data validation (was re self-modifying code)

[Guy Harris]

>   This is for a machine which happily passes descriptors of arrays
> around, and manages to bounds-check array references in parallell
> with the fetch. 

Umm, err, what machine is that?  Doesn't sound like the GE 645 or the
successors that I knew of; as I remember it, the 645 and the HIS 6180 were
fairly "conventional" machines in most regards, with no automatic
bounds-checking for array references.  (Maybe some of the weirdball "indirect
then tag" addressing modes could do this, but I don't think the PL/I compiler
made much use of most of them.)