From: Raul Kompass <kompass AT server1 DOT rz DOT uni-leipzig DOT de>
Newsgroups: comp.os.msdos.djgpp
Subject: ultra precise timing < 80 usecs
Date: Wed, 18 Sep 1996 12:17:34 -0700
Organization: University Leipzig
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To: djgpp AT delorie DOT com
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Hello!

For my experiments I needed a very precise timing and found code (for
Borland-C), that combines the information from timer-interrupts in the
BIOS (1/16 sec's precision IMO) with the value of the counters in the
CTC that provide the timer-interrupt. Thus you have a way to get
ultra-precise time (about 1 usec precision). You cannot control
something at that precision because the interrupts getting fired
regularly occupy chunks of 50 to 90 usecs on a 386 or better PC. But you
can have definitely that precision and you can measure the precision
during the experiments. I would like to have that functionality in
djgpp. There is a solution to speed up the timer-interrupt but I think
it's too complicated and has a lot of overhead. I would like to have the
timer-interrupt as fast as possible (unchanged), and a better
implementation of the 
*****    gettimeofday()     *****
function that does not use the CTC at present.
The code I append below does this in assembler. I can transform it to
gas but I don't know the interrupt-handling in djgpp's protected mode.
You have to disable the timer-interrupt to prevent that you read the
bios-value and before you get the CTC-value the bios and CTC are
updated.
Can anybody help me please with this problem?

*********************************************************************
initializetimer() and restoretimer() aren't necessary I believe because
the CTC is already programmed in the right way. But I'm not sure.

void initializetimer(void)
{

  outp(0x043,0x034);
  _asm\
  {
	jmp short NullJump1

NullJump1:;
  }
  outp(0x040,0x000);
  _asm\
  {
	jmp short NullJump2

NullJump2:;
  }
  outp(0x040,0x000);

}

void restoretimer(void)
{
  outp(0x043,0x036);
  _asm\
  {
	jmp short NullJump1

NullJump1:;
  }
  outp(0x040,0x000);
  _asm\
  {
	jmp short NullJump2

NullJump2:;
  }
  outp(0x040,0x000);

}

#define TimerResolution    1193UL
#define Seconds        (1000*TimerResolution) /* not absolutely exact */

#pragma warn -rvl       /* shut up Turbo C warning about return value */
long readtimer(void)
{
  _asm\
  {
  cli                           /* Disable interrupts */
  mov  dx,020h          /* Address PIC ocw3   */
  mov  al,00Ah          /* Ask to read irr    */
  out  dx,al
  mov  al,00h           /* Latch timer 0 */
  out  043h,al
  in   al,dx            /* Read irr      */
  mov  di,ax            /* Save it in DI */
  in   al,040h          /* Counter --> bx*/
  mov  bl,al            /* LSB in BL     */
  in   al,040h
  mov  bh,al            /* MSB in BH     */
  not  bx                       /* Need ascending counter */
  in   al,021h          /* Read PIC imr  */
  mov  si,ax            /* Save it in SI */
  mov  al,00FFh         /* Mask all interrupts */
  out  021h,al
  mov  ax,040h          /* read low word of time */
  mov  es,ax            /* from BIOS data area   */
  mov  dx,es:[06Ch]
  mov  ax,si            /* Restore imr from SI   */
  out  021h,al
  sti                           /* Enable interrupts */
  mov  ax,di            /* Retrieve old irr  */
  test al,001h          /* Counter hit 0?    */
  jz   done                     /* Jump if not       */
  cmp  bx,0FFh          /* Counter > 0x0FF?    */
  ja   done                     /* Done if so        */
  inc  dx                       /* Else count int req. */
done:;
  mov ax,bx                     /* set function result */
  }
}
#pragma warn .rvl       /* reset Turbo C return value warning */

**********************************************************
thanks

Raul
kompass AT server1 DOT rz DOT uni-leipzig DOT de