The Motorola 68851 is an external

Memory Management Unit A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware unit having all memory references passed through itself, primarily performing the translation of virtual memory addresses to physical a ...

(MMU) which is designed to provide paged memory support for the

68020 The Motorola 68020 ("''sixty-eight-oh-twenty''", "''sixty-eight-oh-two-oh''" or "''six-eight-oh-two-oh''") is a 32-bit microprocessor from Motorola, released in 1984. A lower-cost version was also made available, known as the 68EC020. In keepin ...

using that processor's coprocessor interface. In theory it can be used with other processors such as the

68010 The Motorola MC68010 processor is a 16/32-bit microprocessor from Motorola, released in 1982 as the successor to the Motorola 68000. It fixes several small flaws in the 68000, and adds a few features. The 68010 is pin-compatible with the 68000, ...

by simulating the coprocessor interface in software. Later 68K family processors such as the

68030 The Motorola 68030 ("''sixty-eight-oh-thirty''") is a 32-bit microprocessor in the Motorola 68000 family. It was released in 1987. The 68030 was the successor to the Motorola 68020, and was followed by the Motorola 68040. In keeping with genera ...

, 68040, and

68060 The Motorola 68060 ("''sixty-eight-oh-sixty''") is a 32-bit microprocessor from Motorola released in 1994. It is the successor to the Motorola 68040 and is the highest performing member of the 68000 series. Two derivatives were produced, the 68L ...

have an internal MMU, and will not operate with the 68851, except possibly by simulation of the coprocessor interface. The 68851 was available as an option for the Apple

Macintosh II The Macintosh II is a personal computer designed, manufactured, and sold by Apple Computer from March 1987 to January 1990. Based on the Motorola 68020 32-bit CPU, it is the first Macintosh supporting color graphics. When introduced, a basic sy ...

, and was necessary to run Apple's

A/UX A/UX is Apple Computer's Unix-based operating system for Macintosh computers, integrated with System 7's graphical interface and application compatibility. Launched in 1988 and discontinued in 1995 with version 3.1.1, it is Apple's first officia ...

operating system. On the classic Mac OS,

Connectix Connectix Corporation was a software and hardware company, noted for having released innovative products that were either made obsolete as Apple Computer incorporated the ideas into system software, or were sold to other companies once they becam ...

Virtual was released in early 1989 and used the 68851 to provide virtual memory, which was later integrated into

System 7 System 7, codenamed "Big Bang", and also known as Mac OS 7, is a graphical user interface-based operating system for Macintosh computers and is part of the classic Mac OS series of operating systems. It was introduced on May 13, 1991, by Apple C ...

. Very few cards for the Amiga make use of the 68851 primarily because it can only be used with a small range of processors and most Amigas and accelerator cards use a processor which either has its own MMU or cannot support an MMU. One of the few cards which does use this is the Commodore A2620.

References

See also