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Mail Archives: djgpp/2000/06/11/15:00:44.1

From: "Alexei A. Frounze" <see_below AT the_messasge_body DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: far pointers
Date: Sun, 11 Jun 2000 15:40:54 +0400
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Eli Zaretskii wrote:
> 
> On Sat, 10 Jun 2000, Prashant TR wrote:
> 
> > > V86 was invented because it allowed to use more than 640KB of memory,
> > > by remapping some of the extended memory into the unused addresses
> > > between 640K and 1MB.
> >
> > I don't understand what V86 has to do with remapping of memory.
> 
> It turns on the MMU, without which this remapping is impossible.
> 
> > It's related to the MMU and V86 has nothing to do with the MMU.
> 
> Then please explain why remapping extended memory into the region between
> 640K and 1MB is impossible in plain real mode.

Page translation is possible in PMode only. CR0.PG couldn't be set to 1 if
CR0.PE is 0 or not going to be set with the CR0.PG at the same time. RTFM :)

> Compare this with CWSDPMI that runs on top of HIMEM.SYS.  You do not have
> V86 there, but HIMEM is still used to switch into PM.

Btw, HIMEM.SYS uses BIOS Int 15h fn 87h for data copying. If this thing is
emulated by either Windows or EMM386, HIMEM doesn't really switches to
PMode.

> 
> In other words, having V86 doesn't make the PM and DOS coexistence any
> simpler, at least not by a large margin.

bye.
Alexei A. Frounze
-----------------------------------------
E-mail:   alexfru [AT] chat [DOT] ru
Homepage: http://alexfru.chat.ru
Mirror:   http://members.xoom.com/alexfru

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