How to debug with XMD

Hi,I am a newbie in using EDK. when I debug with XMD in a simple project whose only function is to control the LEDs through gpio,I read the bram using the order "mrd" and set bram using "mwr" in order to make the led off and on,but after I keyin the "mwr" order the value of specified address of bram did not changed at all!! why did this happened? Is this address of bram read-only?if that,how to check the attribute(read-only or read-write) of a certain segment of memory thanks a lot