the need to use the older OS directly. The 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 user chooses to launch the Classic Environment only when needed, launching a "classic" application first launches the Classic Environment, which can be configured to appear in a window resembling the display of a computer booting into Mac OS 9. When the Classic Environment has 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. The 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. These machines cannot boot Mac OS 9 or earlier without the bridging capabilities of the Classic Environment or other software (see SheepShaver). The 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. However, it is not a complete clone of Mac OS 9. The Finder included with Mac OS X v10.2 and later does not support the "Reveal Object" Apple events used by some Mac OS 9 applications, causing the "Reveal In Finder" functionality for those applications to be lost. 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. However, the Classic Environment "resurrected" some older