DOS ain't dead

Hello everyone,

I've a strange problem with my ISR runtime under djgpp/G++.

My code for create interrupt handler
---------------------------- main.cpp -----------------------------------
extern "C" void end_Routine_ISR();
extern "C" void Routine_ISR();
_go32_dpmi_seginfo old_handle, new_handle;

/****** My "ISR handler" function in asm inline *******/
asm (".global _Routine_ISR n _Routine_ISR:n"
// "call _my_context_switching_function"
"iret"
);

asm (".global _end_Routine_ISR n _end_Routine_ISR:n"
"ret"
);
/*******************************************/

void Create_ISR() {
int Vecteur_initialisation = 0x08;

if(_go32_dpmi_get_protected_mode_interrupt_vector(Vecteur_initialisation, &old_handle)!=0)
printf(" ERROR P1n");

nouveau_handle.pm_offset = (long unsigned int) (void*) Routine_ISR;
nouveau_handle.pm_selector = _go32_my_cs();

if(_go32_dpmi_chain_protected_mode_interrupt_vector(Vecteur_initialisation, &new_handle)!=0)
printf(" ERROR P2n");

_go32_dpmi_lock_code((void*) Routine_ISR, (unsigned long) ((unsigned long) &end_Routine_ISR - (unsigned long) &Routine_ISR));
_go32_dpmi_lock_data((void*) Routine_ISR, (unsigned long) ((unsigned long) &end_Routine_ISR - (unsigned long) &Routine_ISR));
}
----------------------------------------------------------------------

I compile this,
gpp main.cpp -O0 -m32 -S

And this is my problem
---------------------------- main.s-----------------------------------
... generated code ...

.global _Routine_ISR
_Routine_ISR:
call _my_context_switching_function().
iret

.global _end_Routine_ISR
_end_Routine_ISR:
ret

... generated code ...
---------------------------------------------------------------------

During my _Routine_ISR interruption if i use this "iret" my program crash with SIGSEGV signal (with or without my "call function"). But if i use "ret" interrupt works.

I want use "iret" in my context switching function , but i can't

According to my Google friend, NT bit flag must set from EFLAG in my context structure, but it's an interrupt created by _go32_dpmi_chain_protected_mode_interrupt_vector() ... WTF

Someone can help me?

Best regards,

---
Sébastien FAVIER

ps: Excuse me for my English level, I'm a French student

You don't need to care about ret/iret in djgpp.
just define your ISR as simple C function:

void covox_playback_isr(void)
{}

and here is sample code how to install your new ISR:

//***************** install Covox playback ISR at timer interrupt ***********
void covox_playback_install(void)
{
  printf("Installing new timer ISR for Covox playback...n");
  // lock the functions and variables
  _go32_dpmi_lock_code(covox_playback_isr,(DWord)sizeof(covox_playback_isr));
  _go32_dpmi_lock_data((void *)&sound_buffer_cnt,(DWord)sizeof(sound_buffer_cnt));
  _go32_dpmi_lock_data((void *)&sound_data_request,(DWord)sizeof(sound_data_request));
  // load the address of the old timer ISR into the OldISR structure
  _go32_dpmi_get_protected_mode_interrupt_vector(INT_TIMER,&timer_isr_old);
  // point NewISR to the proper selector:offset for handler function
  timer_isr_new.pm_offset=(DWord)covox_playback_isr;
  timer_isr_new.pm_selector=_go32_my_cs();
  // chain the new ISR onto the old one so that first the old INT_TIMER ISR will be called, then the new timer ISR
//  _go32_dpmi_set_protected_mode_interrupt_vector(INT_TIMER,&timer_isr_new);
  _go32_dpmi_chain_protected_mode_interrupt_vector(INT_TIMER,&timer_isr_new);
}

//***************** remove Covox playback ISR from timer interrupt chain ****
void covox_playback_uninstall(void)
{
  printf("nRemoving new timer ISR for Covox playback...n");
  // load the old timer ISR back without the new ISR chained on
  _go32_dpmi_set_protected_mode_interrupt_vector(INT_TIMER,&timer_isr_old);
  covox_playback_setup_fs(0);          // set default timer speed
}

As I told, under DPMI host you cannot manipulate IVT on your own will but ratther use DPMI function wrappers to do things correctly. Read DJGPP FAQ documnt for more...

---
DOS gives me freedom to unlimited HW access.