some atomic memory operations

CMPXCHG - Comapre and Exchange

    [lock] cmpxchg  reg, reg/mem
    ^               ^       ^
    |               |       |
    prefix          SRC     DEST

if al == dest:
    ZF      <- 1
    DEST    <- SRC
else:
    ZF      <- 0
    AX      <- DEST

e.g.
            mov         $0,     %al
    lock    cmpxchg     %edx,   some_lock