VLSI VI475

The VLSI VI475, also known as Apple HMMU or HMMU Chip, is a hardware memory management unit (HMMU) that was custom produced by VLSI Technology for Apple Computer to use in its Macintosh II computer. Its Apple part number is 343-0002-1.

Features
The VI475 added backwards compatibility for 24-bit addressing to the 68020 processor because the ROMs in the Macintosh II were not yet 32-bit clean. However, it did not include paged memory management functionality required for virtual memory support like the more expensive 68851.

The VI475 uses only 70 out of the 132 pins in a 68851 socket and can be swapped for a full 68851 to add support for virtual memory, 32-bit addressing (with MODE32 and a FDHD ROM upgrade), and A/UX. These memory management features were later built directly into the 68030 processor used by subsequent Macintosh II series models.

History
A prototype version of the Macintosh II, code named "Paris", used a custom hardware memory management daughtercard as a predecessor to the VI475. This daughtercard occupied the 68851 socket and contained three Programmable Array Logic (PAL) chips produced by Monolithic Memories. This was consolidated into the VI475 upon release of the Macintosh II to the public.