missing _estack

Hello,
I've been trying to figure out how these things are setup. If I use one of the 
sis5xx mainboard configs, I get an error stating that _estack is undefined. 
Where is it supposed to be? Obviously, it has something to do with setting up a 
stack. I just don't understand where it is supposed to come from. Since I'm 
patterning my mainboard (sis530 based) on those as examples. This causes a 
small problem. Can anyone help me?