Backward compatibility (also known as backwards compatibility) is a property of a 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.
A related term from programming jargon is hysterical reasons or hysterical raisins (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 through 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.
Backward compatibility is also employed in video games, in which certain video game systems are able to play games designed for their predecessors. The first video game console to widely support backwards compatibility without additional hardware is the third-generation Atari 7800, which could play most Atari 2600 games. The Atari 5200, the 2600's direct successor, as well as the Intellivision and ColecoVision can also play Atari 2600 games with a particular add-on. The Sega Genesis and Sega Game Gear can play games for the Sega Master System, the Genesis' predecessor, via a special add-on. The three fifth-generation handhelds whose names ended with the word "Color" (Game Boy Color, Neo Geo Pocket Color and the WonderSwan Color) are backwards-compatible with handhelds that are named identically without that word (Game Boy, Neo Geo Pocket and the WonderSwan), and all Nintendo handhelds following the Game Boy have at least one model that is backwards compatible with its immediate predecessor. The PlayStation 2 and PlayStation 3 are backwards compatible with the original PlayStation, and early PS3 models with the Emotion Engine installed can play PS2 games. The original Xbox's first two sequential successors, the Xbox 360 and the Xbox One, can support a fraction of games released for their respective, immediate predecessors via emulation, although some supported Xbox games may not function properly on the Xbox 360. The GameCube's two sequential disc-based successors, the Wii and Wii U, can play games released for their respective, immediate predecessors and support their controllers as well, although the Wii's subsequent model revisions are not backwards compatible. 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.
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 it is anticipated that next generation consoles such as PlayStation 5 and Xbox Series X will 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 higher 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 Sony PlayStation 3, as the first PS3 iteration was expensive to manufacture in part due to including the Emotion Engine from the preceding PS2 in order to run PS2 games, since the PS3 architecture was completely different from the PS2. Subsequent PS3 hardware revisions have eliminated the Emotion Engine as it saved production costs while removing the ability to run PS2 titles, as Sony found out that backward compatibility was not a major selling point for the PS3 in contrast to the PS2. The PS3's chief competitor, the Microsoft Xbox 360, took a different approach to backward compatibility by using software emulation in order to run games from the first Xbox.
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" variations of classic consoles, with built in classic games, have also become more common in recent years, from companies like Sony, Sega and Nintendo.