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From: mir@opera.chorus.fr (Adam Mirowski)
Newsgroups: comp.os.msdos.misc
Subject: Re: Can one prevent emm386 from using VM86 mode ?
Message-ID: <11236@chorus.fr>
Date: 28 Jun 91 18:19:59 GMT
References: <1991Jun28.124744.27009@jet.uk>
Sender: mir@chorus.fr
Reply-To: mir@opera.chorus.fr (Adam Mirowski)
Organization: Chorus systemes, Saint Quentin en Yvelines, France
Lines: 32

In article <1991Jun28.124744.27009@jet.uk>, cm@jet.uk (colin manning) writes:
%% When you run emm386, it always seems to run DOS in virtual 8086 mode.
%% 
%% If you are only using emm386 in order to make upper memory blocks available, 
%% which do not need paging or any other facilities of protected mode in order to 
%% be accessible, is it possible to prevent it moving the 386 to virtual 8086 
%% mode ? I.e have emm386 make UMB's available, but leave the 386 in real mode.
%% 
%% There are two reasons for wanting this:
%% 
%% 1. Virtual 8086 mode is slower for certain operations.
%% 2. You cant run software that likes to switch in and out of protected
%%    mode itself.

No, it is impossible.

First, Intel 386 manuals tell so. :-)

Second, even if you would like to use the method already signaled in
a recent Dr. Dobbs article to access extended memory as an array in
real mode by not flushing a cache (the segment cache) when retuning
from protected mode, it is not possible because the cache for pages
(TLB) has only 32 entries, which is not enough to map the entire real
mode memory. Very quickly a cache miss would occur and would have to
access the page directory and the page tables, which precisely aren't
in its address space anymore...
Anyway, I believe this cache is automatically flushed and disabled when
386 returns to real mode, so I stop these divagations just here. :-)

-- 
Adam Mirowski,  mir@chorus.fr (FRANCE),  tel. +33 (1) 30-64-82-00 or 74
Chorus systemes, 6, av.Gustave Eiffel, 78182 Saint-Quentin-en-Yvelines CEDEX