jQuery.fn.floatdiv = function(location) {
    //判断浏览器版本
    var isIE6 = false;
    var Sys = {};
    var ua = navigator.userAgent.toLowerCase();
    var s;
    (s = ua.match(/msie ([\d.]+)/)) ? Sys.ie = s[1] : 0;
    if (Sys.ie && Sys.ie == "6.0") {
        isIE6 = true;
    }
    var windowWidth, windowHeight; //窗口的高和宽
    //取得窗口的高和宽
    if (self.innerHeight) {
        windowWidth = self.innerWidth;
        windowHeight = self.innerHeight;
    } else if (document.documentElement && document.documentElement.clientHeight) {
        windowWidth = document.documentElement.clientWidth;
        windowHeight = document.documentElement.clientHeight;
    } else if (document.body) {
        windowWidth = document.body.clientWidth;
        windowHeight = document.body.clientHeight;
    }
    return this.each(function() {
        var loc; //层的绝对定位位置
        var wrap = $("<div></div>");
        var top = -1;
        if (location == undefined || location.constructor == String) {
            switch (location) {
                case ("rightbottom"): //右下角
                    loc = { right: "0px", bottom: "0px" };
                    break;
                case ("leftbottom"): //左下角
                    loc = { left: "0px", bottom: "0px" };
                    break;
                case ("lefttop"): //左上角
                    loc = { left: "0px", top: "0px" };
                    top = 0;
                    break;
                case ("righttop"): //右上角
                    loc = { right: "0px", top: "0px" };
                    top = 0;
                    break;
                case ("middletop"): //居中置顶
                    loc = { left: windowWidth / 2 - $(this).width() / 2 + "px", top: "0px" };
                    top = 0;
                    break;
                case ("middlebottom"): //居中置低
                    loc = { left: windowWidth / 2 - $(this).width() / 2 + "px", bottom: "0px" };
                    break;
                case ("leftmiddle"): //左边居中
                    loc = { left: "0px", top: windowHeight / 2 - $(this).height() / 2 + "px" };
                    top = windowHeight / 2 - $(this).height() / 2;
                    break;
                case ("rightmiddle"): //右边居中
                    loc = { right: "0px", top: windowHeight / 2 - $(this).height() / 2 + "px" };
                    top = windowHeight / 2 - $(this).height() / 2;
                    break;
                case ("middle"): //居中
                    var l = 0; //居左
                    var t = 0; //居上
                    l = windowWidth / 2 - $(this).width() / 2;
                    t = windowHeight / 2 - $(this).height() / 2;
                    top = t;
                    loc = { left: l + "px", top: t + "px" };
                    break;
                default: //默认为右下角
                    location = "rightbottom";
                    loc = { right: "0px", bottom: "0px" };
                    break;
            }
        } else {
            loc = location;
          
            var str = loc.top;
            //09-11-5修改：加上top为空值时的判断
            if (typeof (str) != 'undefined') {
                str = str.replace("px", "");
                top = str;
            }
        }
        /*fied ie6 css hack*/
        if (isIE6) {
            if (top >= 0) {
                wrap = $("<div style=\"top:expression(documentElement.scrollTop+" + top + ");\"></div>");
            } else {
                wrap = $("<div style=\"top:expression(documentElement.scrollTop+documentElement.clientHeight-this.offsetHeight);\"></div>");
            }
        }
        $("body").append(wrap);

        wrap.css(loc).css({ position: "fixed",
            z_index: "999"
        });
        if (isIE6) {

            wrap.css("position", "absolute");
            //没有加这个的话，ie6使用表达式时就会发现跳动现象
            //至于为什么要加这个，还有为什么要加nothing.txt这个，偶也不知道，希望知道的同学可以告诉我
            $("body").css("background-attachment", "fixed").css("background-image", "url(n1othing.txt)");
        }
        //将要固定的层添加到固定层里
        $(this).appendTo(wrap);
    });
};