关于lpc1768单片机定时器如何设置初值问题

程序如下：
void delayMs(uint8_t timer_num, uint32_t delayInMs)
{
  if ( timer_num == 0 )
  {
        LPC_TIM0->TCR = 0x02;                /* reset timer */
        LPC_TIM0->PR  = 0x00;                /* set prescaler to zero */
        LPC_TIM0->MR0 = delayInMs * (9000000 / 1000-1);
        LPC_TIM0->IR  = 0xff;                /* reset all interrrupts */
        LPC_TIM0->MCR = 0x04;                /* stop timer on match */
        LPC_TIM0->TCR = 0x01;                /* start timer */

        /* wait until delay time has elapsed */
        while (LPC_TIM0->TCR & 0x01);
  }
  else if ( timer_num == 1 )
  {
        LPC_TIM1->TCR = 0x02;                /* reset timer */
        LPC_TIM1->PR  = 0x00;                /* set prescaler to zero */
        LPC_TIM1->MR0 = delayInMs * (9000000 / 1000-1);
        LPC_TIM1->IR  = 0xff;                /* reset all interrrupts */
        LPC_TIM1->MCR = 0x04;                /* stop timer on match */
        LPC_TIM1->TCR = 0x01;                /* start timer */

        /* wait until delay time has elapsed */
        while (LPC_TIM1->TCR & 0x01);
  }
  return;
}


程序中的9000000是怎么算出来的，求大神解答！！！
谢谢！！