jonl@yubyub.net writes:

>I think it's the direct and indirect interfaces.  Could he be speaking of
>addressing modes?  Do you think for DOS he means CP/M (that never ran on
>PDPs, did it? I thought it was developed there, but never really made it
>past that)...

CP/M was available for the 8080/Z80 architecture, the immediate
8-bit predecessor to the 8086-based line of junk we still have
as PCs today (later, CP/M 86 was available for 8086 PCs aswell
but that was a rather irrelevant part of history.)
He probably saw RT-11 or RSX-11 and mistook it for DOS (DOS, CP/M
and of course VMS all derive their look&feel somewhat from early
DEC systems.)

--mkb