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Date: Sat, 24 Feb 2024 12:05:38 +0100
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Subject: Re: checking int2d amis appstring with djgpp
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On 2024-02-24 01:08, Ozkan Sezer (sezeroz AT gmail DOT com) [via djgpp AT delorie DOT com] wrote:
> On Sat, Feb 24, 2024 at 12:49 AM 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.