MacOS

macOS is a series of operating systems developed by Apple for their Macintosh platform. It was first introduced together with the original Macintosh in 1984. The system from the very beginning made use of a graphical interface, similar to the one used by the earlier Lisa. In its early versions, it only supported running a single application at a time, although it was later extended with cooperative multitasking support via MultiFinder. During Apple's transition from Motorola 68k series to the PowerPC architecture, Mac OS was also ported to the new platform by using emulation, until a new, native OS was developed as a part of the Copland project.

After the failure of Copland, however, Apple was forced to further maintain the original Mac OS codebase, which no longer satisfied the ever-increasing performance and stability requirements. Apple eventually saw the solution in buying out NeXT in 1997, the company founded by Steve Jobs after he was ousted from Apple. As a part of the deal, Apple gained NeXT's object-oriented operating system called NeXTSTEP based on Unix. The Unix core of NeXTSTEP was renamed to Darwin, released as open source and in the following years, it was modernized partially by using code from FreeBSD and other projects.

The classic Mac OS was eventually succeeded by Mac OS X in 2001 with the release of Mac OS X Cheetah. Since then, the system went through several architecture transitions, first from PowerPC to Intel x86 in 2006. In 2020, Apple began another transition, this time from x86 to the ARM-based Apple silicon.

Classic
The original Macintosh System Software is based on the Lisa OS and UI concepts from Xerox PARC. It uses a monolithic architecture but lacks support for memory management, protected memory, access controls, and conflict-free extensions. Only one application at a time could be supported under System Software until version 5 when MultiFinder was introduced. In 1997, Apple rebranded the Macintosh System Software to Mac OS. Mac OS would continue to be developed until the introduction of Mac OS X, after which Apple only delivered minor updates to the classic Mac OS. The final minor updates were designed to ease the transition to OS X. Carbon, an API introduced with Mac OS 8, works as the transitional API for classic Mac OS and Mac OS X applications.

The classic Mac OS was originally written for the Motorola 68k architecture. The 68k architecture was used from 1984 to 1994, after which Apple transitioned to PowerPC. PowerPC would be used until the classic Mac OS's discontinuation in 2002. Classic Mac OS was based on a simple kernel design, which eventually became too limited and crash-prone for applications. In response, Apple bought NeXT and used NeXTSTEP as a base to develop Mac OS X.

The Classic Mac OS can run standalone, and under Mac OS X using the Classic system layer. Only certain versions of Mac OS 9 can run under the Classic system layer, and the layer itself was dropped after Mac OS X Tiger.

UNIX-based
macOS (formerly called Mac OS X until 2012 and then OS X until 2016) is the current incarnation of Mac OS. The first version released was Mac OS X Server in 1999. Unlike the classic Mac OS, it is based on NeXTSTEP and uses its frameworks and architectures. In 2006, Apple switched to Intel-based x86 processors in Macs. Support for PowerPC in Mac OS X was dropped in 2009. In 2012, Apple began to use the OS X moniker to brand Mac OS X alongside their other operating systems (watchOS, tvOS, iOS). In 2016, Apple began to use the macOS moniker to brand OS X alongside their other operating systems (watchOS, tvOS, iOS). In 2020, Apple declared that they would switch over to the Apple Silicon SoC in future Mac computers.