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;

        }

    }

}