Mail Archives: djgpp/2000/06/11/06:53:11
> > Ok, that was not what I really meant. It was supposed to say that V86
> > allows both DOS and protected-mode programs to run *in* protected
> > mode.
>
> But that's also not very accurate: V86 is not PM, although it's
> close.
No, V86 *is* PM, and programs in V86 run at PL3. And that's why you
still have all the protection there inspite of running (simulating)
the dangerous real-mode programs.
> For newbies' sake, I'd suggest to make this distinction very clear (if
> you at all mention V86, which I'm not sure is a good idea).
Ok, so what do you suggest this should be?
But I think I'll need to mention about V86 since that's a special
feature of using 386's PM. But I can make it as short as possible.
> > > 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.
> >
> > Not necessarily. There's no need for any program to make a switch
> > to real mode even when DOS/BIOS calls are made. That's what makes the
> > V86 so special. The 386 generates either a GPF or Exception 6 for
> > instructions IRET, LOCK, POPF, PUSHF, PUSHFD, POPFD, CLI and STI (and
> > of course the privileged instructions). So you can actually emulate
> > all these instructions. A simple V86 stack-frame setup should do the
> > job. That's what I did when I wrote my DOS Extender long ago.
>
> But that's just it: we are not talking about writing an extender. We are
> talking about a tutorial that explains what is protected mode to DJGPP
> users. In that context, catching IRET and its ilk and emulating them is
> not helpful: it doesn't explain anything, only complicates a topic that
> is complicated enough.
No, I didn't mean to include this in the first page of the
tutorial. This would indeed scare away newbies ;-).
But FYI, myself and Alexi did decide to include *everything* about
protected-mode including writing extenders. But then, it definitely
won't come anywhere in the introduction. And moreover, that's still a
long way to go.
> > And the statement does not necessarily mean that coexistence is
> > impossible without V86. It just means that coexistence is possible
> > with the V86 ;-).
>
> If it's possible without it as well, it is misleading to say that V86
> makes it possible.
>
> > Ok, I'll change that line to "The MMU on the 386 provides features
> > that allow Virtual Memory to be implemented". Does it make the
> > tutorial more clear?
>
> Yes, thanks. I suggest to mention the specific features that make it
> possible (the Page Fault exception, among others).
Yes, I'll do that. Thank you for the corrections, Eli.
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