How can I move a div on scroll?

I have to move a div when the the user scrolls, but need to use pure JavaScript.

position: fixed; will not work with the layout. The div's original position is relative to something else. Is there a simple implementation using an event like onscroll, to detect how many pixels the page moved up or down, and change the position of the div accordingly?

The div only needs to move vertically. So if I can detect how many pixels the page has moved I can just add or subtract that to the location of the div.

window.onscroll = function (e) {
  var vertical_position = 0;
  if (pageYOffset)//usual
    vertical_position = pageYOffset;
  else if (document.documentElement.clientHeight)//ie
    vertical_position = document.documentElement.scrollTop;
  else if (document.body)//ie quirks
    vertical_position = document.body.scrollTop;

  var your_div = document.getElementById('some_div');
  your_div.style.top = (vertical_position + 200) + 'px';//200 is arbitrary.. just to show you could now position it how you want
}