ppc inline asm syntax

Hi, i'm writing some inline assembly for ppc.
Grepping linux src code for examples i found syntax that i can't
understand, but as i can see it's frequently used.  For example:

__asm__ __volatile__("lwz%U1%X1 %0,%1;\n"
                              "twi 0,%0,0;\n"
                              "isync" : "=r" (ret) : "m" (*addr));

Can you explain me please  howto interpret 'lwz%U1%X1'  format string
in the assembler template line? Some other of them are:

'stw%U0%X0'   etc...

I can't find any help on this topic on the web.

Thanks a'lot,
Yury.