Mac OS X is an operating system developed, marketed, and sold by Apple Inc., and since 2002 has been included with all new Macintosh computer systems. It is the successor to Mac OS 9, the final release of the "classic"Mac OS, which had been Apple's primary operating system since 1984.
Mac OS X, whose "X" represents the Roman numeral for "10" and is a prominent part of its brand identity, is a Unix-based operating system, built on technologies developed at NeXT between the second half of the 1980s and Apple's purchase of the company in late 1996. Its sixth release Mac OS X v10.5 "Leopard" gained UNIX 03 certification while running on Intel processors.
Apple also produces specialized versions of Mac OS X for use on three of its consumer devices: the iPhone OS for the iPhone,iPod Touch, and iPad, as well as an unnamed version for the Apple TV. Mac OS X is the tenth major version of Apple's operating system for Macintosh computers. Previous Macintosh operating systems were named using Arabic numerals, e.g. Mac OS 8 and Mac OS 9. The letter X in Mac OS X's name refers to the number 10, a Roman numeral.
Mac OS X's core is a POSIX compliant operating system (OS) built on top of the XNU kernel, with standard Unix facilities available from the command line interface. Apple released this family of software as a free and open source operating system named Darwin, but it later became partially proprietary. On top of Darwin, Apple layered a number of components, including the Aqua interface and the Finder, to complete the GUI-based operating system which is Mac OS X.
Mac OS X introduced a number of new capabilities to provide a more stable and reliable platform than its predecessor, Mac OS 9. For example, pre-emptive multitasking and memory protection improved the system's ability to run multiple applications simultaneously without them interrupting or corrupting each other. Many aspects of Mac OS X's architecture are derived from Openstep, which was designed to be portable, to ease the transition from one platform to another. For example, Nextstep was ported from the original 68k-based NeXT workstations to x86 and other architectures before NeXT was purchased by Apple, and OpenStep was later ported to the PowerPC architecture as part of the Rhapsody project.
