how to setup the registor of the triple speed ethernet megacore function ?

http://www.alteraforum.com/forum/attachment.php?attachmentid=10046&stc=1

what is the meaning of mdio_addr0 and mdio_addr1,

I think that we just use one registor (mdio_addr0) ,and we can implement the function to addr 32 PHYes that have been linked to one bus,why there is another registor (mdio_addr1) ?

i am puzzeled by the function of the registor(mdio_addr1),can anybody tell me ???