The Lighthouse of Doom

This repository contain a simple text-based adventure game, implemented twice, once in portable C, and once in Z80 assembly language.

The Z80 version of the game will run under the CP/M operating system, and the humble 48k ZX Spectrum.

My intention was to write a simple text-based adventure game to run under CP/M. Starting large projects in Z80 assembly language from scratch is a bit of a daunting prospect, so I decided to code the game in C first, so that I could get the design right, and avoid getting stuck in too many low-level details initially. Later I ported to the Spectrum, because it seemed like a fun challenge for myself!

Quick links within this README file:

• Play Online
• Plot of the game
• C Implementation
• Z80 Implementation
  • Z80 Changes
  • Memory Map
• Compiling & running it
• Downloading It
• Bugs?

Play Online

Thanks to the excellent jsspeccy ZX Spectrum emulator you can play this game with your browser here:

• https://steve.fi/software/lighthouse/

Game Plot

• You're inside a lighthouse (trapped? doesn't really matter I guess).
• You see a boat on the horizon, heading your way.
• But "oh no!", the lighthouse is dark.
  • The boat will surely crash if you don't turn on the main light.

The game is over, when you either fix the light, or find another solution.

If you do not achieve victory within a turn-limit the boat runs aground, and death will consume you all. (It is a very big boat!)

C Implementation

The implementation is mostly concerned with creating the correct series of data-structures, which are essentially arrays of objects. Because if we can make the game table-based we simplify the coding that needs to be done - we don't need to write per-object handlers anywhere, we can just add pointers to tables/structures.

The C implementation defines most of the important things in the file globals.h such as:

• The structure to define a location.
• The structure to define an object.

The game-state itself is stored in a couple of global variables, there isn't too much state to care about:

• The current location (i.e. index into location-table).
• A list of any items you're carrying.
• The number of turns you've taken.
  • Incremented by one each time you enter a command, be it recognized or not.
• Whether you won/lost.

Z80 Implementation

The Z80 implementation is based upon the C-implementation, with a few small changes.

The implementation uses a simple set of structures:

• A command-table to map input-commands to handlers.
• An item-table to store details about each object in the game.
• A person table to store telephone messages.

The main implementation can be found in the file game.z80, but because we support two targets (CP/M 2.x and the ZX Spectrum) there is a small amount of platform-specific code found in bios.z80.

The Makefile should build everything appropriately for both systems, defining SPECTRUM, and ENTRYPOINT as appropriate.

Z80 Changes

• Along the way I realized that having fixed inventory slots made the coding more of a challenge, so I made the location of each object a property of the object itself.
• The Z80 version has more easter-eggs (Try typing "xyzzy" a few times).
• There is rudimentary support for text-wrapping.
  • Enter WRAP 80 to wrap output around column 80.
  • Enter WRAP to view the current wrapping value.
• There are two victory conditions.
• The game can be built with the text-strings, and game code, protected by simple XOR encryption:
  • This stops users from looking through the binary for hints.
  • Run make release to build both "normal" and "protected" versions of the release.
  • The encrypted versions of the games have an X suffix in their filenames.

Memory Map

Roughly speaking the game consists of two parts:

• The driver/game which is about 3k of code.
• The text of the game along with the state of the world (flags, items carried, etc), which is approximately 9k.

All told the binaries for both CP/M and the ZX Spectrum are approximately 14k.

The layout in memory is basically the same for both variants, however the starting address is different:

• CP/M
  • Game loaded between the range 256-14000
    • [0x0100-0x3500]
  • Copy of state made to 53248 / 0xD000
• ZX Spectrum
  • Game loaded between the range 32768-46000
    • [0x5000-0xB400]
  • Copy of state made to 53248 - 0xD000

When the game starts a copy of the "state" is made to 0xD000 before anything has been modified, and then when the game is started that state is copied back to where it is used. This consists of all content between per_game_state_start and the end of the file. This has the location, flags, and similar, as well as all the static-text which will not change between runs. However copying this region was easier than just copying, or otherwise resetting, the state that changes during play.

No doubt things will change over time, however the (undocumented) BIOS command will show you rough sizes for the various parts of the game.

Compiling & Running It

Ensure you have the pasmo assembler installed, and then use the supplied Makefile to compile the game.

Running make will generate the default targets, if you wish to build only individual things then :

• make game-cpm to build a normal CP/M version.
  • make game-cpm-encrypted to build an encrypted CP/M version.
• make game-spectrum to build the ZX Spectrum version.
  • make game-spectrum-encrypted to build the encrypted ZX Spectrum version.
• make lighthouse will build the C-game for Linux
• make release will build both versions of the CP/M and ZX Spectrum release.

If you have a working golang system you can run the game under this trivial CP/M emulator:

• https://github.com/skx/cpmulator

Downloading It

If you look on our release page you can find the latest stable build.

• For CP/M download lihouse.com to your system, and then run LIHOUSE to launch it.
  • lihousex.com is the encrypted version.
• For the ZX Spectrum download lihouse.tap to your system, and then launch in your favourite emulator.
  • lihousex.tap is the encrypted version.

Bugs?

Report any bugs as you see them:

• A crash of the game is a bug.
• Bad spelling, grammar, or broken punctuation are also bugs.
• Getting into a zombie-state where winning or losing are impossible is a bug.

Steve