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 that examines all references to computer memory, memory, and translates the memory addresses being referenced, known as virtual mem ...

(MMU) which is designed to provide paged memory support for the 68020 using that processor's coprocessor interface. In theory it can be used with other processors such as the

68010 The Motorola MC68010 and Motorola MC68012 are 16/32-bit microprocessors from Motorola, released in 1982 as successors to the Motorola 68000. The 68010 and 68012 added virtualization features, optimized loops and fixed several small flaws to the ...

by simulating the coprocessor interface in software. Later 68K family processors such as the 68030,

68040 The Motorola 68040 ("''sixty-eight-oh-forty''") is a 32-bit microprocessor in the Motorola 68000 series, released in 1990. It is the successor to the 68030 and is followed by the 68060, skipping the 68050. In keeping with general Motorola ...

, and 68060 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 ...

, and was necessary to run Apple's

A/UX A/UX is a Unix-based operating system from Apple Computer for Macintosh computers, integrated with System 7's graphical interface and application compatibility. It is Apple's first official Unix-based operating system, launched in 1988 and disc ...

operating system. On the

classic Mac OS Mac OS (originally System Software; retronym: Classic Mac OS) is the series of operating systems developed for the Mac (computer), Macintosh family of personal computers by Apple Computer, Inc. from 1984 to 2001, starting with System 1 and end ...

Connectix Connectix Corporation was a software and hardware company that 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 became popular. It ...

Virtual was released in early 1989 and used the 68851 to provide virtual memory, which was later integrated into System 7. Very few cards for the

Amiga Amiga is a family of personal computers produced by Commodore International, Commodore from 1985 until the company's bankruptcy in 1994, with production by others afterward. The original model is one of a number of mid-1980s computers with 16-b ...

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