Legacy mode

In computing, legacy mode is a state in which a computer system, component, or software application behaves in a way different from its standard operation in order to support older software, data, or expected behavior. It differs from backward compatibility in that an item in this state will often sacrifice newer features or performance, or be unable to access data or run programs it normally could, in order to provide continued access to older data or functionality. Sometimes it can allow newer technologies that replaced the old to emulate them when running older operating systems.

Examples

* x86-64 processors can be run in one of two states: one provides additional features and performance; the other provides strict compatibility with older operating systems and applications written for the x86 architecture.

* Many PC graphic cards have a special mode that allows them to run on systems that do not have the specific device driver necessary to take advantage of their more sophisticated features.

* Operating systems often have a special mode allowing them to emulate an older release in order to support software applications dependent on the specific interfaces and behavior of that release. Windows XP can be configured to emulate Windows 2000 and Windows 98; Mac OS X can support the execution of Mac OS 9 applications.

* Computer buses emulated through legacy mode:
**Emulated bus (Host bus)
**ISA (PCI)
**PCI (PCI express)
**PS2/RS-232 mouse (USB mouse)
**PS2/AT keyboard (USB keyboard)
**PATA disk drive (USB flash drive, SATA drive)

See also

* Legacy system
* Backward compatibility