An Idea for a Grand Adventure

24 October 2023 (programming retrochallenge retrochallenge2023 retro c64 homelab)

Two weeks before the game jam deadline, I finally had the idea for my main entry. But with most of the first week occupied with work and travel, will the second week be enough to make it a reality?

Years ago, after remaking Time Explorer, I went on to reverse-engineer István Rátkai's subsequent games for the Commodore 64: The Revenge, and New Frontier I and II. This work culminated in a crowd-funded modern Android release. These Android versions were possible because these text adventure games, in quite forward-looking way for 1988, were originally written in a small homebrew bytecode language that was then packaged up with an interpreter. Think of it like a very primitive version of Infocom's Z-machine, but without all the Lisp-y sensitivities.

So anyway, my idea was to take the game's scripts as-is, and implement my own bytecode interpreter for the HomeLab-2. The Commodore 64 has 64 KB of RAM, while the HomeLab only has 16; but surely if I get rid of the multimedia (the per-room graphics and the small handful of short melodies played at key points of the games), the remaining text and scripts can't be that much?

Well it turns out when talking about text adventure games for 8-bit home computers, all that text is actually relatively quite a lot! In The Revenge, for example, just the various messages printed throughout the game fill 16 KB, and then we have 10 more KB of the recognized words and the game script. Clearly, this 26 KB of data will not fit into 16 KB, especially not leaving enough space for the game engine itself and the 256 bytes of game state.

Well, what if we try to be a bit more clever about representation? For example, because of the HomeLab-2's fixed upper-case character set, all that text will be in upper-case and without using any Hungarian accented characters, so we can use something similar to ZSCII to store 3 characters in 2 bytes. This, and some other small tricks like encoding most message-printing in just a single byte instead of a byte to signal that this is message-printing followed by the byte of the message ID, gets us to below 20 KB. Still not good, but better!

It was at this point that I also started writing the engine, to see how big that will be. Of course, at this point I was only able to try it out by only including a subset of the rooms and the game script, but at least it allowed me to get a size estimate for it. And so it came to about 1.5 KB of code to implement everything, plus 256 bytes of game state. Optionally, if we have space left, we can use a second 256 bytes to implement quicksave/quickload.

So what will we do with our budget of 14 KB, given a 20 KB game? Let's find out in the next post.

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