Classic, or Classic Environment, was a hardware and software abstraction layer in Mac OS X that allowed applications compatible with Mac OS 9 to run on the Mac OS X operating system. The name "Classic" is also 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 previous version of the Mac OS. Classic is supported on PowerPC-based Macs running versions of Mac OS X up to v10.4 Tiger, but is not supported on v10.5 Leopard and later, nor on Intel-based Macs running any version of Mac OS X.
Classic was a descendant of Rhapsody's "Blue Box" emulation layer, which served as a proof of concept. It uses a Mac OS 9 System Folder, and a New World ROM file to bridge the differences between the older PowerPC Macintosh platforms and the XNU kernel environment. Classic was a key element of Apple's strategy to replace the "classic" Mac OS (versions 9 and below) with Mac OS X as the standard operating system (OS) used by Macintosh computers by eliminating the need to use the older OS directly.
Classic Environment can be loaded at login (for faster activation when needed later), on command, or whenever a Mac OS application that requires it is launched (to reduce the use of system resources when not needed). It requires a full version of Mac OS 9 to be installed on the system, and loads an instance of that OS in a sandbox environment, replacing some low-level system calls with equivalent calls to Mac OS X via updated system files and the Classic Support system enabler. This sandbox is used to launch all "classic" Mac OS applications—there is only one instance of the Classic process running for a given user, and only one user per machine may be running Classic at a time.
If the "load-when-needed" option is selected, double clicking a "classic" application first launches Classic, which can be configured to appear in a window resembling the display of a computer booting into Mac OS 9. When Classic is finished loading, the application launches. When a "classic" application is in the foreground, the menu bar at the top of the screen changes to look like the older Mac OS system menu. Dialog boxes and other user-interface elements retain their traditional appearance.
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