During a short visit to Bremen in December 1989 John Rosenberg had several discussions with me about computer architecture. Although we had previously worked together for more than a decade in Australia we had not seen each other for over a year, following my move to Bremen in 1988. Meanwhile John was spending a year on study leave at the University of St. Andrews in Scotland with Professor Ron Morrison and his persistent programming research group. From our conversations it was quite clear that John was having a most fruitful time in St. Andrews and was gaining valuable new insights into the...