OCHAKS

From the beginning, Apple deliberately sought to minimize by design the user's conceptual awareness of the operating system as such. Tasks that on other products required a more explicit working knowledge of an operating system would on a Macintosh be accomplished by intuitive mouse gestures and manipulation of graphical control panels. The intention was that the product would thus be more user-friendly and so more easily mastered. This would differentiate it from devices using other operating environments, such as MS-DOSmachines, which were more technically challenging to operate.

The core of the system software was held in ROM, with updates (which would override ROM-resident portions in RAM) typically provided free of charge by Apple dealers on floppy disk. The user's involvement in an upgrade of the operating system was also minimized to running an installer, or simply replacing system files, the simplicity of which again differentiated the product from other offerings.

Early versions of the Mac OS were compatible only with Motorola 68000-based Macintoshes. As Apple introduced computers with PowerPC hardware, the OS was ported to support this architecture as well. Mac OS 8.1 was the last version that could run on a "68K" processor (the 68040). OS X, which has superseded the "Classic" Mac OS, is compatible with only PowerPC processors from version 10.0 ("Cheetah") to version 10.3 ("Panther"). PowerPC and Intel processors are supported in version 10.4 ("Tiger", Intel only supported after an update) and version 10.5 ("Leopard"). 10.6 and later versions support only Intel processors.^[4]

The early Macintosh operating system initially consisted of two pieces of software, called "System" and "Finder", each with its own version number.^[5] System 7.5.1 was the first to include the Mac OS logo (a variation on the original Happy Mac startup icon), and Mac OS 7.6 was the first to be named "Mac OS".

Before the introduction of the later PowerPC G3-based systems, significant parts of the system were stored in physical ROM on the motherboard. The initial purpose of this was to avoid using up the limited storage of floppy disks on system support, given that the early Macs had no hard disk. (Only one model of Mac was ever actually bootable using the ROM alone, the 1991 Mac Classicmodel.) This architecture also allowed for a completely graphical OS interface at the lowest level without the need for a text-only console or command-line mode. Boot time errors, such as finding no functioning disk drives, were communicated to the user graphically, usually with an icon or the distinctive Chicago bitmap font and a Chime of Death or a series of beeps. This was in contrast to computers of the time, which displayed such messages in a mono-spaced font on a black background, and required the use of the keyboard, not a mouse, for input. To provide such niceties at a low level, Mac OS depended on core system software in ROM on the motherboard, a fact that later helped to ensure that only Apple computers or licensed clones (with the copyright-protected ROMs from Apple) could run Mac OS.

Mac OS can be divided into two families:

The "classic" Mac OS is characterized by its total lack of a command line; it is a completely graphical operating system. Versions of Mac OS up through System 4 only ran one application at a time. Even so, it was noted for its ease of use. Mac OS gained cooperative multitasking with System 5, which ran on the Mac SE and Macintosh II. It was criticized for its very limited memory management, lack of protected memory, and susceptibility to conflicts among operating system "extensions" that provide additional functionality (such as networking) or support for a particular device. Some extensions may not work properly together, or work only when loaded in a particular order. Troubleshooting Mac OS extensions could be a time-consuming process of trial and error.

The Macintosh originally used the Macintosh File System (MFS), a flat file system with only one level of folders. This was quickly replaced in 1985 by the Hierarchical File System (HFS), which had a true directory tree. Both file systems are otherwise compatible.

Most file systems used with DOS, Unix, or other operating systems treat a file as simply a sequence of bytes, requiring an application to know which bytes represent what type of information. By contrast, MFS and HFS give files two different "forks". The data fork contains the same sort of information as other file systems, such as the text of a document or the bitmaps of an image file. The resource fork contains other structured data such as menu definitions, graphics, sounds, or code segments. A file might consist only of resources with an empty data fork, or only a data fork with no resource fork. A word processor file could contain its text in the data fork and styling information in the resource fork, so that an application which doesn’t recognize the styling information can still read the raw text.

On the other hand, these forks would provide a challenge to interoperability with other operating systems: how does one copy a dual-forked file into a different file system, or across a file-transfer system, or embed it into email? In copying or transferring a MacOS file to a non-Mac system, the default implementations would simply strip the file of its resource fork. Most data files contained only nonessential information in their resource fork, such as window size and location, but program files would be inoperative without their resources. This necessitated such encoding schemes asBinHex and MacBinary, which allowed a user to encode a dual-forked file into a single stream, or take a single stream so-encoded and reconstitute it into a dual-forked file usable by MacOS.

PowerPC versions of OS X up to and including OS X v10.4 Tiger (support for Classic was dropped by Apple with v10.5 Leopard's release and it is no longer included) include a compatibility layer for running older Mac applications, the Classic Environment. This runs a full copy of the older Mac OS, version 9.1 or later, in a OS X process. PowerPC-based Macs shipped with Mac OS 9.2 as well as OS X. Mac OS 9.2 had to be installed by the user— it was not installed by default on hardware revisions released after the release of OS X 10.4 Tiger. Most well-written "classic" applications function properly under this environment, but compatibility is only assured if the software was written to be unaware of the actual hardware, and to interact solely with the operating system. The Classic Environment is not available on Intel-based Macintosh systems due to the incompatibility of Mac OS 9 with the x86 hardware.

Users of the classic Mac OS generally upgraded to OS X, but many criticized it as being more difficult and less user-friendly than the original Mac OS, for the lack of certain features that had not been re-implemented in the new OS, or for being slower on the same hardware (especially older hardware), or other, sometimes serious incompatibilities with the older OS.^[6] Because drivers (for printers, scanners, tablets, etc.) written for the older Mac OS are not compatible with OS X, and due to the lack of OS X support for older Apple machines, a significant number of Macintosh users continued using the older classic Mac OS.

In June 2005, Steve Jobs announced at the Worldwide Developers Conference keynote that Apple computers would be transitioning from PowerPC to Intel processors and thus dropping compatibility on new machines for Mac OS Classic. At the same conference, Jobs announced Developer Transition Kits that included beta versions of Apple software including OS X that developers could use to test their applications as they ported them to run on Intel-powered Macs. In January 2006, Apple released the first Macintosh computers with Intel processors, an iMacand the MacBook Pro, and in February 2006, Apple released a Mac mini with an Intel Core Solo and Duo processor. On May 16, 2006, Apple released the MacBook, before completing the Intel transition on August 7 with the Mac Pro. To ease the transition for early buyers of the new machines, Intel-based Macs included an emulation technology called Rosetta, which allows them to run OS X software that was compiled for PowerPC-based Macintoshes. Rosetta runs transparently, creating a user experience identical to running the software on a PowerPC machine, though execution is typically slower than with native code. Rosetta was an optional installation in OS X 10.6 Snow Leopard and is not available at all in OS X 10.7 Lion.

OS X, introduced as Mac OS X and renamed OS X in 2012, is the latest version of Apple's operating system. Although it is officially designated as simply "version 10" of the Mac OS, it has a history largely independent of the earlier Mac OS releases.

The operating system is the successor to Mac OS 9 and the "classic" Mac OS. It is however a Unix operating system, based on the NeXTSTEP operating system and the Mach kernel which Apple acquired after purchasing NeXT Computer - with its CEO Steve Jobs returning to Apple at that time. OS X also makes use of the BSD code base. There have been six significant releases of OS X, the most recent being OS X 10.8, referred to as Mountain Lion.

OS X also had six significant releases as OS X Server. The first of these, OS X Server 1.0, was released in beta in 1999. The server versions are architecturally identical to the client versions, with the differentiation found in their inclusion of tools for server management, including tools for managing OS X-based workgroups, mail servers, and web servers, amongst other tools. As of the name change to OS X, OS X Server is no longer sold as a separate operating system, the server tools can be added to OS X, giving the same functionality.

OS X Server was the default operating system for Xserve (which has now been discontinued),^[7] it's an optional feature on the Mac Mini and the Mac Pro, and it's also installable on most other Macs. Unlike the client version, OS X Server can be run in a virtual machine using emulation software such as Parallels Desktop and VMWare Fusion.

OS X is also the basis for iOS, (previously iPhone OS) used on Apple's iPhone, iPod Touch, and iPad.