Functions for window popup

[gio]

I have a number of windows that i want to open up in a seperate window, and i have tried the following code

<script language="javascript">
//<!--
function popup(a url)
{ window.open ("a url","Comments","width=500,height=400,location=0,menubar=0,resizable=1,scrollbars=1,status=0,titlebar=1,toolbar=0") }
//-->
</script>

with the link being <a href="javascript:popup(example.html)">

as you can see, Im a complete newbie to JavaScript

when i click on the link, nothing pops up and nothing happens. there is the little error on page status bar message, so i guess its something to do with my code....