Apple rebranded the system as Mac OS in 1996, starting officially with version 7.6, due in part to its Macintosh clone program.

With the Macintosh 512K, a system extension called the Switcher was developed to use this additional memory to allow multiple programs to remain loaded.

With the Switcher, the now familiar Clipboard feature allowed copy and paste between the loaded programs across switches including the desktop.

By the mid-1990s, however, contemporary operating systems such as Windows NT, OS/2, NeXTSTEP, BSD, and Linux had all brought pre-emptive multitasking, protected memory, access controls, and multi-user capabilities to desktop computers.

The Macintosh's limited memory management and susceptibility to conflicts among extensions that provide additional functionality, such as networking or support for a particular device,[8] led to significant criticism of the operating system, and was a factor in Apple's declining market share at the time.

After two aborted attempts at creating a successor to the Macintosh System Software called Taligent and Copland, and a four-year development effort spearheaded by Steve Jobs's return to Apple in 1997, Apple replaced Mac OS with a new operating system in 2001 named Mac OS X.

It retained most of the user interface design elements of the Classic Mac OS, and there was some overlap of application frameworks for compatibility, but the two operating systems otherwise have completely different origins and architectures.

[9] The Macintosh project started in late 1978 with Jef Raskin, who envisioned an easy-to-use, low-cost computer for the average consumer.

Bill Atkinson, a member of the Apple Lisa team, introduced Raskin to Burrell Smith, a service technician who had been hired earlier that year.

Jobs and a number of Apple engineers visited Xerox PARC in December 1979, three months after the Lisa and Macintosh projects had begun.

He was able to conserve precious ROM space by writing routines in assembly language code optimized with "hacks", or clever programming tricks.

This architecture also allows 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, are 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 is in contrast to MS-DOS and CP/M computers of the time, which display such messages in a mono-spaced font on a black background, and require the use of the keyboard rather than a mouse, for input.

The resource fork contains other structured data such as menu definitions, graphics, sounds, or code segments that would be incorporated into a program's file format on other systems.

As part of Apple's goal of creating a computer with appliance-like simplicity, there is no explicit distinction made between the operating system software and the hardware it runs on.

[22] For the first time, the Macintosh operating system was offered as a distinct retail product that included four 800K disks and three manuals, at a cost of US$49.

It was a major upgrade over System 6, adding a significant user interface overhaul, new applications, stability improvements and many new features.

[23] Accompanying this was a move to 32-bit memory addressing, necessary for the ever-increasing amounts of RAM available to the Motorola 68030 CPU, and 68020 CPUs with a 68851 PMMU.

This process involves making all of the routines in OS code use the full 32-bits of a pointer as an address—prior systems used the upper 8 bits as flags.

To ease the transition, the "Memory" control panel contains a switch to disable this feature, allowing for compatibility with older applications.

Initially planned as Mac OS 7.7, it was renumbered "8" to exploit a legal loophole and accomplish Jobs's goal of terminating third-party manufacturers' licenses to System 7 and shutting down the Macintosh clone market.

[29] Mac OS 8 added a number of features from the abandoned Copland project, while leaving the underlying operating system unchanged.

[34] macOS also makes use of the BSD codebase and the XNU kernel,[35] and its core set of components is based upon Apple's open source Darwin operating system.

It retains the "Platinum" appearance from the Classic Mac OS and even resembles OPENSTEP in places, with the first version to arrive with the new Aqua user interface.

Most well-written "classic" Mac OS applications function properly under this environment, but compatibility is assured only if the software was written to be unaware of the actual hardware and to interact solely with the operating system.

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.

Executor filled a niche market for porting 68k Mac applications to x86 platforms; development ceased in 2002 and the source code was released by the author in late 2008.

[42] QEMU has several advantages over other PowerPC emulators namely supporting a wide range of platforms from Linux to Mac and Windows on current CPU architectures.

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.

Using this method has been said to equal or better the speed of a Macintosh with the same processor, especially with respect to the 68k series due to real Macs running in MMU trap mode, hampering performance.