Date: Tue, 7 Mar 1995 09:08:41 -0300
From: acmq AT beta DOT coe DOT ufrj DOT br (Antonio Carlos Moreirao de Queiroz)
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Re: Interrupt Handling Issues. ???

About handling real-time interrupts:

According to the example somewhere in the into files, the
code to redirect the clock tick interrupt is:

_go32_dpmi_registers regs;
_go32_dpmi_seginfo old_handler,new_handler;

void Interruptor(_go32_dpmi_registers *r)
{
/* The routine that treats the interrupt */
...
}
...
/* To install the handler */
  _go32_dpmi_get_real_mode_interrupt_vector(0x1C,&old_handler);
  new_handler.pm_offset=Interruptor;
  _go32_dpmi_allocate_real_mode_callback_iret(&new_handler,&regs);
  _go32_dpmi_set_real_mode_interrupt_vector(0x1C,&new_handler);
...
 /* To uninstall */
  _go32_dpmi_set_real_mode_interrupt_vector(0x1C,&old_handler);
  _go32_dpmi_free_real_mode_callback(&new_handler);

This appears to work correctly, but I have some questions:
1- How to call the old interrupt handler at the end of the
   new handler?
2- Is it necessary to guarantee that the new handler is
   uninstalled before the program exits, normally or not?
3- Is this really safe?

Antonio Carlos M. de Queiroz