These machines shipped with 2MB RAM, an 80MB HD (huge for the time), and a 68851 pre-installed A/UX required the 68851 to work. I can answer the last part of your question: yes, the Mac II could use a 68851.įor a time, Virginia Tech required incoming Computer Science freshmen to purchase a (heavily discounted) Mac II running A/UX, Apple's first in-house-developed UNIX. It is possible to switch to a real 68851 “PMMU” but it doesn’t increase Mac OS performance unless you want to use virtual memory, and depending on your specific Mac II board, the MMU socket pinout may not match the 68851 and you might have to acquire and use the adapter card mentioned by jeffB. In this case, the 24-to-32 mapping can be turned off. In 24-bit mode, the Mac II can only access 8 MB of ram, so if you want to use more ram, you need to run “32-bit-clean” software which does not use those 8 top address bits. The Mac II has a full 32-bit address bus, so, when running older software which uses those top 8 bits of the address, the 24-to-32 mapping must be turned on so the top 8 address bits are ignored. The original 68000 had only a 24-bit addressing capability, and so on 68000 Macs, the top 8 bits of a 32-bit address do not change what is output on the bus. In 32-bit mode the address is buffered unchanged.Įarly versions of the Mac system software store information in the top 8 bits of pointers (addresses).

In 24-bit mode, the addresses are mapped as shown and the top 8 bits of the address bus are ignored. Your Mac II “HMMU” chip implements the address functionality shown in this image, from Guide to the Macintosh Family Hardware, 2nd edition:

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