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.

Apple deliberately downplayed the existence of the operating system in the early
years of the Macintosh^{[citation needed]} to help make the machine appear more 
user-friendly and to distance it from other operating systems such as MS-DOS,
which was more 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 storing much of the advanced GUI operating
system in the ROM, Apple explored clones while positioning major operating
system upgrades as separate revenue-generating products, first with System 7.
1 and System 7.5, then with Mac OS 7.6 in 1997.

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 68000-class processor (the 68040). Mac OS X, which has
superseded the "Classic" Mac OS, is compatible with both PowerPC and Intel
processors through to version 10.5 ("Leopard"). Version 10.6 ("Snow Leopard")
supports only Intel processors.

Versions

The early Macintosh operating system initially consisted of two pieces of software,
called "System" and "Finder", each with its own version number. System 7.5.2
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 PCs 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)

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 text 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 provide a challenge to interoperability with other operating systems;
copying a file from a Mac to a non-Mac system strips it of its resource fork,
necessitating such encoding schemes as BinHex and MacBinary.

PowerPC versions of Mac OS X up to and including Mac OS X v10.5 Leopard
(support for Classic was dropped by Apple with Snow 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. 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.

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
include 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.

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 10.6, referred to as Snow Leopard. On Apple's October
20th 2010 "Back to the Mac" event, Mac OS X 10.7 Lion was previewed, showing
improvements and additions including a Mac App Store.

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-basedworkgroups, mail
servers, and web servers, amongst other tools. It is currently the default operating
system for the Xserve server hardware, and as an optional feature on the Mac Mini,
as well as being 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.

Mac OS X is also the basis for iOS, (previously iPhone OS) used on Apple's
 iPhone, iPod Touch, and iPad, as well as being the basis for the operating
system used in the Apple TV.

Star Trek

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 where no Mac has gone before"). 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 onOPENSTEP, 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.

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
emulatorcalled 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.
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.

A/UX

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 NeXTStepafter
 Steve Jobs rejoined Apple in 1997.

MAE

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.

MkLinux

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, 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
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.

External links

• MAC OS X -- Official site
• MAC 101 – Apple's introductory guide to the Mac OS.
• FOLKLORE– A site of anecdotes shared by the creators of the first Macintosh.
• The Vintage MAC Museum  – Old Mac System - From System1 to System7
• MAC Myuser  – Web community dedicated to the Mac OS platform of Borneo,
• Malaysia