What's the meaning of keyword "memory" in clobber list?

[Parmenides <mobile.parmenides@xxxxxxxxx>]

Hi,

    For the following function from the Linux kernel, I have two
    questions:

static inline int test_and_set_bit(int nr, volatile unsigned long *
addr)
{
    int oldbit;

    __asm__ __volatile__( LOCK_PREFIX
                          "btsl %2,%1\n\tsbbl %0,%0"
                          :"=r" (oldbit),"+m" (ADDR)
                          :"Ir" (nr) : "memory");
    return oldbit;
}

1. There are two instructions in the inline assemably, namely btsl and
sbbl. But, can the only one LOCK_PREFIX ensure that the operation is
atomic?

2. The clobber list of the inline assembly contains a string of
"memory". What is the meaning of this declaration and why does the
operation need it ? After all, some other operations such as
clear_bit() (see the following code) dose not need "memory" indeed.
The declaration of "memory" appears in source code of kernel
here and there, and bothers me for a long time. So any details about
it will be appreciated.

static inline void clear_bit(int nr, volatile unsigned long * addr)
{
    __asm__ __volatile__( LOCK_PREFIX
                          "btrl %1,%0"
                          :"+m" (ADDR)
                          :"Ir" (nr));
}

Thanks!

-- 
Cogito ergo sum.