In telecommunications and computing, backward compatibility (or backwards compatibility) is a property of an operating system, software, real-world product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system.
Modifying a system in a way that does not allow backward compatibility is sometimes called "" backward compatibility. Such breaking usually incurs various types of costs, such as switching cost.
A complementary concept is forward compatibility; a design that is forward-compatible usually has a roadmap for compatibility with future standards and products.
Full backward compatibility is particularly important in computer instruction set architectures, two of the most successful being the IBM 360/370/390/Zseries families of mainframes, and the Intel x86 family of .
IBM announced the first 360 models in 1964 and has continued to update the series ever since, with migration over the decades from 32-bit register/24-bit addresses to 64-bit registers and addresses.
Intel announced the first Intel 8086/8088 processors in 1978, again with migrations over the decades from 16-bit to 64-bit. (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 of 1974. The Zilog Z80, however, was fully backward compatible with the Intel 8080.)
Fully backward compatible processors can process the same executable as their predecessors, allowing the use of a newer processor without having to acquire new applications or . 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.
In operating systems, upgrades to newer versions are said to be backward compatible if executables and other files from the previous versions will work as usual.
In , backward compatibility may refer to the ability of a compiler for a newer version of the language to accept source code of programs or data that worked under the previous version.
A data format is said to be backward compatible when a newer version of the program can open it without errors just like its predecessor.
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. 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. 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 makers' strategies. The current generation of consoles such as the PlayStation 5 (PS5) and Xbox Series X/S also support this feature as well.
Despite this, it is still possible to bypass some of these hardware costs. For instance, earlier PS2 systems had the core of the original PlayStation (PS1) CPU integrated into the I/O processor for dual-purpose use; it could act as either the main CPU in PS1 mode or it can Clock rate itself to offload I/O in PS2 mode. The original I/O core was replaced with a PowerPC-based core in later systems to serve the same functions, emulating the same functions as the PS1 CPU core. Such an approach can backfire, however, as was the case of the Super Nintendo Entertainment System (Super NES). It opted for the more peculiar 65C816 CPU over the more popular 16-bit microprocessors on the basis that it would allow for easier backwards compatibility with the original Nintendo Entertainment System (NES) due to the 65C816's software compatibility with the 6502 CPU in emulation mode, but ultimately did not proved to be workable once the rest of the Super NES's architecture was designed.
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