checked=false;

function checkedAll(form_name) 
{
	var aa = document.getElementById(form_name);
	 if (checked == false)
          {
           checked = true
          }
        else
          {
          checked = false
          }
	for (var i =0; i < aa.elements.length; i++) 
	{
	 	aa.elements[i].checked = checked;
	}
}