From:	jhheo AT dangun DOT kaist DOT ac DOT kr (Junhyeok Heo)
Subject:	Help: ISR in DJGPP
To:	djgpp AT sun DOT soe DOT clarkson DOT edu
Date:	Wed, 3 Aug 1994 18:33:21 +0900 (KST)

Please help me !!

Situation:
      I used DJGPP 1.11.maint5
      I intercept the timer interrupt and set my new timer handler 
Problem:
      I wanna call the old timer interrupt handler 
        in my new timer handler
      But i don't know how to do this, 
        because of the difference of selector 
        between the old timer interrupt handler and my new handler
      I can't use _go32_dpmi_chain_protected_mode_interrupt_vector()
        because i wanna explicitly call the old timer handler 
        when some condition is satisfied.
      I used _go32_dpmi_simulate_fcall_iret() 
        but after my new handler was called successfully a few times
        and then the violation of segmentation  has occured.

Sample Program:

--------------cut here--------------------------------------------------
#include <stdio.h>
#include <pc.h>
#include <dos.h>
#include <sys/types.h>
#include <dpmi.h>

volatile int new_tc;
_go32_dpmi_seginfo old_handler, new_handler,rm_old;
_go32_dpmi_registers rm_regs;
union REGS r;

void tic_handler()
{
  new_tc ++;
  _go32_dpmi_simulate_fcall_iret(&rm_regs); 
}

int main()
{
  int old_tc = 0, iter=0;

  printf("grabbing timer interrupt\n");

  _go32_dpmi_get_protected_mode_interrupt_vector(8, &old_handler);

  _go32_dpmi_get_protected_mode_interrupt_vector(8, &rm_old);
  rm_regs.x.cs=rm_old.rm_segment;
  rm_regs.x.ip=rm_old.rm_offset;

  new_handler.pm_offset = (int)tic_handler;
  new_handler.pm_selector = _go32_my_cs();
  _go32_dpmi_set_protected_mode_interrupt_vector(8,&new_handler);

  printf("Trapping timer tics, press any key to stop\n");
  while (!kbhit())
  {
    if (old_tc != new_tc)
    {
      printf("iter = %d,  tics = %d\n", iter, new_tc);
      old_tc = new_tc;
    }
    iter++;
  }
  getkey();

  printf("releasing timer interrupt\n");
  _go32_dpmi_set_protected_mode_interrupt_vector(8, &old_handler);

  return 0;
}

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