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From: "A. Wik (awik32 AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
Date: Sun, 25 Feb 2024 19:28:07 +0000
Message-ID: <CALPW7mSqDTJcOs0JywP8Q8OPQHsj-qVR8dQsAyCkZUo2OPo_kw@mail.gmail.com>
Subject: Re: DJGPP documentation updates
To: djgpp AT delorie DOT com
Content-Type: text/plain; charset="UTF-8"
Reply-To: djgpp AT delorie DOT com

On Sun, 25 Feb 2024 at 17:11, Eli Zaretskii (eliz AT gnu DOT org) [via
djgpp AT delorie DOT com] <djgpp AT delorie DOT com> wrote:
>
> > >
> > > What exactly is the difference between a physical address below 1MB
> > > and the linear address there?
> >
> > I'm not sure now if I understood it correctly. But my understanding of
> > the spec is that DPMI host which supports paging may page-out also
> > linear addresses below 1 MB.
>
> Really?  IME, no DPMI host does that.  DOS memory is always mapped
> 1:1, AFAIR.

Windows maps VMs one at a time into the low 1 MB, although the memory
that was in use when Windows started is replicated (at the same linear
address) in all the VMs.  A VM is approximately a "DOS box", but there
is always a so-called "system" VM in which (usually) the GUI is
running.

There is also expanded memory (LIM EMS) that involves mapping the
"expanded" memory into or out of a program's low 1 MB address space.

-Albert.