Decompiling GOET0580

This is a step-by-step description of how to extract the A-code source code of Mike Goetz’s 580-point Adventure expansion (taxonomized in the Adventure Family Tree as GOET0580).

The IF Archive has two versions of this game:

Download, unzip, and compile the advdis tool out of the former. (This tool is copyright 2000 John Elliott — a completely different John Elliott from the one quoted in yesterday’s post!)

mkdir -p /tmp/work ; cd /tmp/work
curl -O
cd advdis
less advdis.doc  # Always read the instructions first!
gcc advdis.c -o advdis

Now you can disassemble the A-code from versions B00, B01, and B02! You should find no differences between B00 and B01. Version B02 adds the computer center and two new treasures, in addition to fixing many typos.

cd ../adv-B00
../advdis/advdis -t > disassembled.txt
cd ../adv-B01
../advdis/advdis -t > disassembled.txt
cd ../adv-B02
../advdis/advdis -t > disassembled.txt

Disassembling version B03 (an open question)

Now download and disassemble version B03:

mkdir adv-b03 ; cd adv-b03
curl -O
../advdis/advdis -t > disassembled.txt

This disassembly is mostly garbage. We can fix the first bunch of errors by borrowing version B02’s “” file.

cp ../adv-b02/ .
../advdis/advdis -t > disassembled.txt

The next problem is that all the messages are encrypted. This clearly has something to do with the 384-byte “DECODE.DAT” file sitting in this directory, but I don’t know what to do with it.

It seems to me that the messages are also compressed, because e.g. INFROMOUT (a 109-character message) occupies only 43 bytes in version B03. (And they’re compressed more aggressively than in version B02, because version B02’s INFROMOUT occupies 86 bytes.)

So, I’m pretty much stymied at this point. Maybe someone reading this blog post will have an idea of how to proceed with decrypting the messages in version B03. We know (most of) the plaintext; we have the ciphertext; we have some (but not all) knowledge of the compression algorithm; but we don’t know the encryption algorithm at all (except insofar as one might reverse-engineer it from the Z80 machine code in “”). That’s a job for Not Me.

Postscript, 2020-07-06: While I haven’t made any progress on decrypting the munged data back into A-code, the question is now pretty much moot, because I’ve gotten in touch with Mike Goetz and received a copy of the original A-code source files (and the original munger and interpreter, both written in K&R C) for Goetz’s version C01.1, circa March 1987. These files are now on their way to the Interactive Fiction Archive and I’ll update this post again once they’re available.

Postscript, 2020-07-29: The files are now available in the IF Archive! The path is /games/cpm/

Posted 2020-06-30