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Date: Sat, 10 Jun 2000 09:45:00 +0200
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From: "Eli Zaretskii" <eliz AT is DOT elta DOT co DOT il>
To: Prashant TR <prashant_tr AT yahoo DOT com>
CC: djgpp AT delorie DOT com
In-reply-to: <m3og5amiom.fsf@vsnl.net.in> (message from Prashant TR on 10 Jun
	2000 06:54:57 +0530)
Subject: Re: far pointers
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> From: Prashant TR <prashant_tr AT yahoo DOT com>
> Newsgroups: comp.os.msdos.djgpp
> Date: 10 Jun 2000 06:54:57 +0530
>
> >   That means programs like DOS can coexist with protected-mode programs. 
> > 
> > I don't think V86 has anything to do with RM and PM programs coexisting. 
> > perhaps I'm missing something in your reasoning.
> 
> That statement is right. Because without V86 mode, you wouldn't be
> able to run any of your DOS programs without swicthing back to real
> mode (which is terribly slow). And when a program run in V86 mode, it
> *is* running in PMode at PL3. So this statement is right.

Sorry, I still don't follow your reasoning; the above paragraph
contains several contradictions.

First, you *can* run protected-mode programs on top of plain real-mode
DOS: that's what CWSDPMI does, for example, when it runs without EMM
or QEMM installed; the speed of the mode switch is not really relevant
here.  This fact alone seems to go against your text, which suggest
that V86 allows DOS and PM programs to coexist (meaning that without
V86, such coexistence is impossible).

Second, even in V86 mode, a protected-mode program that calls
real-mode DOS services needs to make an explicit PM-to-RM switch, or
it will crash.  So there's nothing in V86 per se that allows or makes
it simpler for PM programs to coexist with DOS.  Faster, yes, but not
simpler.

In fact, I think that V86 should not be mentioned at all in the kind
of tutorial that you are writing.  It is not really important in the
context of DJGPP, but it does complicate things and confuses newbies.
I don't expect many DJGPP users to write a V86 monitor ;-)

> >   The memory management unit (MMU) on the 386 supports virtual memory
> > 
> > This is inaccurate: VM is not implemented by MMU, but by software that hooks
> > the Page Fault exception and pages memory in and out as needed.
> 
> The word "support" is the key here.

An MMU can hardly ``support'' something that it doesn't know about.
IMHO, the paragraph I commented about does not help to understand what
the MMU is.  It might confuse people into thinking that MMU has some
important part in the implementation of VM, which is not true.