Darwin
Darwin is an open source POSIX compliant computer operating system released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NEXTSTEP, FreeBSD, and other free software projects.
Darwin forms the core set of components upon which Mac OS X and iPhone OS are based. It can also be run as a standalone operating system (although Apple no longer provides a distribution). It is compatible with the Single UNIX Specification version 3 (SUSv3) and POSIX UNIX applications and utilities.
Darwin's heritage began with NeXT's NEXTSTEP operating system (later known as OPENSTEP), first released in 1989. After Apple bought NeXT in 1997, it announced it would base its next operating system on OPENSTEP. This was developed into Rhapsody in 1997 and the Rhapsody-based Mac OS X Server 1.0 in 1999. In 2000, Rhapsody was forked into Darwin and released as open-source software under the Apple Public Source License (APSL), and components from Darwin are present in Mac OS X today.
Up to Darwin 8.0, 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 computers as a standalone operating system. Minor updates were released as packages that were installed separately. Darwin is now only available as source code, except for the ARM variant, which has not been released in any form separately from iPhone OS.
Darwin is built around XNU, a hybrid kernel that combines the Mach 3 microkernel, various elements of BSD (including the process model, network stack, and virtual file system), and an object-oriented device driver API called I/O Kit.
Some of the benefits of this choice of kernel are the Mach-O binary format, which allows a single executable file (including the kernel itself) to support multiple CPU architectures, and the mature support for symmetric multiprocessing in Mach. The hybrid kernel design compromises between the flexibility of a microkernel and the performance of a monolithic kernel.
Darwin currently includes support for both 32-bit and 64-bit variants of the PowerPC and Intel x86 processors used in the Mac and Apple TV as well as the 32-bit ARM processor used in the iPhone and iPod Touch.
It supports the POSIX API by way of its BSD lineage and a large number of programs written for various other UNIX-like systems can be compiled on Darwin with no changes to the source code.
Darwin and Mac OS X both use I/O Kit for their drivers and therefore support the same hardware, file systems, and so forth. Apple's distribution of Darwin included proprietary (binary-only) drivers for their AirPort wireless cards.
Darwin does not include many of the defining elements of Mac OS X, such as the Carbon and Cocoa APIs or the Quartz Compositor and Aqua user interface, and thus cannot run Mac applications. It does, however, support a number of lesser known features of Mac OS X, such as mDNSResponder, which is the multicast DNS responder and a core component of the Bonjour networking technology, and launchd, an advanced service management framework.
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