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From: vandys@sequent.com (Andrew Valencia)
Newsgroups: comp.unix.wizards
Subject: Re: BSD virtual memory mngmt algorithm. Was: Performance Tuning Ultrix 4.1
Keywords: paging swapping fast large load BSD
Message-ID: <1991May11.144054.17281@sequent.com>
Date: 11 May 91 14:40:54 GMT
References: <1991Apr30.160331.16215@milton.u.washington.edu> <1991May7.065338.1027@ubeaut.enet.dec.com> <16121@smoke.brl.mil> <13087@dog.ee.lbl.gov>
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Organization: Sequent Computer Systems, Inc.
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torek@elf.ee.lbl.gov (Chris Torek) writes:

>Well no: while the old VM was rather icky and somewhat painful to
>modify for 3-level schemes such as the i386 and 68030, it did work
>pretty well for anything that had hardware PTEs.

	Actually the '030 can do both 2- and 3-level tables.  The kicker
was the '040 which can only do 3-level.  Rumor has it that they did this
because they felt they didn't have the real estate to do it both ways on
the chip.  They went for 3-level because they felt it better addressed the
low end--low end, 68040, go figure.  I probably don't have the complete
story, could someone from Moto flesh this out?

	The i386 is only 2-level page tables.  I guess you could call
it 3-level if you count the segmentation.  But most companies just make
that a flat 32-bit segment and forget about it.

				Andy Valencia
				vandys@sequent.com

Disclaimer: these are only my opinions, and probably completely
	wrong at that.