writing to register

Why doesn't this work?

//from system.h:

USER_LOGIC_REGISTER_BASE 0x00920850

//my code:

IOWR(USER_LOGIC_REGISTER_BASE, 0, 0x00 )

reg = IORD(USER_LOGIC_REGISTER_BASE,0)

Can anyone tell me what I'm doing wrong?

Thanks.

Newbie