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From: np4@prism.gatech.EDU (POMPONIO,NICHOLAS A)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: Fast Timer Interrupts, anyone?
Message-ID: <15116@hydra.gatech.EDU>
Date: 12 Oct 90 19:03:35 GMT
References: <39119@ucbvax.BERKELEY.EDU>
Organization: Georgia Institute of Technology
Lines: 81

In article <39119@ucbvax.BERKELEY.EDU> scole@janus.Berkeley.EDU (Steven Cole) writes:
>I'm wondering if anyone out there can help me...
>
>I'm looking for a code fragment that sets up timer interrupts
>approximately every 125 usec on a vanilla PC.  From what I understand,
>this is possible by resetting the timer chip that causes INT 08, but
>unfortunately I don't know just how to do this.  (I.e., is this
>register memory mapped or does it take some obscure I/O operation?)
>
>Thanks for any pointers,
>

I used the following snippit of code to speed the clock interrupts. (I hope 
that you are using a very fast PC, because an XT that I used previously
could take up to 125 usec just to respond to the interrupt!) The code is
in Microsoft C 5.1. I gathered bits and pieces from some working code, and
hopefully the details you need are here. If not, let me know by e-mail.

--------------------------------------------------------------------------

static void (interrupt * old_intr8)();
static void interrupt system_timer ();

/*  Timer (8253) values */

#define  TIMER_CONTROL_PORT  0x43
#define  TIMER_0_PORT        0x40
#define  TIMER_LOAD0_CMD     0x36
#define  DIVIDE_BY           2  /* Speed up clock int's by this factor */
#define  TIMER_0_COUNT       0x10000 / DIVIDE_BY
#define  TIMER_0_DEFAULT     0u

static void  install_interrupts ()
{
/*
     This routine installs the interrupt handler routine below and reprograms
     the timer channel 0
*/ 
   auto union {
      unsigned int   word;
      unsigned char  bytes [2];
   } split;

   old_intr8 = _dos_getvect (0x08);
   split.word = TIMER_0_COUNT;

   _disable ();

   outp (TIMER_CONTROL_PORT, TIMER_LOAD0_CMD);
   outp (TIMER_0_PORT, split.bytes [0]);
   outp (TIMER_0_PORT, split.bytes [1]);
   _dos_setvect (0x08, system_timer);

   _enable ();
}

#pragma check_stack(off)
void interrupt system_timer ()
{
/*
   This routine is the interrupt handler. The check_stack pragmas are needed
   only if you plan to call another function from this handler.
*/
   static int  divide_by = DIVIDE_BY;

   if (--divide_by == 0)   {
      divide_by = DIVIDE_BY;
      (* old_intr8)();
   }
   else   {
      outp (0x20, 0x20);   /* non-specific end-of-interrupt reset */
      _enable ();
   }
/*  Do something here */
}
#pragma check_stack(on)
-- 
POMPONIO,NICHOLAS A
Georgia Institute of Technology, Atlanta Georgia, 30332
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