The T.H.E. multiprogramming system — as designed and implemented by Dijkstra, Bron, Habermann, Hendriks, Ligtmans, and Voorhoeve — was studied by Mike McKeag during his visit to Eindhoven some time during the first half of 1971. Subsequently, McKeag went back to his university in Belfast (in Hoare's research group) where he wrote a report of the T.H.E. system. That report was sent to Dijkstra on 22 July 1971 with a cover letter stating:
Dear Professor Dijkstra,
I enclose a copy of the report I have written following my most enjoyable visit to Eindhoven earlier this year.
There are still a number of points that I have not completely understood and so I am writing to Dr. Bron in the hope that he will enlighten me. Subsequently I hope to produce a short supplement to the report to correct any mistakes and to amplify the description and evaluation of various aspects of the system.
I am sorry you will not be able to attend our Seminar in August but I look forward to meeting Dr. Bron and Mr. Martin again.
Also McKeag's supplement (mentioned in the above passage) eventually reached Dijkstra.
Without delving into the details of McKeag's report here, it seems clear that the structure underlying Dijkstra's T.H.E. system is, at least in part, a continuation of what Dijkstra had carried through with his ALGOL translator in 1960 (which is described in my paper). In his T.H.E. system, Dijkstra introduced separate levels of concern. Each level corresponds with one process and each such process operates entirely in terms of processes (and resources) at lower levels, unaware of what processes (and resources) exist at higher levels. For example, at the lowest level, the dispatching of processor time to processes occurs and above this level the actual number of processors in the system is irrelevant. One level higher, the mapping of the one-level virtual store onto the two-level physical store takes place. The implication is that all higher processes work in terms of one large shared virtual store, they are not aware of the underlying fact that the physical store contains a drum and a core store. This is very similar to the Dijkstra-Zonneveld compiler for ALGOL.