//创建遮罩层
			function AlertMsg(){
			  
			/**第一步：创建DIV遮罩层。*/
				var sWidth,sHeight;
				sWidth = window.screen.availWidth;
				//屏幕可用工作区高度： window.screen.availHeight;
				//屏幕可用工作区宽度： window.screen.availWidth;
				//网页正文全文宽：     document.body.scrollWidth;
				//网页正文全文高：     document.body.scrollHeight;
				if(window.screen.availHeight > document.body.scrollHeight){  //当高度少于一屏
					sHeight = window.screen.availHeight;  
				}else{//当高度大于一屏
					sHeight = document.body.scrollHeight;   
				}
				//创建遮罩背景
				//var maskObj = document.createElement("div");
				//maskObj.setAttribute('id','BigDiv');
				//maskObj.style.position = "absolute";
				//maskObj.style.top = "0";
				//maskObj.style.left = "0";
				//maskObj.style.background = "#777";
				//maskObj.style.filter = "Alpha(opacity=30);";
				//maskObj.style.opacity = "0.3";
				//maskObj.style.width = sWidth + "px";
				//maskObj.style.height = sHeight + "px";
				//maskObj.style.zIndex = "10000";
				//document.body.appendChild(maskObj);

		      
				/**第二步：动态设置div的上边距和左边距，使弹出div居中打开。*/
				var MyDiv =document.getElementById("MyDiv");
				var MyDiv_w = getStyle(MyDiv,"width");
				var MyDiv_h = getStyle(MyDiv,"height");
			    
				MyDiv_w = parseInt(MyDiv_w); //去掉 单位 "px"
				MyDiv_h = parseInt(MyDiv_h);

				var width = pageWidth(); 
				var height = pageHeight();
				var left = leftPosition();
				var top = topPosition();

				var Div_topposition = top + (height / 2) - (MyDiv_h / 2); //计算上边距
				var Div_leftposition = left + (width / 2) - (MyDiv_w / 2); //计算左边距

				MyDiv.style.left = Div_leftposition + "px";  //拼接上 单位"px"
				MyDiv.style.top =  Div_topposition + "px";
				MyDiv.style.display = "block";  //设置弹出div显示
			}

				/**第三步：关闭窗口和遮罩层。*/
			function CloseDiv(){
				//var Bigdiv = document.getElementById("BigDiv");
				var Mydiv = document.getElementById("MyDiv");
				//document.body.removeChild(Bigdiv); 
				Mydiv.style.display="none";
			}

		// 计算当前窗口的宽度 //
		function pageWidth(){
					return window.innerWidth != null ? window.innerWidth : document.documentElement && document.documentElement.clientWidth ? document.documentElement.clientWidth : document.body != null ? document.body.clientWidth : null;
		}

		// 计算当前窗口的高度 //
		function pageHeight(){
					return window.innerHeight != null? window.innerHeight : document.documentElement && document.documentElement.clientHeight ? document.documentElement.clientHeight : document.body != null? document.body.clientHeight : null;
		}

		// 计算当前窗口的上边滚动条//
		function topPosition(){
					return typeof window.pageYOffset != 'undefined' ? window.pageYOffset : document.documentElement && document.documentElement.scrollTop ? document.documentElement.scrollTop : document.body.scrollTop ? document.body.scrollTop : 0;
		}

		// 计算当前窗口的左边滚动条//
		function leftPosition(){
					return typeof window.pageXOffset != 'undefined' ? window.pageXOffset : document.documentElement && document.documentElement.scrollLeft ? document.documentElement.scrollLeft : document.body.scrollLeft ? document.body.scrollLeft : 0;
		}

		//解决外嵌样式style , 用js获取不到的问题。
		function getStyle(elem, name){
					if(elem.style[name])
						return elem.style[name];
					else if(elem.currentStyle)	//ie
						return elem.currentStyle[name];
					else if(document.defaultView && document.defaultView.getComputedStyle){	//w3c
						name = name.replace(/([A-Z])/g,"-$1");
						name = name.toLowerCase();
						
						var s = document.defaultView.getComputedStyle(elem,"");
						return s && s.getPropertyValue(name);
					} else
						return null
		}