Motorola 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.