Date: Sun, 11 Jun 2000 16:08:22 +0530 (IST)
Message-Id: <200006111038.QAA10511@bgl2.vsnl.net.in>
From: Prashant TR <prashant_tr AT yahoo DOT com>
To: eliz AT is DOT elta DOT co DOT il
CC: pmode AT egroups DOT com, djgpp AT delorie DOT com
In-reply-to: <Pine.SUN.3.91.1000611090952.22597L-100000@is> (message from Eli
	Zaretskii on Sun, 11 Jun 2000 09:15:37 +0300 (IDT))
Subject: Re: far pointers
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> > > 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.

No, I think you're mistaken. Switching to V86 does *not* turn on the
MMU. The only thing you need for running a V86 task is 32-bit ISRs for
H/W interrupts. If you need to remap, you have to setup the page
tables and turn on the MMU yourself.

> > 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.

Because the MMU isn't active. It doesn't work in real mode. OTOH, what
you said would be one of the options that the OS developer could
add (and is generally implemented). But this doesn't happen
automatically. The OS ultimately will have to find out where and how
much of the 640K and 1M it can give to real mode programs.

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

It doesn't make it simpler. Not a doubt about that. But it does add
protection to your system.