pointer -> long conversion

[DBrown at HI-MULTICS.ARPA]

  Well, on a machine with 3-word pointers the operation refrains from
sign-extending so as not to accidentally create a long which points
somewhere else when converted back to a pointer.
  Probably the criteria shoul be "do what you must, but try to retain
corectness".  This implies not sign-extending (so as to avoid a
non-"pure" copy, but it also implies that a loss of significance should
be visible to the programmer somehow.  How you're going to do that I
wouldn't know.
 --dave (unix hack on a 'bun) brown