‘The half minute which we daily devote to the winding-up of our watches is an exertion of labour almost insensible; yet, by the aid of a few wheels, its effect is spread over the whole twenty-four hours.’

Confusion: PDP-10 Zork

Benjamin Slade

I grew up playing Infocom, Magnetic Scrolls, and Level 9 text adventures, with the Zork trilogy, the Enchanter trilogy, Planetfall, Wishbringer, The Guild of Thieves, The Pawn, Knight Orc, and Silicon Dreams being particularly prominent in my memory (somewhat re-activated through recent listening to the Eaten by a Grue podcast). I would have played all of these on an Atari 8bit or ST computer, and didn’t have any access to anything like a mainframe, and so never actually played the original Zork, which was written in the Lisp-derived MDL language (which formed the basis for the MDL-subset Infocom-specific ZIL language used for their subsequent offerings) for the DEC PDP-10.

Fortunately, some years ago, Matthew Russotto created Confusion, “a MDL interpreter which works just well enough to play the original Zork all the way through” and runs on Linux/Unix and presumably macOS. I’d tried to install this from Arch’s AUR a couple of years ago, without any luck. Inspecting the package build, the AUR Confusion tries to get around errors by using an ancient version of gcc. I found that a few fairly trivial fixes were enough to get it to compile with a recent version of gcc, and so was finally able to try the original Zork:

The most fun I always had in text adventures was exploring, and I’ve always found Infocom’s various “time-limit” elements (batteries running out, health deteriorating, &c.) somewhat irritating. I remembered that there’s a torch in Zork I, and that is also true in the original Zork, and I managed to obtain it early on so as to save my (trusty brass) lamp’s batteries. I then cheated somewhat wildly (though, to be fair, a lot of the puzzles I had figured out once upon a time in Zork I or Zork II; and the bank puzzle and some of the others I wouldn’t have patience for anymore).

The original code has some bugs in it which made things more interesting.

Narrow Ledge
You are on a narrow ledge overlooking the inside of an old dormant
volcano.  This ledge appears to be about in the middle between the
floor below and the rim above. There is an exit here to the south.
There is a very large and extremely heavy wicker basket with a cloth
bag here. Inside the basket is a metal receptacle of some kind.
Attached to the basket on the outside is a piece of wire.
The basket contains:
 A cloth bag
 A braided wire
 A receptacle
 The receptacle contains:
  A newspaper
 A blue label
There is a small hook attached to the rock here.

> untie braided wire from hook

 "First arg to NTH must be structured"
Atom REP has neither LVAL nor GVAL

I had a similar issue involving the result of a disagreement with a suspicious-looking individual holding a bag. It turns out (thanks to Matthew Russotto for the following information) that this has to do with issues in saving and restoring files, and (failure of) either properly recording or decoding certain values. The balloon issue has to do with a record about the object burning in the receptacle. It is saved as BINF!-FLAG, which should be a boolean-type flag, but at some point it became an object (recording what is burning) and apparently isn’t decoded properly on a restore. Saving-and-restoring during a battle with the suspicious-looking individual produces a similar error to the balloon-burnable error, due to the THIEF-ENGROSSED!-FLAG (which apparently really is a flag) not being saved properly. The upshot (for a player) is that you shouldn’t save during either of these bits of the game.

With these additional lurking grues out of the way, I was able to make it to the end of the game:

Speaking of saving, MDL Zork only has a single restore file. It is stored in the MTRZORK directory and is named ZORK.SAVE. Of course you can create additional saves by copying out this save file to different names.

If you too want to play the original Zork using the original PDP-10 MDL source code, you should be able to build the MDL interpreter necessary from the release at Matthew Russotto’s site (where my minor patches should be incorporated): http://www.russotto.net/git/mrussotto/confusion/releases or from the git repo with my patches at https://gitlab.com/emacsomancer/confusion-mdl.

Or, the easiest of all, install it via Guix (which can installed on top of your current distro as a standalone package manager), as I have created a Guix package which was accepted upstream (the package name in Guix is confusion-mdl).

Since I played through this right before designing the propositional logic homework for my Semantics class, my students are currently “enjoying” doing Zork-flavoured propositional logic translations, e.g.:

  1. The thief only steals your things when you’re in the cellar.
  2. Your sword glows blue whenever the thief or the troll is near and never any other time.
  3. Whether or not you are lost in a maze of twisty little passages (all alike), the grues are lurking in the darkness.
  4. The grues will eat you unless you bring a light source.

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Comment by MKlein on Fri Sep 6, 2019 12:30 UTC
Awesome story. This all seems so long ago (which it really is). Thanks for sharing! A question: Was the save feature already implemented in the PDP-10 version or was it included later for the home releases of Zork1?
Comment by B. Slade on Mon Apr 22, 2019 22:39 UTC

Arg, the commenter ate the syntax. At the prompt you should type the following command, but surrounded by angle brackets: FLOAD “run.mud”.

See the Gitlab readme for more details.

Comment by B. Slade on Mon Apr 22, 2019 22:31 UTC
@Chris: You should be able to resolve this by re-generating the image. If you navigate to the mdlzork directory and run mdli with no parameters and then type at the prompt, it should regenerate.
Comment by Chris Stacy on Mon Apr 22, 2019 22:05 UTC

When I run it, I get:

Built-in table size does not match Unable to read built_in table -1 Initial restore failed

Not sure how to proceed. I have a transcript of everything from downloading all the pieces, to invoking mdli on the SAVE file, if that’s helpful to you. I assume I am running into a well-known and simple problem.