int main(void)
{
  *rcc_ahb1enr |= (1 << 3); // enable PortD's clock

  // assume powerup conditions, moder is all 0's
  // set bit-8 as output
  uint32_t moder = portd->MODER;
  moder |= (1 << 16);
  moder &= ~(1 << 17);
  portd->MODER = moder;

  while (1) {
    portd->ODR |= (1 << 8); // led-on
    sleep(500);
    portd->ODR &= ~(1 << 8); // led-off
    sleep(500);
  }
}