[TUHS] PDP-7 UNIX filesystem

DMR explained how PDP-7 UNIX was used in "The Evolution of the Unix Time-sharing System" but having played with it myself, I stumbled in a couple of cases and found it a bit awkward to use. Maybe someone (ken and doug?) can shed some light on "elaborate set of conventions" that dmr mentioned. My questions are these: you cannot execute a program if you're in a directory you can't write into. I asked Warren about this when I first tried pdp7 unix and he explained it to me: the shell creates a link to the binary and executes it. If it can't write into the current directory, it fails to create the link and hence can't execute the program. How was this handled in practice? did users have write permissions on all directories? did you just stay in your directory all the time? . and .. Was this introduced first with PDP-11 unix or did the convention start on the PDP-7 already? It certainly seems to be the case with . but how about ..? the dd directory seems to take on the role of a sort of root directory and the now discovered program pd actually creates a file .. (haven't tried to understand what it does though yet) What does dd stand for, dotdot? directory directory? aap