[Simh] Questions about PDP-7

[Bob Supnik]

I think the answer is two characters per 18b word, right aligned in a 9b 
"byte".

This is based on the assembly code at the end of the first PDF:

a.out:
     <a.>; <ou>; <t 040; 040040

Clearly it's just two characters per word (; denotes end of statement to 
this assembler). 040040 is octal for space-space in ASCII, one character 
per 9b in octal.

/Bob

On 2/25/2016 12:00 PM, simh-request at trailing-edge.com wrote:
> Message: 1
> Date: Thu, 25 Feb 2016 08:04:01 +1000
> From: Warren Toomey<wkt at tuhs.org>
> To:simh at trailing-edge.com
> Subject: [Simh] Questions about PDP-7
> Message-ID:<20160224220401.GA8375 at minnie.tuhs.org>
> Content-Type: text/plain; charset=us-ascii
>
> 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 athttp://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 athttps://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