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From: "Ozkan Sezer (sezeroz AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
Date: Sat, 24 Feb 2024 14:20:02 +0300
Message-ID: <CAA2C=vCJ+vZcgVRq3DBJUyKQzsJhZajhTp4KapMGCqaRHHVDFg@mail.gmail.com>
Subject: Re: checking int2d amis appstring with djgpp
To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

On Sat, Feb 24, 2024 at 2:06 PM J.W. Jagersma (jwjagersma AT gmail DOT com)
[via djgpp AT delorie DOT com] <djgpp AT delorie DOT com> wrote:
>
> >>>     __asm__ __volatile__ ("movw %%dx,%0":"=m"(addr.segment));
> >>>     __asm__ __volatile__ ("movw %%di,%0":"=m"(addr.offset16));
> >>
> >> This doesn't look right - you can't assume gcc preserves registers
> >> between asm blocks.
> >
> > OK, should have been like this then, yes?
> >
> > __asm__ __volatile__ (
> >   "movb %2,   %%ah\n"
> >   "xorb %%al, %%al\n"
> >   "int  $0x2D\n"
> >   "cmpb $0xFF,%%al\n"     /* is this a free multiplex? */
> >   "jz  1f\n"
> >   "xorw %%dx, %%dx\n"     /* it is, return NULL pointer */
> >   "xorw %%di, %%di\n"
> >   "1:\n"
> >   "movw %%dx, %0\n"
> >   "movw %%di, %1\n"
> >   : "=m"(addr.segment),
> >     "=m"(addr.offset16)
> >   :  "m"(mx) );
>
> Almost.  You would also need to list the registers you modified,
> otherwise gcc will make the wrong assumptions about their contents.  So:
>
>   /* ... */
>   : "=m"(addr.segment),
>     "=m"(addr.offset16)
>   :  "m"(mx)
>   : "ax", "dx", "di");
>
> Although if I were to call a protected-mode interrupt, I would do the
> control flow in C, so the asm can be simplified to:
>
>   unsigned ax = mx << 8;
>   __dpmi_raddr addr;
>
>   asm
>   (
>     "int $0x2D"
>     : "+a" (ax),            /* ax */
>       "=d" (addr.segment),  /* dx */
>       "=D" (addr.offset16)  /* di */
>   );
>
>   if ((ax & 0xff) != 0xff)
>     continue;
>
>   /* ... */
>
> But this is not very relevant to your question anymore, of course :)
>
> Last version of the code you posted looks good to me, I would expect
> that to work.

Thanks!