[Simh] Questions about PDP-7

[Warren Toomey]

Hi all, recently we unearthed a set of pages that contain assembly listings
of the PDP-7 Unix kernel and some associated user-mode programs. See the files
0*.pdf at http://www.tuhs.org/Archive/PDP-11/Distributions/research/McIlroy_v0/

There's a move afoot to see if we can bring this up on a real PDP-7 and also
on SimH. I've set up a mailing list if anybody wants to help out, and my
initial Github repository is at https://github.com/DoctorWkt/pdp7-unix

I've got some PDP-7 questions and also some SimH PDP-7 questions. What is
the format of disk blocks that SimH stores for the 18-bit systems? In other
words, how are the 18-bit words in each block stored in the bytes of the
host systems? At some point we will need to write a tool to generate a
filesystem, so this information will be crucial.
 
A general PDP-7 question: how are ASCII strings stored in memory? Are
they 7-bit or 8-bit? I'm assuming that two ASCII characters are stored
in one 18-bit word, but which bits go where? If I had the string "abcd",
which bits go where in the two 18-bit words?

Thanks in advance for all your help, and also thanks to RMS and others
who wrote Simh as it's been an invaluable tool to keep the PDP-11 and
Vax Unix systems going.

Cheers, Warren