var xPos = 20;
var yPos = document.body.clientHeight;
var step = 1;
var delay = 30; 
var height = 0;
var Hoffset = 0;
var Woffset = 0;
var yon = 0;
var xon = 0;
var pause = true;
var interval;

dd.style.top = yPos;

function changePos(){
	width = document.body.clientWidth;
	height = document.body.clientHeight;
	Hoffset = dd.offsetHeight;
	Woffset = dd.offsetWidth;
	dd.style.left = xPos + document.body.scrollLeft;
	dd.style.top = yPos + document.body.scrollTop;
	if (yon){
		yPos = yPos + step;
	}else{
		yPos = yPos - step;
	}
	if (yPos < 0){
		yon = 1;
		yPos = 0;
	}
	if (yPos >= (height - Hoffset)){
		yon = 0;
		yPos = (height - Hoffset);
	}
	if (xon) {
		xPos = xPos + step;
	}else{
		xPos = xPos - step;
	}
	if (xPos < 0){
		xon = 1;
		xPos = 0;
	}
	if (xPos >= (width - Woffset)) {
		xon = 0;
		xPos = (width - Woffset);
	}
}

function start(){
	dd.visibility = "visible";
	interval = setInterval('changePos()', delay);
}

function pause_resume() {
	if(pause) {
		clearInterval(interval);
		pause = false;
	}else{
		interval = setInterval('changePos()',delay);
		pause = true;
	}
}

start();