The MC68451 is a

Motorola Motorola, Inc. () was an American Multinational corporation, multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent p ...

(now

Freescale Freescale Semiconductor, Inc. was an American semiconductor manufacturer. It was created by the divestiture of the Semiconductor Products Sector of Motorola in 2004. Freescale focused their integrated circuit products on the automotive, embed ...

)

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 ad ...

(MMU), which was primarily used in conjunction with the

MC68010

microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...

The MC68451 supported a 16 MB address space and provided a

MC68000 The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand") is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Sector ...

or a MC68010 with support for memory management and protection of memory against unauthorized access. The block size was variable, so it was usually used for segment-based memory management. It supported the mapping of up to 32 memory segments or pages of a variable size from logical to physical addresses. To allow more segments or pages, the simultaneous use of multiple MC68451 MMUs was supported In combination with a MC68010, the MC68451 permitted the realization of

virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very l ...

. With the earlier

, this was not possible due to the way the

treated memory access errors, i.e. processor state could not always be properly restored after a page fault; two MC68000s would be required, with the main CPU pausing when it got a memory access error, and the other CPU servicing the page fault. The limitation to 32 segment table entries per MMU made systems based on a MC68010 and a MC68451 slow, as they often had to modify the segment table due to its small size. H. Berthold AG used 12 MC68451 MMUs together with their UNOS variant vBertOS. Others (e.g.

Sun Microsystems Sun Microsystems, Inc. (Sun for short) was an American technology company that sold computers, computer components, software, and information technology services and created the Java programming language, the Solaris operating system, ZFS, the ...

Convergent Technologies Convergent Technologies was an American computer company formed by a small group of people who left Intel Corporation and Xerox PARC in 1979. Among the founders were CEO Allen Michels, VP Engineering Bob Garrow, head of marketing Kal Hubler, and ...

) used their own proprietary MMUs instead of the MC68451.

References

68k architecture 68451 {{compu-stub

See also

References