Performs a conditional atomic compare and exchange operation on a
  quadword.  The quadword pointed to by source is read and compared
  with the quadword old_value.  If they are equal, the quadword
  new_value is written into the quadword pointed to by source.  The
  read and write is performed atomically, with no intervening access
  to the same memory region.

  The function returns 1 if the write occurs, and 0 otherwise.

  Syntax:

       int __CMP_SWAP_QUAD (volatile void *source, int old_value, int
       new_value);