<input type="checkbox">
<input type="checkbox">
<input type="checkbox">
<input type="checkbox">
<input type="checkbox">
<input type="checkbox">
<input type="checkbox">
<input type="checkbox">
<input type="checkbox" name="checkbox2" value="Check All" onClick="mm()">

<script language=javascript>
function mm()
{
var a = document.getElementsByTagName("input");
if(a[0].checked==true){
for (var i=0; i<a.length; i++)
if (a[i].type == "checkbox") a[i].checked = false;
}
else
{
for (var i=0; i<a.length; i++)
if (a[i].type == "checkbox") a[i].checked = true;
}
}
</script>

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