Backward compatibility

In technology, for example in telecommunications and computing, a device or technology is said to be backwards (or downwards) compatible if it allows input generated by older devices. A standard, for example a data format or a communication protocol, is said to allow backward compatibility, if products designed for the new standard can receive, read, view or play older standards or formats.

This should not be confused with forward compatibility, which implies that old devices allow (or are expected to allow) data formats generated by new (or future) devices, perhaps without supporting all new features. A standard supports forward compatibility if older product versions can receive, read, view or play the new standard.

For example, the introduction of FM stereo transmission, or color television, allowed backward compatibility since new receivers could receive monophonic or black-and-white signals generated by old transmitters. It also allowed forward compatibility, since old (and new) monophonic FM radio receivers and black-and-white TV sets still could receive a signal from a new transmitter.

In programming languages, backwards compatibility refers to the ability of a compiler for version N of the language to accept programs or data that worked under version N - 1. (By this definition, if version N - 1 and other previous versions were also backward compatible, which is often the case, then by the principle of recursion, version N will also accept input that worked under any prior version after the latest one that was not non–backward compatible. However, in practice, features are often deprecated and support is dropped in a later release - yet still thought of as backwards compatible.) In other contexts, a product or a technology is said to be backward compatible when it is able to fully take the place of an older product, by inter-operating with products that were designed for the older product.

Description
Backward compatibility is a relationship between two components, rather than being an attribute of just one of them. More generally, a new component is said to be backward compatible if it provides all of the functionality of the old component.

Backward compatibility is the special case of compatibility in which the new component has a direct historical ancestral relationship with the old component. If this special relationship does not exist then it not usually spoken of as "backward" compatibility but is instead just "compatible"&mdash;a consistent interface allowing interoperability between components and products that were each developed separately.

Data does nothing in the absence of an interpreter, so the notion of compatibility does not apply to document files, it only applies to software. In the case of a program that creates document files, a new version of that program ("v2") is said to be backward compatible with the old version of the program ("v1") when it can both read and write documents that work with v1. Everything that v1 could do must also be possible with v2, including saving documents that can be read by v1 (which is something that v1 could do.)

If a newer software version cannot save files that can be read by the older version, it is not backward compatible with the older version, although it may provide an irreversible upgrade capability for the old files. This situation has often been used strategically by software vendors to force customers to purchase upgrades since, over time, the number of data files usable by an old version diminishes at a rate proportional to the number of other customers that have upgraded (assuming that all customers generate files at the same the average rate.)

Levels of compatibility vary. In software, binary compatibility and source compatibility are distinguishable. Binary compatibility means that programs can work correctly with the new version of this library without requiring recompilation. Source compatibility requires recompilation but no changes to the source code.

Many platforms rely on emulation, the simulation of an older platform in software, to achieve backward compatibility.

Compatibility checking
Approaches for checking compatibility between the client program and the server component include:
 * Check by version number;
 * Check by an interface definition language (IDL)
 * Check by just-in-time test runs (the client program gives some example inputs to the server component to see if the component returns the desired example outputs).

Examples

 * PCI Express 2.0 is backward compatible with PCI Express 1.1.
 * The NTSC colour broadcast system was engineered by RCA to be backward compatible with black-and-white NTSC television sets.
 * A computer system is backward compatible if it is able to work with software or accessories designed for the system it is meant to replace.
 * The IBM 7080 transistorized computer was backward compatible with all models of the IBM 705 vacuum tube computer.
 * The Apple Macintosh PowerPC-based models are backward compatible with the earlier Macintosh computers using the Motorola 68000 CPU, through emulation implemented in the Mac OS; this means that PowerPC Macs can run all programs for the 68000 Macs (but not vice-versa&mdash;that would be forward compatibility.)
 * The PlayStation Portable, while not being truly backward compatible with any other system, can emulate most PlayStation games when disc images are downloaded via the PlayStation Network.
 * The Atari 7800 is backward compatible with most Atari 2600 games.
 * The Game Boy Advance line (except the Game Boy Micro) is backward compatible with previous Game Boy systems, meaning all Game Boy and Game Boy Color titles are playable on this system.
 * The Nintendo DS and the Nintendo DS lite are backward compatible with all Game Boy Advance games, but only in single-player mode, due to the system's lack of a GBA link cable port. The exception to this would be the newly released Nintendo DSi, which is not compatible with GBA games or any peripheral using the Slot 2, for example Guitar Hero: On Tour because it has lost the cartridge port for GBA games.
 * The PlayStation 2 is backward compatible with most of the original PlayStation library.
 * The initial PlayStation 3 model is backward compatible with most PlayStation and PlayStation 2 games. This is provided by the inclusion of the original Emotion Engine chip that is built inside the PS2. This form of compatibility is only available to the first series of consoles that launched across North America, Japan, Hong Kong and Taiwan, the later launch of the system in many PAL areas (Europe, Oceania, Asia) lacked the chip provided in the original units and instead relied on software emulation (which greatly reduced the number of playable titles) for backwards compatibility. Newer versions of the console (PAL and NTSC 80GB CECHKxx and 160GB CECHPxx models) lack the PS2 chips and PlayStation 2 emulation software; therefore no production models of the PlayStation 3 are capable of playing PlayStation 2 games.
 * All of Intel x86 processors – Core 2, Core, Pentium 4, Pentium III, Pentium II, Pentium Pro, Pentium, 80486, 80386, 80286, 80186 and 8086 – are backward binary compatible with their predecessors, because they can execute machine code programs written for an earlier processor. (There are actually a few obscure exceptions to this.)  These processors are not backward hardware compatible, because the later CPUs cannot replace the earlier ones in a CPU socket on a circuit board.
 * The Mega Drive/Genesis is not directly backward compatible with the Master System, however, with the Power Base Adapter (a mostly passive device) it was able to play most Master System games, and Master System controllers can be used with the Mega Drive.
 * The Xbox 360 is backward compatible with some Xbox games via hardware emulation.
 * The Wii is backward compatible with all games from the previous Nintendo system, the Nintendo GameCube, due to it being based on the PowerPC, the same base as the latter. It is compatible with GameCube controllers and memory cards but not with other peripherals such as the Game Boy Player. Some games originally released for older consoles – the NES, SNES, Nintendo 64, Sega Genesis, Turbografx 16 and Neo Geo – are available to play on the Wii via emulation, however the Wii cannot be said to be backward compatible with these systems as the software in its original format (i.e. cartridges) cannot be used.
 * Other examples of backward-compatible software
 * Microsoft Windows contains application compatibility shims to make the platform compatible with most software from earlier 32-bit and 16-bit versions (e.g. Civilization (circa 1991, designed for Windows 3.0) running on Windows Vista). XP, Vista and Server x64 versions drop 16-bit support for reliability while maintaining 32-bit support. Windows versions at least until Vista also maintain the ability to run 16-bit DOS programs (with technical limitations which give rise to many exceptions) so Windows can be said to be backward-compatible with MS-DOS, its predecessor.  (Windows versions before Windows 95 actually ran on top of and required DOS; Win 95 through Me incorporated their own versions of DOS.)
 * Microsoft Word 2000 was backward compatible with Word 97 because it could read and write files in Word 97 format, with the understanding that features unique to Word 2000 would not appear in Word 97. The same applied to Word 2002 (the version of Word in Office XP) and Word 2003.
 * By adding the proper external hardware, many consoles can become backward compatible. This includes:
 * The Atari 5200 can play Atari 2600 games by adding the "Atari VCS Cartridge Adapter".
 * The ColecoVision can play Atari 2600 games by adding the "Expansion Module #1".
 * The Intellivision can play Atari 2600 games by adding the "System Changer".
 * Although the Sega Mega Drive/Genesis is backward compatible with the Sega Master System, due to a different cartridge format games can not be played without adding a pass-through cart. The Mega Drive also supports Master System controllers.
 * The Sega Game Gear can play Sega Master System games by adding a "Master Gear".
 * The Super Nintendo Entertainment System can play games for the Nintendo Entertainment System by adding a Super 8 (video game accessory).
 * The Nintendo 64 can play games for the Nintendo Entertainment System and Super Nintendo Entertainment System by adding a Tristar 64, although this add-on was not licenced by Nintendo.
 * Several computer operating systems have various methods of running software originally designed for older versions or other OSes:
 * Windows NT and successors have various subsystems to run legacy applications. MS-DOS and Win16 subsystems (only on i386) can run some applications for those platforms, and it has an OS/2 subsystem for running CLI OS/2 applications.