Classic Environment - Compatibility and Performance

Compatibility and Performance

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 September 2002. These machines cannot boot Mac OS 9 or earlier without the bridging capabilities of Classic or other software (see SheepShaver). Classic is not supported on versions of Mac OS X that run on Apple systems incorporating Intel microprocessors, which includes all new shipping Apple computers, as of September 2006. The last version of Mac OS to support Classic on PowerPCs was Mac OS X v10.4 "Tiger".

Classic's compatibility is good, provided the application using it does not require direct access to hardware or engage in full-screen drawing. However, it is not a perfect clone of a Mac OS 9 computer. The Finder included with Mac OS X v10.2 and later does not support the "Reveal Object" Apple event 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 10.2 would often fail to draw window frames of Classic applications correctly, and after Classic windowing was made double buffered in Mac OS X v10.3, some older applications and games sometimes failed to update the screen properly, such as the original Macintosh port of Doom. However, Classic "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.

Classic's performance is also generally acceptable, with a few exceptions. Most of an application is run directly as PowerPC code (which would not be possible on Intel-based Macs). Motorola 68k code is handled by the same Motorola 68LC040 emulator that Mac OS 9 uses. Some application functions are actually faster in Classic than under Mac OS 9 on equivalent hardware, due to performance improvements in the newer operating system's device drivers. These applications are largely those that use heavy disk processing, and were often quickly ported to Mac OS X by their developers. On the other hand, applications that rely on heavy processing and which did not share resources under Mac OS 9's co-operative multitasking model will be interrupted by other (non-Classic) processes under Mac OS X's preemptive multitasking. The greater processing power of most systems that run Mac OS X (compared to systems intended to run Mac OS 8 or 9) helps to mitigate the performance degradation of Classic's emulation.