This is chapter 8 from Capability-Based Computer Systems, a book by Henry Levy published by Digital Press. Here's the TOC:
http://homes.cs.washington.edu/~levy/capabook/
I remember reading this chapter in the mid 80's when I was starting to develop business software for this minicomputer. The System/38 hardware and OS were not well understood outside of IBM, and Levy's book gave me some sense of what was going on under the hood to make the system secure.