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;

                        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);

        });

};