From: Joshua Hale <jgh AT dcs DOT ed DOT ac DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Timer Interrupt
Date: Tue, 07 Apr 1998 20:02:54 +0100
Organization: University
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To: djgpp AT delorie DOT com
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Can anyone help me with the timer interrupt under DOS? I'm trying to
play a sample using the timer interupt on a Sound Blaster, but I have
a problem that I think is caused by my timer interrupt taking too long.
I have a 66Mhz 486, and whenever I reset the timer frequency to more
than about 10Khz it degrades system performance, at about 11KHz 
brings the whole thing to a stand still. I'm sure I should be able
to call the timer interrupt at 11KHz, even 44Khz? (neccesary for higher
quality samples)

Here's the code I was using to set it up:

#include <stdio.h>
#include <go32.h>
#include <dpmi.h>
#include <dos.h>
#include <conio.h>


#define TIMERISR	0x1C
#define LOW_BYTE(n) (n & 0x00ff)
#define HIGH_BYTE(n) ((n>>8)&0x00ff)


long counter;
_go32_dpmi_seginfo OldTimer,NewTimer;

Timer_ISR(_go32_dpmi_registers*) {
  counter += 1;
}

void SetupTimerISR()
{
  // Record old timer interrupt
  _go32_dpmi_get_protected_mode_interrupt_vector(TIMERISR,&OldTimer);
  // Store new timer interrupt
  NewTimer.pm_offset = (int)Timer_ISR;
  NewTimer.pm_selector=_go32_my_cs();
  // Set new timer interrupt
  disable();
  _go32_dpmi_allocate_iret_wrapper(&NewTimer);
  _go32_dpmi_set_protected_mode_interrupt_vector(TIMERISR,&NewTimer);
  enable();
}

void SetTimerFrequency(float hz)
{ //NOTE: Lowest frequency is 18.2 hz
  float base=1193180.0;
  unsigned byte;

  base/=hz;
  byte=(unsigned) base;
  printf("Value of byte: %d, low: %d, high: %d\n",byte, LOW_BYTE(byte),
HIGH_BYTE(byte));
  disable();
  outp(0x43,0x3c);
  outp(0x40, LOW_BYTE(byte));
  outp(0x40,HIGH_BYTE(byte));
  enable();
}



void ReturnISR()
{
  // Reset interrupt to original value
  disable();
  _go32_dpmi_set_protected_mode_interrupt_vector(TIMERISR,&OldTimer);
  _go32_dpmi_free_iret_wrapper(&NewTimer);
  // Reset timer frequency to 18.2Hz
  outp(0x43,0x3c);
  outp(0x40,LOW_BYTE(0xFFFF));
  outp(0x40,HIGH_BYTE(0xFFFF));
  enable();
}


void main() {
  long f = 100;

  printf("Enter frequency (>18.2): ");
  scanf("%ld",&f);
  printf("Starting\n");
  SetupTimerISR();
  printf("Set up timer\n");
  SetTimerFrequency(f);
  printf("Reset timer frequency\n");
  counter = 0;
  getch();
  ReturnISR();
  printf("%d clicks counted\n",counter);
  getch();
}


Any help with this would be greatly appreciated. 

Thanks,

Josh.