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From: ga@herts.rdg.dec.com (Giles Atkinson)
Newsgroups: comp.arch
Subject: Re: VAX and VMS development (was Re: Graphics Primitives)
Message-ID: <1990Nov22.113536.19655@hollie.rdg.dec.com>
Date: 22 Nov 90 11:35:36 GMT
References: <11030@pt.cs.cmu.edu> <42992@mips.mips.COM> <513@ctycal.UUCP> <1990Nov21.191759.23254@Think.COM> <23425@grebyn.com>
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Reply-To: ga@herts.rdg.dec.com (Giles Atkinson)
Organization: Digital Equipment Corporation
Lines: 29


In article <23425@grebyn.com>, ckp@grebyn.com (Checkpoint Technologies) writes:
|> 
|> The VAX MMU has a "modified" bit for page table entries, but no
|> "referenced" bit.  VMS uses a "soft page fault" (the page is invalidated
|> for the user, but remains in core; if a fault occurs, the page is
|> revalidated quickly) to discover reference frequency.  Interestingly
|> enough, the "modified" bit is not used by VMS; pages are always
|> written out, modified or not. (I, also, have taken the
|> VMS internals course!)

Actually pages are not 'always written out', but VMS does sometimes write
out pages which have not been modified.   This happens to pages which are
writeable and not shared and is done because the same memory location in the
executive (BAK array entry) is used to store the original disk location of the
page's data and its allocated backing store in the paging file.
Lazy allocation of backing store occurs when the page is first brought in.

BSD does the same, but also does it for read-only pages!

I think the lack of a reference bit was probably seen as advantageous given
the original VMS memory management concept of each process paging within a
fixed quota of physical memory.    A weak substitute for a reference bit is
simulated on most VAX processors by polling the TLB before replacing a page.

(I have taught the VMS internals course more often than I care to remember)

No signature.
Standard diclaimer here.