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From:	zandro_arceo AT support DOT trendmicro DOT com (IeperNaum)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Hooking INT 2F
Date:	5 Mar 2002 21:01:55 -0800
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Eli Zaretskii <eliz AT is DOT elta DOT co DOT il> wrote in message news:<Pine DOT SUN DOT 3 DOT 91 DOT 1020305122716 DOT 19973C-100000 AT is>...
> On 5 Mar 2002, IeperNaum wrote:
> 
> > i tried to build the sample tsr, and it worked. then i changed the
> > interrupt from int 8 to 2f, modified the int2f handler to make the
> > program beep.
> > 
> > but it did not work.
> 
> Int 2Fh is a software interrupt, not a hardware interrupt.  Software 
> interrupts are not reflected to protected-mode handlers (says the DPMI 
> spec), so you need either to hook them in real mode or to use real-mode 
> callbacks, as explained in section 18.9 of the DJGPP FAQ list.

thanks for the info.

now i was able to make it call the handler. 
another question. i am currently working on a network redirector
project. so i need to hook int 2f. but now i have a problem. i read on
this group that DJGPP has no provision for interrupt chaining in real
mode. how can i work things out.

i tried to use _go32_dpmi_simulate_fcall_iret() but it did not work.

i will greatly appreciate any help.

thanks.

<code excerpt>

_go32_dpmi_seginfo old_vector;

void int2f(_go32_dpmi_registers* r)
{
  printf("INT 2F Invoked r->h.ah = %X\r\n", r->h.ah);

  if (r->h.ah != 0x11)
  {
    printf("Do Control\r\n", r->h.ah);
    //other processing here    return;
  }

  r->x.ss = NULL;
  r->x.sp = NULL;
  r->x.flags = NULL;
  
  r->x.cs = old_vector.rm_segment;
  r->x.ip = old_vector.rm_offset;
  _go32_dpmi_simulate_fcall_iret(r);
}

 int main(int argc, char* argv[])
 {
   __dpmi_regs r;
   _go32_dpmi_seginfo info;
   
   _go32_dpmi_get_real_mode_interrupt_vector(0x2f, &old_vector);
   info.pm_offset = (unsigned long)int2f;
   _go32_dpmi_allocate_real_mode_callback_iret(&info, &g_r);
   _go32_dpmi_set_real_mode_interrupt_vector(0x2f, &info);

   r.x.ax = 0x252F;
   r.x.ds = info.rm_segment;
   r.x.dx = info.rm_offset;
   __dpmi_int(0x21, &r);


   _write(2,"Installing TSR\r\n",16);
   //__djgpp_exception_toggle(); 	// Only needed if exceptions linked

   r.x.ax = 0x3100;
   r.x.dx = 256 / 16;		// paragraphs
   __dpmi_int(0x21, &r);

   printf("Hey\n");

   _go32_dpmi_set_real_mode_interrupt_vector(0x2f, &old_vector);
   _go32_dpmi_free_real_mode_callback(&info);


 }

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