// JavaScript Document// JavaScript Document
var x = 50,y = 60//调节初始的位置
var a = 1, b = 0
var step =1.0   //速度

var obj=document.getElementById("ad") //ad为层的id 
function ad() {
     var L=T=0
     var R= document.body.clientWidth-obj.offsetWidth
     var B = document.body.clientHeight-obj.offsetHeight
     obj.style.left = x + document.body.scrollLeft
     obj.style.top = y + document.body.scrollTop
     x = x + step*(a?1:-1) 
     if (x < L) { a=1; x = L} 
     if (x > R){ a =0; x = R} 
     y = y + step*(b?1:-1) 
     if (y < T) { b =1; y = T } 
     if (y > B) { b = 0; y = B } 
}
var itl= setInterval("ad()", 1) 
obj.onmouseover=function(){clearInterval(itl)} 
obj.onmouseout=function(){itl=setInterval("ad()", 1)}