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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.micro,net.arch
Subject: Re: 386 Family Products
Message-ID: <6129@utzoo.UUCP>
Date: Sat, 9-Nov-85 19:37:17 EST
Article-I.D.: utzoo.6129
Posted: Sat Nov  9 19:37:17 1985
Date-Received: Sat, 9-Nov-85 19:37:17 EST
References: <129@intelca.UUCP> <392@aum.UUCP> <225@l5.uucp>
Organization: U of Toronto Zoology
Lines: 17

> ... if I understand the Intel literature correctly, you
> can do paging on top of emulating the 8086 in "virtual 86 mode."  Then
> you can set the page(s) where the screen is to "not present," and then do
> a trap every time it is accessed.  Alternatively, you could set that page
> as "read-only," so that you would do a trap only on writes.  According to
> Intel, this is not as fast as one might like...

"not as fast as one might like" is the understatement of the century,
actually.  This is a serious performance problem in virtual-machine work
when the machine has memory-mapped i/o devices.  For the screen, you might
be able to live with a scheme in which the system scanned the page table
every 60th of a second to identify "screen" pages which had been modified,
and then did something appropriate with them.  Trapping every screen-update
write is a performance disaster.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry