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From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.arch
Subject: Re: Is handling off-alignment important? (was Re: RISC hard to program?)
Message-ID: <2357@crdos1.crd.ge.COM>
Date: 23 Jul 90 12:19:55 GMT
References: <104037@convex.convex.com> <8840014@hpfcso.HP.COM>
Reply-To: davidsen@crdos1.crd.ge.com (bill davidsen)
Organization: GE Corp R&D Center, Schenectady NY
Lines: 24

In article <8840014@hpfcso.HP.COM> dgr@hpfcso.HP.COM (Dave Roberts) writes:

| What if page 2 is paged out?  When does the CPU notice that it's paged
| out and what does it do about it?  Does it abort the transaction
| entirely, bring page 2 back in, and restart?  If so, what if, because
| of the page replacement algorithm, page 1 gets blown away by bringing
| in page 2?  

  You simply restart the instruction. Using an LRU scheme the 1st page
would not get paged out unless the physical mapping was 1 page/process.
As a reasonable constraint you want 8 pages/process anyway, so you avoid
this.

  Note 1: yes "simply" is a relative term in this case, relative to doing
half the instruction and then handling the fault.

  Note 2: pages for code, stack, source of copy instruction, dest of
copy instruction. If you allow unalligned access you need two pages in
each area to handle access over a boundary, that totals eight. No, I
wouldn't want to actually run a system with that little memory.

-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
            "Stupidity, like virtue, is its own reward" -me