Mac OS is the trademarked name for 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.
Apple deliberately downplayed the existence of the operating system in the early years of the Macintosh to help make the machine appear more user-friendly and to distance it from other operating systems such as MS-DOS, which were portrayed as arcane and technically challenging. Much of this early system software was held in ROM, with updates typically provided free of charge by Apple dealers on floppy disk. As increasing disk storage capacity and performance gradually eliminated the need for fixing much of an advanced GUI operating system in ROM, Apple explored cloning while positioning major operating system upgrades as separate revenue-generating products, first with System 7 and System 7.5, then with Mac OS 7.6 in 1997.
Earlier versions of the Mac OS were compatible only with Motorola 68000-based Macintoshes. As Apple introduced computers with PowerPC hardware, the OS was upgraded to support this architecture as well. Mac OS X, which has superseded the "Classic" Mac OS, is compatible with both PowerPC and Intel processors.
Until the advent 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. A fatal software error, or even a low-level hardware error discovered during system startup (such as finding no functioning disk drives), was communicated to the user graphically using some combination of icons, alert box windows, buttons, a mouse pointer, and the distinctive Chicago bitmap font. Mac OS depended on this 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.
The Mac OS can be divided into two families of operating systems:
The "classic" Mac OS is characterized by its total lack of a command line; it is a completely graphical operating system. Noted for its ease of use and its cooperative multitasking, 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 can 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 represented what type of information. By contrast, MFS and HFS gave files two different "forks". The data fork contained 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 contained 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 text file could contain its text in the data fork and styling information in the resource fork, so that an application, which didn’t recognize the styling information, could still read the raw text. On the other hand, these forks provided a challenge to interoperability with other operating systems; copying a file from a Mac to a non-Mac system would strip it of its resource fork, necessitating such encoding schemes as BinHex and MacBinary.
Classic Application Support was shipped with Mac OS X with PowerPC Macs until early 2006. However, Intel-based Macintoshes cannot run the Classic system or applications, nor can PowerPC models while they are running Mac OS X v10.5 Leopard.
Mac OS X brought Unix-style memory management and pre-emptive multitasking to the Mac platform. It is based on the Mach kernel and the BSD implementation of UNIX, which were incorporated into NeXTSTEP, the object-oriented operating system developed by Steve Jobs' NeXT company. The new memory management system allowed more programs to run at once and virtually eliminated the possibility of one program crashing another. It is also the second Macintosh operating system to include a command line (the first is the now-discontinued A/UX, which supported classic Mac OS applications on top of a UNIX kernel), although it is never seen unless the user launches a terminal emulator.
However, since these new features put higher demands on system resources, Mac OS X only officially supported the PowerPC G3 and newer processors, and now has the additional requirement of built-in USB (10.3) and FireWire (10.4)).
For over three years, Mac OS X has become faster with every release - faster on the same hardware. PowerPC builds of Mac OS X 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 Macintoshes due to the incompatibility of Mac OS 9 with the x86 hardware, and was removed completely on Mac OS X 10.5 Leopard.
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. 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 have still continued using the older classic Mac OS. But by 2005, it has been reported that almost all users of systems capable of running Mac OS X are doing so, with only a small fraction still running the 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. 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 include an emulation technology called Rosetta, which allows them to run (at reduced speed) pre-existing Mac OS X native application software that was compiled only for PowerPC-based Macintoshes.
One interesting historical aspect of the classic Mac OS was a relatively unknown secret prototype Apple started work on in 1992, code-named "Star Trek" (as in "to boldly go"). The goal of this project was to create a version of 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 a PC. Some of the code from this effort was reused when porting the Mac OS later to PowerPC.
The project was short lived, being canceled only one year later in early 1993. There are two theories for the cancellation: 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.
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.
A notable exception was the Executor commercial software product from Abacus Research & Development, the only product that exclusively used 100% reverse engineered code without the use of Apple technology. It ran extremely fast 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 project is now defunct.
Emulators using Mac ROM images offered near complete Mac OS compatibility and later versions offered excellent performance as modern x86 processor performance increased exponentially.
Unfortunately most of the Mac user base had already started moving to the PowerPC platform that offered excellent classic Mac 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.
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.
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.