A friend asked me the other day, ï¿¼Iï¿¼m still using Tiger, but am buying a new Mac. Will my applications run under Leopard?ï¿¼My answer: You almost always can run Mac applications written for Mac OS X 10.x on newer versions of the operating system. You can also keep on using old hardware peripherals.
While there are some exceptions, they tend to be very old applications, like QuarkXPress 6.52, which Quark supports for all version of Mac OS X up through 10.3 Panther ï¿¼ but does not under 10.4 Tiger or later. Quark strongly recommends that Tiger and 10.5 Leopard users move to QuarkXPress 7.31. (You can still use QuarkXPress 6.52 on Tiger and Leopard, but not everything works, and if you have problems, Quark wonï¿¼t help you.)
Why do most things work fine? Because while Apple always adds new capabilities to each Mac OS Xï¿¼s APIs (applications programming interfaces), it rarely takes old ones away. There are exceptions, of course, and some APIs are deprecated (thatï¿¼s the technical term for ï¿¼rendered obsolete.ï¿¼) That can stop programs from working, or from working properly.
For example, Carbon is Appleï¿¼s term for a low-level set of 32-bit APIs that handle graphics, and Apple has indicated that itï¿¼s phasing it out in favor of its Cocoa 32/64-bit APIs. Applications written for Carbon will still work on Mac OS X, but may not forever.
When users have compatibility problems when moving from one version of Mac OS X to another, itï¿¼s often because Apple also distributes new versions of its bundled applications. Leopard has updated versions of applications like the Safari Web browser and Mail client than Tiger, and in some cases there may be compatibility issues with some Web sites or third-party plug-ins.
Fortunately, the under-the-covers differences between Mac OS X versions are fairly minor. By contrast, Windows users have a tougher time There are very big differences between, say, Windows 95/98, Windows NT 4.0, Windows 2000, Windows XP and Windows Vista that can affect a large number of applications.
While Windows Vista runs nearly all Windows XP applications, it has trouble with hardware devices that havenï¿¼t been upgraded to work with Vista, and it doesnï¿¼t run many Windows 98 programs.
By comparison, Mac OS X 10.5 Leopard will almost certainly run every Mac OS X 10.2 Jaguar app, and supports applications that work with any version of Mac OS X. Thereï¿¼s no reason to believe that this will change with Mac OS X 10.6 ï¿¼ whatever it will be called. (And no, it probably wonï¿¼t be named ThunderCat.)