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To: Tenchusatsu who wrote (69588)	1/30/2002 9:06:03 PM
From: Dan3	Read Replies (2) \| Respond to of 275872

Re: What does PMMU mean, anyway? It is simply impossible that Tenchusatsu hasn't heard of a paged memory management unit. Who are you and why are you posting from his account? :-) The Macintosh II was a 68020 based machine running at 16 MHz RAM was 1 MB - 20 MB using standard 30-pin SIMMs, had an optional PMMU, and standard FPU. Hard disk was 20 MB - 80 MB, and floppy drive was 800K (dual 800K was optional). It was not 32-bit clean. Later Apple offered an upgrade for the 1.44 MB Superdrive for the Mac II. This changed the ROMs, added a PMMU (Paged Memory Management Unit - for virtual memory use), and 1.44 MB floppy drive(s), and made the machine 32-bit clean. acornworld.net

To: Tenchusatsu who wrote (69588)	1/31/2002 1:38:21 AM
From: dale_laroy	Read Replies (1) \| Respond to of 275872

>What does PMMU mean, anyway?< Well, without clearly defining what a page is, the term PMMU is relatively meaningless. For example, the Macintosh II used the 68851 PMMU while maintaining compatibility with the "segmented" memory management of the 68000 based Macintosh through the use of custom memory management hardware. I thought this was sort of rediculous because the 68851 PMMU supported five levels of programmable page sizes with early termination of table walk. Theoretically, the lowest two levels could have been set at the smallest page size of the DRAMs (allowing for up to two different DRAM page sizes), the next level up at the 32KB segment size that Apple handled in hardware, the next level up at the 16MB address space of a NuBus slot and the next level up at the 256MB address space of a NuBus Superslot. This would have allowed each memory structure from an active DRAM page to an entire NuBus Superslot to be handled using a single table entry terminated at the appropriate level. Now that was a PMMU! Directory based cache coherency is probably a reasonable description of what I was talking about.