The kernel of Darwin is XNU, a hybrid kernel which uses OSFMK 7.3 (Open Software Foundation Mach Kernel) from the OSF, various elements of FreeBSD (including the process model, network stack, and virtual file system), and an object-oriented device driver API called I/O Kit. As of January 2023, Apple no longer mentions Darwin by name on its Open Source website and only publishes an incomplete collection of open-source projects relating to macOS and iOS. Darwin is now only available as source code. Minor updates were released as packages that were installed separately.

Up to Darwin 8.0.1, Apple released a binary installer (as an ISO image) after each major Mac OS X release that allowed one to install Darwin on PowerPC and Intel x86 systems as a standalone operating system. In 2000, the core operating system components of Mac OS X were released as open-source software under the Apple Public Source License (APSL) as Darwin the higher-level components, such as the Cocoa and Carbon frameworks, remained closed-source. At the time, interim CEO Steve Jobs alluded to British naturalist Charles Darwin by announcing 'because it's about evolution'. In 1999, Apple announced it would release the source code for the Mach 2.5 microkernel, BSD Unix 4.4 OS, and the Apache Web server components of Mac OS X Server. This was developed into Rhapsody in 1997, Mac OS X Server 1.0 in 1999, Mac OS X Public Beta in 2000, and Mac OS X 10.0 in 2001. After Apple bought NeXT in 1996, it announced it would base its next operating system on OPENSTEP. The heritage of Darwin began with Unix derivatives supplemented by aspects of NeXT's NeXTSTEP operating system (later, since version 4.0, known as OPENSTEP), first released in 1989. Starting with Leopard, macOS has been certified as compatible with the Single UNIX Specification version 3 (SUSv3). It is composed of code derived from NeXTSTEP, BSD, Mach, and other free software projects' code, as well as code developed by Apple.ĭarwin is mostly POSIX-compatible, but has never, by itself, been certified as compatible with any version of POSIX. It previously existed as an independent open-source operating system, first released by Apple Inc. Darwin is the core Unix operating system of macOS (previously OS X and Mac OS X), iOS, watchOS, tvOS, iPadOS and bridgeOS.

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