Throughout the early 1990s, Apple had tried to create a 'next-generation' OS to succeed its classic Mac OS through the Taligent, Copland and Gershwin projects, but all were eventually abandoned. Its graphical user interface was built on top of an object-oriented GUI toolkit using the Objective-C programming language. The kernel of NeXTSTEP is based upon the Mach kernel, which was originally developed at Carnegie Mellon University, with additional kernel layers and low-level user space code derived from parts of BSD. There, the Unix-like NeXTSTEP operating system was developed, before being launched in 1989. The heritage of what would become macOS had originated at NeXT, a company founded by Steve Jobs following his departure from Apple in 1985. In 2020, Apple began the Apple silicon transition, using self-designed, 64-bit ARM-based Apple M1 processors on the latest Macintosh computers. In 2006, Apple transitioned to the Intel architecture with a line of Macs using Intel Core processors. MacOS has supported three major processor architectures, beginning with PowerPC-based Macs in 1999. After sixteen distinct versions of macOS 10, macOS Big Sur was presented as version 11 in 2020, and macOS Monterey was presented as version 12 in 2021. Apple shortened the name to 'OS X' in 2012 and then changed it to 'macOS' in 2016 to align with the branding of Apple's other operating systems, iOS, watchOS, and tvOS. Ī prominent part of macOS's original brand identity was the use of Roman numeral X, pronounced 'ten' as in Mac OS X and also the iPhone X, as well as code naming each release after species of big cats, or places within California. Apple's mobile operating system, iOS, has been considered a variant of macOS.

All releases from Mac OS X 10.5 Leopard and after are UNIX 03 certified, with an exception for OS X 10.7 Lion.

The first desktop version, Mac OS X 10.0, was released in March 2001, with its first update, 10.1, arriving later that year.

During this time, Apple cofounder Steve Jobs had left Apple and started another company, NeXT, developing the NeXTSTEP platform that would later be acquired by Apple to form the basis of macOS. MacOS succeeded the classic Mac OS, a Macintosh operating system with nine releases from 1984 to 1999. Within the market of desktop and laptop computers it is the second most widely used desktop OS, after Microsoft Windows and ahead of ChromeOS. It is the primary operating system for Apple's Mac computers. MacOS ( / ˌ m æ k oʊ ˈ ɛ s/ previously Mac OS X and later OS X) is a Unix operating system developed and marketed by Apple Inc.

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