🚀 go-pugleaf

I am having trouble getting a timer interrupt routine to work.  Can anybody
tell me why the following piece of code doesn't work (it's written in
Mark Williams C):

#include <bios.h>
#include <xbios.h>

int tick=0;		/* clock counter */

clock_vector()
{
  tick++;
}

main()
{
  int control, data;
  
  /*  have the user enter values for control and data */
  printf("Enter timer A control register value: "); scanf("%d",&control);
  printf("Enter timer A data register value: "); scanf("%d",&data);
  /* Next install the interrupt handler for timer A */
  Xbtimer(0, control, data, clock_vector);
  /* Enable Timer A interrupts */
  Jenabint(MFP_TIMA);
  /* loop until a key is pressed printing the timer tick value */
  while(!Cconis())
    printf("tick = %d\n",tick);
  Cconin();
  Jdisint(MFP_TIMA);
}

I have read the section of the Abacus Atari Internals book about what
values should go into the timer control register, but when I set control
to a value which I think should correctly start the timer, my program
bombs with from 2 to 4 bombs, and the system always hangs.  
Can anyone tell me what magic values should be passed to Xbtimer, or if
that is not the problem, what am I doing wrong?
			-- Scott Morrison
			morrison@berkeley.edu