Compatibility mode

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A compatibility mode is a software mechanism in which a computer's operating system emulates an older processor, operating system, and/or hardware platform in order to allow obsolete software to remain compatible with the computer's newer hardware or software.

This differs from a full-fledged emulator in that an emulator typically creates a virtual hardware architecture on the host system, rather than simply translating the older system's function calls into calls that the host system can understand.

Examples include Classic Mode in Mac OS X and Windows XP's compatibility mode, which both allow applications designed for older versions of the operating system to run.