>Also, ITS had the neat feature of detaching job trees: You could login, get your own HACTRN (the hacked-up debugger ITS used as a shell), run a few other programs which would then be children of that HACTRN job, and detach the whole tree and logout. When you logged back in, you could re-attach the tree and carry on like nothing happened. It's kinda like screen or tmux.

You could also detach any particular job sub-tree, and other users could reattach it. Useful for passing a live ZORK or LISP or EMACS back and forth between different logged-in users. "Here, can you fix this please?"

There was also a :SNARF command for picking a sub-job out of a detached tree (good for snarfing just your EMACS from your old HACTRN/DDT tree left after you disconnected, and attaching it to your current DDT).

https://github.com/PDP-10/its/blob/master/src/sysen1/ddt.154...

It helped that ITS had no security whatsoever! But it had some very obscure commands, like $$^R (literally: two escapes followed by a control-R).

There was an obscure symbol that went with it called "DPSTOK" ("DePoSiT OK", presumably) that, if you set it to -1, allowed you to type $$^R to mess with other people's jobs, dynamically patch their code, etc. (The DDT top level shell had a built-in assembler/debugger, and anyone could read anybody else's job's memory, but you needed to use $$^R to enable writing).

Since ITS epitomized "security through obscurity", the magic symbol DPSTOK was never supposed to be spoken of or written down, except in the source code. But if you found and read the source code, then you passed the test, and deserved to know!

There was a trick if you wanted to set DPSTOK in your login script (which everyone could read), or if somebody was OS output spying on you (which people did all the time), and you wanted to change their prompt or patch some PDP-10 instructions into their job without them learning how to do it back to you.

The trick was to take advantage of the fact that DPSTOK happened to come right after BYERUN. So you could set BYERUN/-1, which everybody does (to run "BYE" to show a joke or funny quote when you log out), then type a line feed to go to the next address without mentioning it name, then set that to -1 anonymously.

So knowing the name and incantation of the secret symbol implied you'd actually read the DDT source code, which meant you had high moral principles, and were qualified to hate unix, which didn't let you do cool stuff like that. ;)

https://github.com/larsbrinkhoff/its-archives/blob/master/em...

From: "Stephen E. Robbins" <stever@ai.mit.edu>
Date: Thu, 21 Dec 89 13:25:56 EST
To: CENT%AI.AI.MIT.EDU@mintaka.lcs.mit.edu
Subject: Where unix-haters-request is

Date: Wed, 20 Dec 89 22:13:42 EST
From: "Pandora B. Berman" <CENT%AI.AI.MIT.EDU@mintaka.lcs.mit.edu>

....Candidates for entry into this august body must either prove their
worth by a short rant on their pet piece of unix brain death, or produce
witnesses of known authority to attest to their adherence to our high
moral principles..

Does knowing about :DDTSYM DPSTOK/-1 followed by $$^R qualify as attesting
to adherence of high moral principles?

- Stephen

Here are the symbols in the source:

https://github.com/PDP-10/its/blob/master/src/sysen1/ddt.154...

BYERUN: 0 ;-1 => RUN :BYE AT LOGOUT TIME.

DPSTOK: 0 ;-1 => $$^R OK on non-SYS jobs

Here's the $$^R handling code that slyly prints out " OP? " to pretend it didn't understand you. (In case anybody's watching!)

https://github.com/PDP-10/its/blob/master/src/sysen1/ddt.154...

N2ACR: SKIPN SYSSW ;$$^R
jrst n2acr0 ; Not the system?
SETOM SYSDPS
jrst n2acr9

n2acr0: skipn dpstok ;Feature enabled?
jrst n2acr9 ; nope
skipe intbit(U) ;Is it foreign?
jrst n2acr9 ; no, either SYS (special) or our own (OK anyway!)
movei d,%URDPS
iorm d,urandm(u) ;turn on winnage
n2acr9: 7NRTYP [ASCIZ/ OP? /]