how to mount /dev/ram0 to /

I made a new uRamdisk for the mini bubinga board,
change uboot's bootargs like this
bootargs root=/dev/ram rw console=ttyS0,115200 ip=192.168.0.200:192.168.0.19:::bubinga:eth0:off panic=1
==>
bootargs root=/dev/ram0 rw console=ttyS0,115200 ip=192.168.0.200:192.168.0.19:::bubinga:eth0:off panic=1

after boot up,excute df to see the mount point:
~ # df
Filesystem           1k-blocks      Used Available Use% Mounted on
~ # 
nothing was mounted.
the fatab in /etc is like this:
/dev/ram0               /               ext2    defaults        1 1
none                    /proc           proc    defaults        0 0

anything wrong?
thank you.

tony