Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!cs.utexas.edu!rutgers!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.mac
Subject: Re: Mach platform w/small pages?  (MacII/uVAX?)
Message-ID: <7860@cbmvax.UUCP>
Date: 8 Sep 89 15:35:55 GMT
References: <11682@polya.Stanford.EDU>
Organization: Commodore Technology, West Chester, PA
Lines: 33

in article <11682@polya.Stanford.EDU>, wilson@carcoar.Stanford.EDU (Paul Wilson) says:
> Keywords: Mach, Macintosh II, page size, virtual memory, PMMU, 68851
> Xref: cbmvax comp.sys.mac:41273 comp.os.mach:45

> Does anybody know offhand the page size on the MacII?  My understanding
> is that the MC68851 can handle any page size that's a power of 2 multiple
> of 512 bytes;  the choice of page size is up to the system designer.

You can set the minimum page size between 256 bytes and 32k, in power of 2
increments, for both 68851 and 68030 MMUs.  That's the minimum page -- since
the MMU reads a translation tree, larger effective page sizes are no problem;
the tree can terminate at any level.  The 68030 also provides two registers
for transparent translation, useful for I/O spaces or other non-translated
space that you'd rather not work into your MMU tables.

> So anyway, if the page size is fixed in hardware, what is it?

If you're upward looking on such a system, it's probably better to think in
terms of 4k or 8k pages; the 68040 MMUs are limited to those two sizes.

> (Oh yeah -- Apple has its own propriety PMMU that's a subset of the
> 68851 -- maybe that fixes the page size, at least for some MacII's?)

Far as I know, the Apple MMU does two things: [1] Wraps the 32 bit address into
24 bits so that native Mac OS stuff works, and [2] holds the place that the
68851 will go.  It doesn't really page or anything; they require a 68851 to
support any sort of translation (UNIX) or even virtual memory under the Mac OS.

>     Paul
-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
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