The Great Escape

This project reverse engineers Denton Designs' classic ZX Spectrum 48K game The Great Escape from a tape image of the original game into a cross-referenced and annotated HTML disassembly and full assembly source code, using SkoolKit.

The Game

The Great Escape is a 1986 isometric 3D prison break game for the 48K ZX Spectrum where you play a POW trying to escape from a nazi prison camp. It's one of the best-regarded Spectrum games and I want to know how the authors managed to pack all that magic into the 48K ZX Spectrum.

The Disassembly

Read the current version of the disassembly here. It's not only an assembly listing: it has decoded game graphics, characters, items, animations and various other details about the game's workings.

Note that the disassembly may lag behind the head of the source tree: it might not feature the very latest changes until I push an updated build.

Chat

Goals

My first goal is to build a fully documented disassembly of the game. SkoolKit lets us build both an annotated assembly listing suitable for rebuilding an exact image of the original game and a detailed cross-referenced disassembly in HTML format.

The second is to transform the annotated assembly into C source code which behaves exactly like the original game. The work-in-progress reimplementation project lives in this repository and the two projects proceed in tandem.

SkoolKit

SkoolKit is the peerless Python toolkit for disassembling ZX Spectrum software written by Richard Dymond.

In addition to producing the cross-referenced HTML disassembly SkoolKit can also produce an annotated assembly listing. We can use this to reassemble the game to a .TAP file which you can load into an emulator. You can use it to alter the game and fix 30-year-old bugs!

Building the Cross-Referenced Disassembly

• First, install SkoolKit
• Then make a local clone of this repository:
  • git clone https://github.com/dpt/The-Great-Escape.git
• Finally change to the repository and build the HTML format disassembly:
  • cd <cloned repo>
  • make disasm

If all's well you will see output like:

$ make disasm
tap2sna.py --output-dir build @TheGreatEscape.t2s && mv build/TheGreatEscape.z80 build/TheGreatEscape.pristine.z80
Downloading http://www.worldofspectrum.org/pub/sinclair/games/g/GreatEscapeThe.tzx.zip
Extracting The Great Escape.tzx
Writing build/TheGreatEscape.z80
mkdir -p build
sna2skool.py --hex --sft TheGreatEscape.sft build/TheGreatEscape.pristine.z80 > TheGreatEscape.skool
Using skool file template: TheGreatEscape.sft
skool2html.py --hex --asm-labels --rebuild-images TheGreatEscape.skool
Using ref files: TheGreatEscape.ref, TheGreatEscapeBugs.ref, TheGreatEscapeChangelog.ref, TheGreatEscapeFacts.ref, TheGreatEscapeGame.ref, TheGreatEscapeGlossary.ref, TheGreatEscapeGraphics.ref
Parsing TheGreatEscape.skool
Output directory: build/TheGreatEscape
Copying /usr/local/lib/python3.7/site-packages/skoolkit/resources/skoolkit.css to skoolkit.css
Copying TheGreatEscape.css to TheGreatEscape.css
Copying static-images/BarbedWire.png to static-images/BarbedWire.png
Copying static-images/GameWindow.png to static-images/GameWindow.png
Copying static-images/JoystickControls.png to static-images/JoystickControls.png
Writing disassembly files in asm
Writing maps/all.html
Writing maps/routines.html
Writing maps/data.html
Writing maps/messages.html
Writing maps/unused.html
Writing buffers/gbuffer.html
Writing reference/bugs.html
Writing reference/changelog.html
Writing reference/facts.html
Writing reference/glossary.html
Writing graphics/glitches.html
Writing Intro.html
Writing Controls.html
Writing Completion.html
Writing Characters.html
Writing Items.html
Writing Masks.html
suggested width 17 > actual 16
Writing RoomObjects.html
Writing Rooms.html
Writing Map.html
Writing index.html

• Open up build/TheGreatEscape/index.html in your browser and dive in.

Building Runnable Games

• To build a .tap for loading into an emulator:
  • make tap will build build/TheGreatEscape.tap
• Or use make z80 instead to build a .z80 image if you prefer.

Any of the above steps will invoke its prior step automatically.

To Edit, Rebuild and Run the Game

• make skool -- to build the .skool file
• Edit TheGreatEscape.skool
• make tap
• Reload build/TheGreatEscape.tap in your emulator

Building the Assembly Source

If skool files are not to your taste and you prefer a regular assembly listing: make asm will build build/TheGreatEscape.asm. This can then be passed into Pasmo, for instance, to build a binary.

Current State of the Project

The reverse engineering of the game is now complete, but work will continue to improve the accuracy and readability of the disassembly.

Presentations & Write-ups

Here's the slides from a presentation I delivered to my colleagues about the project in January 2016.

There's a more recently updated version too which were written for a 2020 presentation about the project to the ABug user group: http://abug.org.uk/index.php/2020/07/04/the-great-escape-risc-os-conversion-david-thomas/

In 2019 I did a big write-up about the project on my website: http://www.davespace.co.uk/the.great.escape/