function myDrag(obj){    obj.onmousedown=function(e){        var e=e||window.event;        var diffX=e.clientX-this.offsetLeft;        var diffY=e.clientY-this.offsetTop;        document.onmousemove=function(e){            var e=e||window.event;            var left=e.clientX-diffX;            var top=e.clientY-diffY;            var leftMax=document.documentElement.clientWidth-obj.offsetWidth;            var topMax=document.documentElement.clientHeight-obj.offsetHeight;            if(left<0){                left=0;            }else if(left>leftMax){                left=leftMax;            }            if(top<-20){                 top=-20;            }else if(top>topMax){                top=topMax;            }            obj.style.left=left+"px";            obj.style.top=top+"px";        }        document.onmouseup=function(){        document.onmousemove=null;        }    }}