Classic, also known as the Classic environment or Classic mode, is an abstraction layer that allows most legacy applications written for Mac OS 9 and earlier to function under PowerPC versions of Mac OS X. It was supported from early versions of Mac OS X Public Beta through Mac OS X 10.4.11 as a vital part of the transition from classic Mac OS to OS X.
Mac OS X is based on NeXTSTEP, which uses a Unix-based kernel that is inherently incompatible with classic Mac applications. During its early development under the code name Rhapsody, a proof-of-concept called "Blue box" demonstrated that backwards compatibility could be provided through a native application that would run the older software in a sandbox environment. Any problematic legacy program that would normally cause Mac OS 9 to crash could be contained and only require relaunching of the Classic environment without bringing down the main system.
During beta testing of Mac OS X, the minimum version of classic Mac OS to be supported was 9.0.4, which needed to be installed separately. Stability was inconsistent and improved with successive public beta releases. Another transition technology called Carbon was also introduced, which allowed applications which took advantage of the API to run on both Mac OS 9 and PowerPC versions of Mac OS X.
Mac OS X 10.5, which was part of the Intel transition, dropped support for classic applications entirely. Alternatives for modern versions of macOS include emulators such as Basilisk II, Mini vMac, QEMU, and SheepShaver.
- Mac OS X: Codename “Jaguar”, Apple Computer. Archived 2002-06-04.
- Mac OS X - Technologies: Elegant Power., Apple Computer. Archived 2001-10-05.
- Mac OS X Beta not worth installing for most users by Henry Norr, Tampa Bay Times. 2005-09-28.
- Mac OS X Public Beta Review by Jason Buck, MacOpz. Archived 2001-04-30.
- Mac OS 9.1 to be 'Classic' environment in OS X by Dennis Sellers, Macworld. 2001-02-02.
- Low End Mac’s Compleat* Guide to Mac OS 9 by Charles W. Moore, Low End Mac. 2013-12-01.
- Classic at Apple (archived 2001-11-25)
- ARCHIVED: In Mac OS X, what is the Classic environment, and how do I activate it? at Indiana University
- Understanding Mac OS 9.2, the Classic Environment, and Classic Applications at eTutorials
- Classic environment at Wikipedia