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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 11:56:28 +0300
Message-ID: <CAA2C=vCiD_Uq25grxhsz+A_T_eTdFpO3x6jxQkme+tqqUEyTFA@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

Hello Eli,

On Sat, Feb 24, 2024 at 10:03 AM Eli Zaretskii (eliz AT gnu DOT org) [via
djgpp AT delorie DOT com] <djgpp AT delorie DOT com> wrote:
> It is better to avoid inline assembly at all.  I'm guessing Watcom did
> that for speed or something, but does speed really matter here?

Most certainly no, speed is not a concern here. It's my ignorance in
attempting to translate watcom code

> >   r_addr = ((uint32_t)addr.segment<<4) + (uint32_t)addr.offset16;
> >   if (!r_addr) return -1;
>
> Instead of the above bit juggling to get r_addr, you could simply test
> both segment and offset to be zero.  After all, how else you'd get
> zero in the linear address?

Indeed

> >     appstring = (char *)real2ptr(r_addr);
[...]
> You cannot access conventional (below 1MB) memory like that.  You need
> to use one of the techniques described in the node "Xfer" in the
> DJGPPFAQ Info manual, to fetch the 5 bytes to protected-mode memory
> first.  In your case, _dosmemget seems to be the best method.

Thanks.  So: is the following correct? Does it match the intention at
https://github.com/wbcbz7/sndlib-watcom/blob/master/sndlib.cpp#L41-L71

int sndlib_sbemu_detect(void)
{
  __dpmi_raddr addr;
  __dpmi_regs  regs;
  uint32_t r_addr;
  char appstring[16];
  int mx;

  /* check for INT2D vector == NULL */
  __dpmi_get_real_mode_interrupt_vector(0x2D, &addr);
  if (!addr.segment || !addr.offset16) return -1;

  /* scan all multiplexes of INT 2D */
  for (mx = 0; mx < 256; mx++)
  {
    memset(&regs, 0, sizeof(regs));
    regs.h.ah = mx;
    __dpmi_int(0x2D, &regs);
    if (regs.h.al != 0xFF)
      continue;

    /* check for SBEMU application string */
    r_addr = ((uint32_t)regs.x.dx<<4) + (uint32_t)regs.x.di;
    if (!r_addr) continue;
    dosmemget(r_addr, 16, appstring);
    if (memcmp(appstring + 8,"SBEMU",5) == 0)
      return mx;
  }

  return -1;
}