega planned its exit from the hardware market, chairman Isao Okawa approached Microsoft chairman Bill Gates to implement Dreamcast on their upcoming Xbox, but negotiations fell through when Gates refused to provide Internet connectivity, a feature that Okawa felt was essential.
There are several incentives for a company to implement backward compatibility. Backward compatibility can be used to preserve older software that would have otherwise been lost when a manufacturer decides to stop supporting older hardware. Classic video games are a common example used when discussing the value of supporting older software. The cultural impact of video games is a large part of their continued success, and some believe ignoring backward compatibility would cause these titles to disappear. Backward compatibility also acts as an additional selling point for new hardware, as an existing player base can more affordably upgrade to subsequent generations of a console. This also helps to make up for lack of content in the early launch of new systems, as users can pull from the previous console's large library of games while developers slowly transition to the new hardware.
One example of this is the Sony PlayStation 2 (PS2) which was backward compatible with games for its predecessor PlayStation (PS1). While the selection of PS2 games available at launch was small, sales of the console were nonetheless strong in 2000–2001 thanks to the large library of games for the preceding PS1. This bought time for the PS2 to grow a large installed base and developers to release more quality PS2 games for the crucial 2001 holiday season.
Additionally, and despite not being included at launch, Microsoft slowly incorporated backward compatibility for select titles on the Xbox One several years into its product life cycle. Players have racked up over a billion hours with backward compatible games on Xbox, and the newest generation of consoles such as PlayStation 5 and Xbox Series X/S also support this feature. A large part of the success and implementation of this feature is that the hardware within newer generation consoles is both powerful and similar enough to legacy systems that older titles can be broken down and re-configured to run on the Xbox One. The backward compatibility program not only supports the previous generation Xbox 360, but also titles from the original Xbox system. Some titles are even given slight visual improvements and additional levels at no cost to the user. This program has proven incredibly popular with Xbox players and goes against the recent trend of studio made remasters of classic titles, creating what some believe to be an important shift in console maker's strategies.
The literal costs of supporting old software is considered a large drawback to the usage of backward compatibility. The associated costs of backward compatibility are a larger bill of materials if hardware is required to support the legacy systems; increased complexity of the product that may lead to longer time to market, technological hindrances, and slowing innovation; and increased expectations from users in terms of compatibility. Because of this, several gaming consoles chose to phase out backward compatibility toward the end of the console generation in order to reduce cost and briefly re-invigorate sales before the arrival of newer hardware.
A notable example is the contrast between Sony's hardware-based implementation of backward compatibility in earlier versions of the PlayStation 2 versus the PlayStation 3. In the PS3, PS2 hardware served no purpose in PS3 mode. In the PS2, a CPU core identical to that of the PS1 served a dual purpose, either as the main CPU in PS1 mode, or upclocking itself to offload I/O in PS2 mode. Such an approach can backfire, however, as in the case of the Super Nintendo, which opted for the peculiar 65C816 over more popular 16-bit microprocessors, on the basis that it would allow easy compatibility with the earlier Nintendo Entertainment System, but NES compatibility ultimately did not prove workable once the rest of the SNES's architecture was designed.
However, with the current decline in physical game sales and the rise of digital storefronts and downloads, some believe backward compatibility will soon be as obsolete as the phased-out consoles it supports. Many game studios are re-mastering and re-releasing their most popular titles by improving the quality of graphics and adding new content. These remasters have found success by appealing both to nostalgic players who remember enjoying the original versions when they were younger, and to newcomers who may not have had the original system it was released on. For most consumers, digital remasters are more appealing than hanging on to bulky cartridges and obsolete hardware. For the manufacturers of consoles, digital re-releases of classic titles are a large benefit. It not only removes the financial drawbacks of supporting older hardware, but also shifts all of the costs of updating software to the developers. The manufacturer gets a new addition to their system with strong name recognition, and the studio does not have to completely develop a game from the ground up. Officially licensed, "plug and play mini" emulators of classic consoles, with built in classic games, have also become more common in recent years, from companies like Sony, Sega and Nintendo.