the three security architectures

[Jonathan S. Shapiro]

Since this isn't relevant to the bulk of your note, I'm answering it
separately:

First, an aside about Letwin's OS/2 story: it is true that you don't need to
know where the DLL is mapped. That information is stored in the segment
descriptor table, which the program doesn't have access to. What you need to
know is the index in the segment descriptor table of the mapped segment. All
addresses on that machine are of the form

    segment index : offset relative to segment

On the pre-80286 machines, the segment index was in fact the segment base
address divided by 16. On 80286 and later it was the integer index into the
descriptor table.