Apple Wiki
Apple Wiki
Motorola XC68851RC16 memory management unit

A pin grid array (PGA) version of the 68851.

The Motorola 68851 is a paged memory management unit (MMU) that was produced by Motorola's Semiconductor Products Sector for use with the Motorola 68020 processor.

The 68851 allows a 68020 processor to support virtual memory. The only Macintosh model with a socket to accept a 68851 was the 68020-based Macintosh II. However, the socket was shipped with a lower-cost VLSI VI475 custom hardware memory management unit (HMMU) which did not support virtual memory like the more expensive 68851.

The 68851 succeeded the Motorola 68451 memory management unit, which was designed for the 68010 processor. This memory management functionality was later built directly into the 68030 processor used by subsequent models.[1]


  1. Frequently Asked Questions (FAQ) by Robert Boys, comp.sys.m68k. 1996-01-06.

External links[]

Motorola 68k series processors
Central processing units : 68000 · 68010 · 68020 · 68030 · 68040 · 68050 · 68060
Floating-point units :   (n/a)  ·   (n/a)  · 68881 · 68882 · (SANE)
Memory management units :   (n/a)  · 68451 · 68851
Other :                           (VI475)
Superseded by PowerPC processors in March 1994
Utility stub shuffle This article is a stub. You can help by expanding it.