轮询显示提示框

<script type="text/javascript">
        var iTimerID=null;
        var mytime=null;

        window.onload = function() {
            iTimerID = window.setInterval("popmsg(\"dd\")", 20000);
        }

        var oPopup = window.createPopup();
        var popTop = 0;
        var flag = false;
        var end = false;
        function popmsg(msgstr) {
            var winstr = "<table id='openwindow' width='100%' border='0' cellpadding='0' cellspacing='0' "
                + "style='border-right-width: 1px;border-bottom-width: 1px;border-left-width: 1px;border-right-style: "
                + "solid;border-bottom-style: solid;border-left-style: solid;border-right-color: #3D7045;border-bottom-color: #3D7045;"
                + "border-left-color: #3D7045;'><tr><td height='30' style='font-size:12px'>&nbsp;消息提醒："
                + "</td></tr><tr><td height='99' align='center' bgcolor='#FAFAFA' style='font-size:12px'>" + msgstr + "</td></tr></table>";
            oPopup.document.body.innerHTML = winstr;
            mytime = window.setInterval("popshow();", 50);
        }
        function popshow() {

            if (popTop > 500) {
                popTop = 500;
                flag = true;
            }
            if (popTop <= 0) {
                popTop = 0;
            }
            if (popTop < 130) {
                oPopup.show(screen.width - 160, screen.height, 150, popTop);
            }
            else if (popTop < 140) {
                oPopup.show(screen.width - 160, screen.height - popTop, 150, 130);
            }
            if (!flag) {
                popTop += 5; //控制速度
            }
            else {
                popTop -= 5;
                if (popTop <= 0) {
                    flag = false;
                                    if (mytime != null) {
                                        window.clearTimeout(mytime);
                                        mytime = null;
                                    }

                }
            }
          

        }
    </script>