var showad = true;
var Toppx = 60;            
var AdDivW = 200;        
var AdDivH = 300;        
var PageWidth = 800;    
var MinScreenW = 1024;    
var ClosebuttonHtml = '<div align="right" style="position:absolute; top:0px; right:0px; margin:2px; padding:2px;z-index:2000;"><a href="javascript:;" onclick="hidead()" style="color:red;text-decoration:none;font-size:12px;">關(guān)閉</a></div>'
var AdContentHtml = '<div align="center" style="color:green;font-size:23px;font-family:黑體;"> <a  target="_blank"><img src="bsnlch.jpg" width="200" height="300" border="0"></a></div>';
document.write ('<div id="Javascript.LeftDiv" style="position: absolute;border: 1px solid #336699;background-color:#EEEEE2;z-index:1000;width:'+AdDivW+'px;height:'+AdDivH+'px;top:-1000px;word-break:break-all;display:none;">'+ClosebuttonHtml+'<div>'+AdContentHtml+'</div></div>');
document.write ('<div id="Javascript.RightDiv" style="position: absolute;border: 1px solid #336699;background-color:#EEEEE2;z-index:1000;width:'+AdDivW+'px;height:'+AdDivH+'px;top:-1000px;word-break:break-all;display:none;">'+ClosebuttonHtml+'<div>'+AdContentHtml+'</div></div>');

function scall(){
if(!showad){return;}
if (window.screen.width<MinScreenW){
alert("臨時(shí)提示：\n\n顯示器分辨率寬度小于"+MinScreenW+",不顯示廣告");
showad = false;
document.getElementById("Javascript.LeftDiv").style.display="none";
document.getElementById("Javascript.RightDiv").style.display="none";
return;
}
var Borderpx = ((window.screen.width-PageWidth)/2-AdDivW)/2;
document.getElementById("Javascript.LeftDiv").style.display="";
document.getElementById("Javascript.LeftDiv").style.top=document.body.scrollTop+Toppx;
document.getElementById("Javascript.LeftDiv").style.left=document.body.scrollLeft+Borderpx;
document.getElementById("Javascript.RightDiv").style.display="";
document.getElementById("Javascript.RightDiv").style.top=document.body.scrollTop+Toppx;
document.getElementById("Javascript.RightDiv").style.left=document.body.scrollLeft+document.body.clientWidth-document.getElementById("Javascript.RightDiv").offsetWidth-Borderpx;
}
function hidead()
{
showad = false;
document.getElementById("Javascript.LeftDiv").style.display="none";
document.getElementById("Javascript.RightDiv").style.display="none";
}
window.onscroll=scall;
window.onresize=scall;
window.onload=scall;

這一段是什么意思？？