为什么获取不到js全局变量--高分跪求大神帮我解决

我要获取的确是 A.js 中全局变量 b,各位可以将下面代码拷下去,分成两个文件, A.js test.html.然后点击 test.html里面会生成一个 iframe 框架,但我要获取 父窗体中 b 变量,但 IE Firefox 提示 b 未定义.跪求大神帮我解决,解决问题者给高分.万分感谢.

JS A页面:
function A()
{
this.x = 6;
this.y = 7;
}
var b = 5;

HTML 页面:
<html>
<head>
<script type="text/javascript" src="./A.js"></script>
<script type="text/javascript">
var x = 5;
var a = new A();
</script>
</head>
<body>
<input id = "readme" type = "text" value="helloworld"></input><br>
<script>
window.onload=function(){
var str="<div><iframe width=400 height=300 frameborder='yes' scrolling='no' style='position:absolute'></iframe></div>";
document.write(str);

var mystr = "";
mystr += "<html>";
mystr += " <head>";
mystr += " <script type='text/javascr" + "ipt' defer>";
mystr += " function func(){";
mystr += " var x=parent.b;document.getElementsByTagName('input')[0].value=x;"; mystr += " }";
mystr += " </" + "scr" + "ipt>";
mystr += " </head>";
mystr += " <body>";
mystr += " <input type='button' value='' onclick='func();'></input>";
mystr += " </body>";
mystr += "</head>";

window.frames[0].document.write(mystr);
window.frames[0].document.close();}
</script>
<p>一些老的浏览器不支持 iframe。</p>
<p>如果得不到支持，iframe 是不可见的。</p>
</body>
</html>



为什么不能获取不到 b 全局变量?