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From: "A. Wik (awik32 AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
Date: Mon, 26 Feb 2024 08:23:27 +0000
Message-ID: <CALPW7mRAZHoai1B6xp_qMsC77wOJOPp6mZ1ueNceF-w=eW80WQ@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 19:59, Eli Zaretskii (eliz AT gnu DOT org) [via
djgpp AT delorie DOT com] <djgpp AT delorie DOT com> wrote:
>
> > 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
> >
> > 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:
> > >
> > > > 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.
>
> I think there's a misunderstanding here.  The issue at hand is whether
> the DOS memory's linear address, as it appears within a certain VM,
> can ever have the value above 1MB.  If the linear address is _always_
> below 1MB, then for all practical purposes it is the same as the
> "physical address", since what the VM does under the hood is none of
> the business of the DJGPP programmer.

Linear V86 addresses are obviously below 1 MB, but (via the page
tables) they can be (and in Wndows often are) mapped to any physical
address, including above 1 MB.

-Albert.