Message-ID: <003401c09558$17818a00$128baad0@0021682320>
From: "Jeremiah" <xdebugx AT emeraldis DOT com>
To: <djgpp AT delorie DOT com>
References: <Pine DOT SUN DOT 3 DOT 91 DOT 1010212094219 DOT 12969L AT is> <96917s$guccd$1 AT ID-57378 DOT news DOT dfncis DOT de> <7263-Mon12Feb2001211331+0200-eliz AT is DOT elta DOT co DOT il>
Subject: Re: inline asm
Date: Mon, 12 Feb 2001 19:58:32 -0500
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Reply-To: djgpp AT delorie DOT com

> > From: "Alexei A. Frounze" <dummy_addressee AT hotmail DOT com>
> > Newsgroups: comp.os.msdos.djgpp
> > Date: Mon, 12 Feb 2001 11:00:07 -0500
> > >
> > > On Sun, 11 Feb 2001, Jeremiah wrote:
> > >
> > > > How do i call interrupts with inline assembly in djgpp compiler.
like:
> > > >  int 25h
> > >
> > > You don't need inline assembly for that, use __dpmi_int instead.  The
> > > DJGPP FAQ shows how to do that in section 18.2 and other sections of
> > > chapter 18.
> > >
> > > Issuing an INT instruction in assembly is not a good idea with DJGPP,
> > > since your program runs in protected mode, so the INT instruction is
> > > handled by protected-mode handler first.  Who knows what that will do?
> >
> > TechHelp says this (on int 31h fn 300h = simulate real-mode interrupt):
> > -------8<--------
> > The DPMI host automatically handles all INT nn opcodes to provide
> > BIOS and DOS services transparently.  I'm not sure why you would
> > use this fn.
> > -------8<--------
> > So what's the problem?
>
> If this is what TechHelp says, perhaps you should toss it and read the
> DJGPP FAQ (sections 18.1 and 18.2) instead ;-)  ``References'' like
> this really make a disservice to a programmer by proliferating
> disinformation.
>
> First of all, Int 25h requires a pointer to a buffer in DS:BX, and the
> buffer needs to be in the low memory (below 1MB mark), since real-mode
> addressing cannot reach higher addresses.  In contrast, a protected-mode
> program has all its buffers above 1MB mark--problem no.1.
>
> In addition, different DPMI environments have different exciting
> problems with specific interrupts when they are invoked as INT nn
> instruction in protected mode.  For example, Int 2Fh, if issued in
> protected mode on certain configurations of Windows 9X wedges the DOS
> box so hard that the only thing you can do is kill the VM--problem no.2.
>
> Going through function 300h of Int 31h (i.e. via __dpmi_int) does not
> have all these problems.
>
> In other words, Int 31h/AX=0300h was _designed_ to handle real-mode
> interrupts, and the designers intended that programs will use it to
> call real-mode services.  By contrast, INT nn issued in protected mode
> was _never_ designed to handle real-mode services, so they work or
> don't work by sheer coincidence, luck, or the lack thereof.
>
So how do i call intrupt 25h.  Is there a way to make djgpp run in real
mode.  Or do i use __dpmi_int instead and if so do i use inline asm to set
the bx, cx and dx registers that this intterupt uses, and what the syntax
for using __dpmi_int?