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-$(this).height()/2;   
                    top=t;   
                    loc={left:l+"px",top:t+"px"};   
                    break;   
                default://默认为右下角   
                    location="rightbottom";   
                    loc={right:"0px",bottom:"0px"};   
                    break;   
            }   
        }else{   
            loc=location;   
            alert(loc.bottom);   
            var str=loc.top;   
            //09-11-5修改：加上top为空值时的判断   
            if (typeof(str)!= 'undefined'){   
                str=str.replace("px","");   
                top=str;   
            }   
        }   
        /*fied ie6 css hack*/  
        if(isIE6){   
            if (top>=0)   
            {   
                wrap=$("<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   
            //$("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);   
    });   
};