Keep calling on a function while mouseover

Keep calling on a function while mouseover

how do I keep calling a function on mouseover while the mouse is hovered
over an html element for example:
<script>
function a(){
"special code hear"
}
</script>
<div onmouseover('a()')>&nbsp;</div>
How can I keep calling the function a while the mouse is hovered over the
div instead of having it call the function once.