In line ASM magic? What is this?

[George Anzinger <george@mvista.com>]

I am trying to understand what is going on in the following code.  The
reference for %2, i.e. "m"(*__xg(ptr)) seems like magic (from
.../include/i386/system.h).  At the same time, the code "m" (*mem) from
the second __asm__ below (my code) seems to generate the required asm
code.  Before I go with the simple version, could someone tell me why? 
Inquiring minds want to know.

struct __xchg_dummy { unsigned long a[100]; };
#define __xg(x) ((struct __xchg_dummy *)(x))

		__asm__ __volatile__(LOCK_PREFIX "cmpxchgl %b1,%2"
				     : "=a"(prev)
				     : "q"(new), "m"(*__xg(ptr)), "0"(old)
				     : "memory");


	__asm__ __volatile__(
                             LOCK "cmpxchgl %1,%2\n\t"
                             :"=a" (result)
                             :"r" (new),
                              "m" (*mem),
                              "a0" (test)
                             : "memory");


George
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