The ADD_INTERLOCKED function adds the value of an expression  to
  the  value of a variable, stores the newly computed value in the
  variable, and returns an integer value:  -1 if the new value  is
  negative, 0 if it is zero, and 1 if it is positive.

  Syntax:

     ADD_INTERLOCKED( e, v )

  The type of the expression 'e'  must  be  assignment  compatible
  with  that  of  the  variable  'v'.  The variable 'v' must be an
  integer or an unsigned subrange; 'v'  must  have  an  allocation
  size  of  two bytes and must be aligned on a word boundary.  The
  type of 'e' must be assignment compatible with that of 'v'.