The following is based on a newsgroup posting.  I (Paul Hsieh) have made 
modifications where I thought there was a need for clarification:
=============================================================================

From: alexad3@icebox.iceonline.com (Alexander J. Russell)
Newsgroups: rec.games.programmer
Subject: Re: I need some example sources (C/C++) of how to setup a TimerIRQ handler....
Date: Tue, 10 Oct 1995 03:40:21 GMT
Organization: ICE Online
Message-ID: <DG7rH9.BrC@news.iceonline.com>

>Simon van Tongeren (savant@euronet.nl) wrote:
>: Hello,
>
>: Is the someone who can supply me with some sources of how
>: I can setup a TimerIRQ handler (in borland C/C++ 3.1)????
>
>: Any help, suggestions, source files are very welcome (prefered
>: by e-mail).
>
>: Thanx,
>: Simon.
Here is an example:
/*

    timer.c

    Copyright 1994, August 23 by Alec Russell, ALL rights reserved
    Permission granted to use ths code as you wish.

    Created - 1994/8/23

    Used to speed up main timer to 60 Hz

    History:
        New file

*/

#include <stdio.h>
#include <dos.h>


#define ULONG unsigned long

#pragma inline

volatile unsigned long fast_tick, slow_tick;   /* "volatile" is essential */
static void interrupt (far *oldtimer)(void);   /* BIOS timer handler */

void deinit_timer(void);

/*
   You don't have to call the old timer, but if you don't you have to 
   write some code to cleanup in de-init that fixes DOS's internal 
   clock.  This can usually be done by reading back the CMOS time.

   Its considered 'good form' to call the old int.  If everyone does, 
   then everything that other TSR's etc...  may have installed will 
   also work.

   If you skip the little chunk of ASM code- the out 20- you WILL 
   LOCKUP all interupts, and your computer.

   Anyways, this test replacement just increments a couple of long 
   ints.
*/
/* ---------------------- new_timer() ------------------- August 23,1994 */
static void interrupt new_timer(void)
{
   asm cli
   fast_tick++;

   if ( !(fast_tick & 3) ) /* call old timer ever 4th new tick */
      {
      oldtimer();
      slow_tick++;
      }
   else
      {
      /* The following sends an End of Interrupt to the PIC */
      asm {
          mov al, 20h
          out 20h, al
          }
      }

   asm sti
}

/* see that 1st line of inline asm?

   to set whatever clock speed you want load
   bx with 1193180/x where x is the clock speed you want in Hz.

*/
/* ---------------------- init_timer() ------------------ August 23,1994 */
void init_timer(void)
{

   slow_tick=fast_tick=0l;
   oldtimer=getvect(8); // save old timer

   asm cli

   // speed up clock
   asm {
        mov     bx,  19886     //  set the clock speed to 60Hz (1193180/60)
        mov     al,  00110110b
        out     43h, al
        mov     al,  bl
        out     40h, al
        mov     al,  bh
        out     40h, al
      }

   setvect(8, new_timer);

   asm sti

}


/* ---------------------- deinit_timer() ---------------- August 23,1994 */
void deinit_timer(void)
{
   asm cli

   // slow down clock   1193180 / 65536 = 18.2, but we use zero

   asm {
      xor bx,  bx          // min rate 18.2 Hz when set to zero
      mov al,  00110110b
      out 43h, al
      mov al,  bl
      out 40h, al
      mov al,  bh
      out 40h, al
      }

   setvect(8, oldtimer);  // restore oldtimer

   asm sti

}


/* ---------------------- waitt() --------------------- February 11,1995 */
void waitt(ULONG t)
{
   t+=fast_tick;

   while ( t > fast_tick ) 
      {
      /* empty */
      }
}


#define TEST 1

#if TEST

#include <stdio.h>
#include <bios.h>


/* ---------------------- get_fast() -------------------- August 23,1994 */
ULONG get_fast(void)
{
   return(fast_tick);
}

/* ---------------------- get_slow() -------------------- August 23,1994 */
ULONG get_slow(void)
{
   return(slow_tick);
}

/* ---------------------- main() ------------------------ August 23,1994 */
void main(void)
{
   ULONG t1, t2;

   printf("init timer\n");
   init_timer();

   while ( !bioskey(1) )
      {
      t1=get_fast();
      t2=get_slow();
      printf("fast %lu slow %lu\n", t1, t2);
      }

   deinit_timer();
}


#endif

/* ------------------------------ EOF -------------------------------- */

The AnArChIsT - Anarchy! NOT Chaos! aka Alec Russell
alexad3@icebox.iceonline.com