javascript 怎么来控制iframe的滚动条？

<html>
<head>
<title>系统一览图</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<script language="JavaScript">
<!--
function scrolltop(){
alert(document.body.scrollHeight);
alert(document.body.clientHeight);
document.body.scrollTop =document.body.scrollHeight- document.body.clientHeight;
}
//-->
</script></head><body onLoad="scrolltop()">
<script language="JavaScript">
<!--
var timer=false;
document.ondblclick=scrollscreen;
function scrollscreen(){
var bdy=document.body;
if(bdy.clientHeight+bdy.scrollTop>=bdy.scrollHeight){
alert("already bottom! refresh now!");
window.location.reload();
return ;
}
bdy.scrollTop += 200;
window.setTimeout(scrollscreen,500);
}
for(var i=0;i<800;i++)
document.write("&nbsp;"+i+"<br>");
//-->
</script>
</body></html>这是一个控制滚动条的代码,你根据需要改一下好了.