Mac OS X versions were internally code-named after jungle cats. Programs can also be developed in Java and BSD. Carbon is used for applications that can run on both OS X and earlier Mac OS machines, and Classic is the API prior to Mac OS X. Cocoa is the native OS X interface, derived from OpenStep.

There have been five programming interfaces (APIs) for writing Mac OS X applications. However, PowerPC applications could run on Intel Macs via an emulator (see Rosetta). In 2006, Apple switched from the PowerPC to the Intel platform, and Classic support was dropped in the Intel version of OS X. Classic was not preloaded on new Macs, but was available for installation from the system disks. Prior to Mac OS X 10.5 (Leopard), OS X ran legacy OS 9 and earlier applications via its Classic software, which was a full copy of OS 9 that ran as an OS X process. The heart of OS X is the open source, POSIX-compliant Darwin kernel, which includes an enhanced BSD 4.4 operating system and Mach 2.5 microkernel.

OS X added protected memory, pre-emptive multitasking, multithreading and symmetric multiprocessing (SMP) to the Mac world. For the first time, the rich set of Unix commands became available to all Mac developers (A/UX was an earlier Unix OS for the Mac but was not widely used). Based on Unix and featuring an entirely redesigned user interface, OS X was a major departure from the previous OS 9 system. OS X Server was introduced in 1999, and the client version came out in 2001. As a result, all the names of Apple operating systems have the same OS suffix (macOS, iOS, iPadOS, tvOS). (Mac OS 10) The operating system from Apple for the Mac family from 2001 until the fall of 2016, at which time the Sierra OS Version 10.12 began using the rebranded 'macOS' moniker.

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