status状态栏实现字符串走动

<script type="text/javascript" language="javascript">
 var i = 0;
 var str="                好好学习 天天向上";
 function display(){
  var temp = str.substr(i);//从str的i索引开始截取到最后  
  window.status = temp;
  i++;
  if(i==str.length){
   i=0; 
  }
   
 }
 function startShow(){
  window.setInterval("display()",500);
 } 
</script>