I know approximately zero PDP-7 assembly, but this looks like it might be “print directory”, equivalent to pwd today? It seems to open its parent “dotdot” directory and write it out.
I don't know PDP7 assembly either, but I don't see how it could stand for "previous directory" (and be equivalent to today's `cd -`). Any command to change directory needs to be done in-process by the shell, so it would need to be implemented as a builtin, not a standalone executable.
So "print directory" or (print something about the) "parent directory" seem more likely to me.
In the earliest versions of Unix, the shell didn't fork to run programs. It would exec the program, and then the program would exec the shell instead of exiting. Makes sense, right? That's what you'd do if you didn't have an operating system at all, and it's how, for example, CP/M worked. So you could write "cd" as a non-built-in command.
But it sounds like the other commenters have figured out that it's "pack directory".
After staring at the listing for a while, I think it is a kind of garbage collector for removing unlinked files from directory listings ("pack directory"?). As best as I can work out, the code does:
open ..
loop
read a directory entry into tbuf
if we read 0 bytes (eof presumably), break loop
if tbuf[0] == '\0', go back to start of loop
append tbuf to dir
done
close ..
reopen .. with creat
write the stuff we built up into dir to ..
close ..
exit
Looking at this again, your psuedocode seems to be pretty reasonable. However, I'm still left wondering why this command would be necessary: doesn't it just read the directory entry into memory and write it back out again? I'm also curious why "sys write" takes .. directly when all the other calls seem to need dotdot.
If my reading is right, it filters out any with a leading null-byte before writing back out. My guess is that that is how they implemented unlink/rm - just write zeroes over the entry.
As for the sys write: my current hypothesis is that .. there is a placeholder for an argument which is written to by the preceding dac .+4. ('"If a program can't rewrite its own code", he asked, "what good is it?"'). But I can't make sense of what it's actually putting there - I'd guess length, but it looks like ~(dir - 2) + 8.
That took me a while. The ‘8’ in ‘tad 8' is the contents of memory location 8, i.e. the destination pointer in the memory copy, so it's ~(dir - 2) + dst. And since -x = ~x + 1 (there being no negation instruction), that works out to the length, dst - dir + 1.
dir appears to be copied into from tbuf at some point. It puts dir-1 into memory location 8 at the start, and then the code around the 2: section I'm pretty sure is a memcpy-ish loop like:
c1 = -8;
*9 = tbuf - 1;
do {
*(++(*8)) = *(++(*9));
} while(++c1 != 0);
goto 1b; // b for backwards?
It seems that memory locations 8-15 are auto-indexing[0], so the lac/dac i increments the pointed-at location before use, and isz is "increment and skip next instruction if zero".
I think ‘dotdot’ is what is called ‘dd’ there (the corresponding symbol in ls is still named ‘dd'). I don't think the concept of a parent directory existed yet. Maybe ‘pd’ is something like ‘prepare directory’, constructing the dd/dotdot entry.
But the program appears to fail if dotdot can't be opened for reading, and it appears to scan through it doing reads first. Then it closes and does a creat, which is expected to succeed also. I'm suspecting that at this stage of development, open may have been used for reading and creat for writing (including to an existing file).
From TFS¹, creat() on an existing file truncates it (i.e. to length zero, as O_TRUNC), and only the superuser can do this to a directory. open() has read/write flag bits as now.
I am leaning toward Pete_D's ‘pack directory’ idea.
> Unidentified program.
> Might “pd” stand for “previous directory”?
I know approximately zero PDP-7 assembly, but this looks like it might be “print directory”, equivalent to pwd today? It seems to open its parent “dotdot” directory and write it out.