> From: [...] Richard A. O'Keefe
> The DEC-10 machines had a scheme called Unimplemented User-Operations
> (UUOs) which IIRC were used heavily in some Pop implementations.
> I have a vague notion that "extracodes" may go back to Atlas.

I have a vague feeling you're right.  I'll ask my father - one of the folks
who worked on the London Atlas - next time I talk to him.  If not, we still
have a good few of the Atlas folks around at the University of Manchester.

		- Peter (old by proxy :-)