Classic Environment, usually referred to as Classic, is a hardware and software abstraction layer in PowerPC versions of Mac OS X that allows most legacy applications compatible with Mac OS 9 to run on Mac OS X The name "Classic" is sometimes used by software vendors to refer to the application programming interface available to "classic" applications, to differentiate between programming for Mac OS X and the classic version of the Mac OS.
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Classic Environment is supported on PowerPC-based Macintosh computers running versions of Mac OS X up to 10.
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Classic Environment is a descendant of Rhapsody's "Blue Box" virtualization layer, which served as a proof of concept.
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The Classic Environment was created as a key element of Apple's strategy to replace the classic Mac OS with Mac OS X as the standard operating system used by Macintosh computers by eliminating the need to use the older OS directly.
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Classic Environment can be loaded at login, on command, or whenever a Mac OS application that requires it is launched.
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Classic Environment provides a way to run "Classic" applications on Apple's G5 systems as well as on most G4 based computers sold after January 2003.
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Classic Environment's compatibility is usually sufficient for many applications, provided the application using it does not require direct access to hardware or engage in full-screen drawing.
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Early releases of Mac OS X would often fail to draw window frames of Classic applications correctly, and after the Classic Environment's windowing was made double buffered in Mac OS X Panther, some older applications and games sometimes failed to update the screen properly, such as the original Macintosh port of Doom.
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However, the Classic Environment "resurrected" some older applications that had previously been unusable on the Macintosh Quadra and Power Macintosh series; this is because Mac OS X replaced Mac OS 9's virtual memory system with a more standard and less fragile implementation.
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Classic Environment's performance is generally acceptable, with a few exceptions.
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