VS 2010 - How To Select ALL Gridview Rows Not Just Rows Displayed

I have a gridview which right now is getting loaded with four pages of data. There are 50 rows per page. My problem is I need the "select all" button on my page to select 180 rows, not just the 50 on the page. This is my javascript:

Code:
function sel() {
var frm = document.forms[0];
isSelected = !isSelected;
for (var i = 0; i < frm.elements.length; i++) {
var e = frm.elements[i];
if (e.type == 'checkbox' && e.name.indexOf("ckSelected") > -1)
e.checked = isSelected;
}
}

I am more a C# person than javascript, but I am deducing that I need something other than document.forms[0] to represent the entire gridview. Or perhaps, I can't do this in javascript at all. Do I need server code to run and set a session variable, then when the user clicks the button to do something with all his selected rows (it goes to a detail page), query that session variable to see if it's all rows instead of iterating through each gridview row individually to see if the checkbox is checked?