Mac OS

Mac OS
Mac OS
Mac OS logo
Company / developer Apple
OS family Mac OS (System 1–7, Mac OS 8–9)
Unix (Mac OS X)[1][2][3]
Working state Publicly released
Source model Proprietary software (with open source components)
Latest stable release 10.7.2 "Lion" build 11C74 [+/−]
Latest unstable release None at this time [+/−]
License Proprietary EULA
Official website

Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. (formerly Apple Computer, Inc.) for their Macintosh line of computer systems. The Macintosh user experience is credited with popularizing the graphical user interface. The original form of what Apple would later name the "Mac OS" was the integral and unnamed system software first introduced in 1984 with the original Macintosh, usually referred to simply as the System software.



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-DOS machines, 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). Mac OS X, which has superseded the "Classic" Mac OS, is compatible with only PowerPC processors from version 10.0 ("Cheetah") to version 10.3 ("Panther").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"). Versions 10.6 ("Snow Leopard") and later 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 Classic model.) 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 Mac OS Classic family, which was based on Apple's own code
  • The Mac OS X operating system, developed from Mac OS Classic family, and NeXTSTEP, which was UNIX-based.

"Classic" Mac OS (1984–2001)

Original 1984 Macintosh desktop

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 as BinHex 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 Mac OS X up to and including Mac 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 Mac OS X process. PowerPC-based Macs shipped with Mac OS 9.2 as well as Mac 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 Mac 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 Mac 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 Mac OS X, and due to the lack of Mac 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 Mac 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 iMac and 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 Mac 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 Mac OS X 10.6 Snow Leopard and is not available at all in Mac OS X 10.7 Lion.

Mac OS X

Mac OS X is the newest of Apple Inc.'s Mac OS line of operating systems. 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 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 this time. Mac OS X also makes use of the BSD code base. There have been six significant releases of the client version, the most recent being Mac OS X 10.7, referred to as Lion.

As well as the client versions, Mac OS X has also had six significant releases as a server version, called Mac OS X Server. The first of these, Mac 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 Mac OS X-based workgroups, mail servers, and web servers, amongst other tools. It 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, Mac OS X Server can be run in a virtual machine using emulation software such as Parallels Desktop and VMWare Fusion.

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

Star Trek

Star Trek (as in "to boldly go where no Mac has gone before") was a relatively unknown secret prototype Apple started work on in 1992, which goal was to create a version of the classic Mac OS that would run on Intel-compatible x86 personal computers. The project was instigated by Novell, Inc., who were looking to integrate their DR-DOS with the Mac OS UI as a retort to Microsoft's Windows 3.0. The Apple/Novell team (fourteen engineers from the former, four from the latter) was able to get the Macintosh Finder and some basic applications, like QuickTime, running smoothly on Windows. Some of the code from this effort was reused later when porting the Mac OS to PowerPC.[8]

The project was cancelled only one year later in early 1993. There are two theories for the project's short life: the first is that Apple's board canceled further development upon realizing that going with Star Trek would mean an entirely new business model and one that would likely see a notable drop in Apple's lucrative hardware sales; and the second is that an x86 Mac OS was not commercially viable in the early nineties because Microsoft's contracts for Windows 3.1 forced PC manufacturers to pay a royalty to Microsoft for every computer shipped, regardless of what operating system it contained.[9]

A further complication was that Star Trek was designed to be source-level compatible, not binary compatible, with the Mac OS. Mac applications would therefore have to be recompiled or rewritten by their developers to run on the x86 architecture, and there was much skepticism as to exactly how much work this would entail.

Fifteen years after Star Trek, support for the x86 architecture was officially included in Mac OS, and then Apple transitioned all desktop computers to the x86 architecture. This was not the direct result of earlier Project Star Trek efforts. The Darwin underpinning used for Mac OS X 10.0 and later included support for the x86 architecture. The remaining non-Darwin portion of Mac OS X (based on OPENSTEP, which ran on Intel processors) was released officially with the introduction of x86 Macintosh computers.

68000 emulation

Although the Star Trek software was never released, third-party Macintosh emulators, such as vMac, Basilisk II, and Executor, eventually made it possible to run the classic Mac OS on Intel-based PCs. These emulators were restricted to emulating the 68000 series of processors, and as such most couldn't run versions of the Mac OS that succeeded 8.1, which required PowerPC processors. Most also required a Mac ROM image or a hardware interface supporting a real Mac ROM chip; those requiring an image are of dubious legal standing as the ROM image may infringe on Apple's intellectual property.

A notable exception was the Executor commercial software product from Abacus Research & Development, the only product that used 100% reverse engineered code without the use of Apple technology. It ran extremely quickly but never achieved more than a minor subset of functionality. Few programs were completely compatible and many were extremely crash-prone if they ran at all. Executor filled a niche market for porting 68000 classic Mac applications to x86 platforms; development ceased in 2002 and the source code was released by the author in late 2008.[10]

Emulators using Mac ROM images offered near complete Mac OS compatibility and later versions offered excellent performance as modern x86 processor performance increased exponentially.

Most of the Mac user base had already started moving to the PowerPC platform that offered backward compatibility on 8.xx & 9.xx operating systems along with faster PowerPC software support. This helped ease the transition to PowerPC-only applications while prematurely obsolescing 68000 emulators and the Classic-only applications they supported well before these emulators were refined enough to compete with a real Mac.

PowerPC emulation

At the time of 68000-emulator development, PowerPC support was difficult to justify not only due to the emulation code itself but also the anticipated wide performance overhead of an emulated PowerPC architecture vs. a real PowerPC based Mac. This would later prove correct with the start of the PearPC project even years later despite the availability of 7th & 8th generation x86 processors employing similar architecture paradigms present in the PowerPC. Many application developers were also creating and releasing both 68000 Classic and PowerPC versions concurrently helping to negate the need for PowerPC emulation. PowerPC Mac users who could technically run either obviously chose the faster PowerPC applications. Soon Apple was no longer selling 68000-based Macs and the existing installed base started to quickly evaporate. Despite the eventual excellent 68000-emulation technology available they proved never to be even a minor threat to real Macs due to their late arrival and immaturity even several years after the release of much more compelling PowerPC based Macs.

The PearPC emulator is capable of emulating the PowerPC processors required by newer versions of the Mac OS (like Mac OS X). Unfortunately, it is still in the early stages and, like many emulators, tends to run much slower than a native operating system would.

During the transition from PowerPC to Intel processors, Apple realized the need to incorporate a PowerPC emulator into Mac OS X in order to protect its customers' investments in software designed to run on the PowerPC. Apple's solution is an emulator called Rosetta. Prior to the announcement of Rosetta, industry observers assumed that any PowerPC emulator running on an x86 processor would suffer a heavy performance penalty (e.g., PearPC's slow performance). Rosetta's relatively minor performance penalty therefore took many by surprise.

Another PowerPC emulator is SheepShaver, which has been around since 1998 for BeOS on the PowerPC platform, but in 2002 was open sourced with porting efforts beginning to get it to run on other platforms. Originally it was not designed for use on x86 platforms and required an actual PowerPC processor present in the machine it was running on similar to a hypervisor. Although it provides PowerPC processor support, it can only run up to Mac OS 9.0.4 because it does not emulate a memory management unit.

Other examples include ShapeShifter(by the same programmer that conceived SheepShaver), Fusion and iFusion. The latter ran classic Mac OS with a PowerPC "coprocessor" accelerator card. Using this method has been said to equal or better the speed of a Macintosh with the same processor, especially with respect to the m68k series due to real Macs running in MMU trap mode, hampering performance.

Macintosh clones

Several computer manufacturers over the years have made Macintosh clones capable of running Mac OS, notably Power Computing, UMAX and Motorola. These machines normally ran various versions of classic Mac OS. Steve Jobs ended the clone-licensing program after returning to Apple in 1997.

In 2008, a manufacturing company in Miami, FL called Psystar Corporation, announced a $499 clone that comes with a barebones system that can run Mac OS X 10.5 Leopard. Threatened with legal battles, Psystar originally called the system OpenMac and have since changed it to Open Computer. Apple filed a lawsuit with the company and asked that Psystar be ordered to stop producing clone systems, recall every system sold, and pay monetary damages.[11] Eventually, Apple prevailed in court, and the Open Computer's production was ceased. Psystar itself appears to be defunct now, as the company's website is gone.


In 1988, Apple released its first UNIX-based OS, A/UX, which was a UNIX operating system with the Mac OS look and feel. It was not very competitive for its time, due in part to the crowded Unix market. A/UX had most of its success in sales to the U.S. government, where UNIX was a requirement that Mac OS could not meet. Mac OS X later incorporated code from the UNIX-based NeXTStep after Steve Jobs rejoined Apple in 1997.


The Macintosh Application Environment (MAE) was a software package introduced by Apple Computer in 1994 which allowed users of certain Unix-based computer workstations to run Apple Macintosh application software.

MAE used the X Window System to emulate a Macintosh Finder-style graphical user interface. The last version, MAE 3.0, was compatible with System 7.5.3.

MAE was available for Sun Microsystems SPARCstation and Hewlett-Packard systems. It was discontinued on May 14, 1998.


Announced at The 1996 World Wide Developers Conference (WWDC), MkLinux is an open source computer operating system started by the OSF Research Institute and Apple Computer in February 1996 to port Linux to the PowerPC platform, and thus Macintosh computers. In the summer of 1998, the community-led MkLinux Developers Association took over development of the operating system. MkLinux is short for "Microkernel Linux," which refers to the project's adaptation of the Linux kernel to run as a server hosted atop the Mach microkernel. MkLinux is based on version 3.0 of Mach.

Mac OS on non Apple-labeled computers

Though a violation of Apple's EULA,[12] running Mac OS X operating systems compiled for x86 on a non-Apple PC is possible using various kernel modifications, third-party and community drivers, and emulation methods. For example, the PC-EFI[13] project emulates the Extensible Firmware Interface that is normally present on Apple's Intel-based Macs, allowing Mac OS X to be installed on non-Apple hardware.


  1. ^ "The Open Brand - Register of Certified Products - Mac OS X 10.5 on Intel-based Macintosh computers". The Open Group. Retrieved November 22, 2009. 
  2. ^ "The Open Brand - Register of Certified Products - Mac OS X 10.6 on Intel-based Macintosh computers". The Open Group. Retrieved November 22, 2009. 
  3. ^ "Apple page on UNIX". Apple Inc. Retrieved November 5, 2008. 
  4. ^ "Mac OS X 10.6 Snow Leopard Installation and Setup Guide". Apple Inc. Retrieved April 6, 2010. 
  5. ^ Macintosh: System Software Version History
  6. ^ "Mac OS X 10.1". 
  7. ^ "XServe has been discontinued". Retrieved 29 December 2010. 
  8. ^ Hormby (2005)
  9. ^ Caldera Sues Microsoft
  10. ^ "Executor source code". 
  11. ^ Paul McDougall (2008-07-16). "Apple Demands Recall Of Psystar Mac Clones". InformationWeek. Retrieved 2010-12-30. 
  12. ^ "EnglishLicense" (PDF). Retrieved 2010-05-10. 
  13. ^ "PC EFI Enables Vanilla OS X Kernel Boot on White Box PCs - InsanelyMac Forum". Retrieved 2010-05-10. 


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