Getting my HomeLab-2 sea legs

21 October 2023 (homelab programming retrochallenge retrochallenge2023 retro haskell)

Previously, we left off our HomeLab-2 game jam story with two somewhat working emulators, and a creeping realization that we still haven't written a single line of code.

Actually, it was a bit worse than that. My initial "plan" was to "participate in the HomeLab-2 game jam", with nothing about pesky details such as:

• What game do I want to make?
• What technology will I use?
• How will I find time to do it, given that I'm spending all of September back home in Hungary?

I found the answers to these questions in reverse order. First of all, since for three of the five weeks I've spent in Hungary, I was working from home instead of being on leave, we didn't really have much planned for those days so the afternoons were mostly free.

Because the HomeLab-2 is so weak in its processing power (what with its Z80 only doing useful work in less than 20% of the time if you want to have video output), and also because I have never ever done any assembly programming, I decided now or never: I will go full assembly. Perhaps unsurprisingly, perhaps as a parody of myself, I found a way to use Haskell as my Z80 assembler of choice.

This left me with the question of what game to do. Coming up with a completely original concept was out of the quesiton simply because I lack both the game designing experience as well as ideas. Also, if there's one thing I've learnt from the Haskell Tiny Games Jam, it is that it's better to crank out multiple poor quality entries (and improve in the process) than it is to aim for the stars (a.k.a. that pottery class story that is hard to find an authoritative origin for). Another constraint was that neither of my emulators supported raster graphics, and I was worried that even if they did, it would be too slow on real hardware; so I wanted to come up with games that would work well with character graphics.

After a half-hearted attempt at Tetris (which I stopped working on when someone else has already submitted a Tetris implementation), the first game I actually finished was Snake. For testing, I just hacked my emulator so that on boot, it loads the game to its fixed stating address, then used CALL from HomeLab BASIC to start it. This was much more convenient than loading from WAV files; doubly so because it took me a while to figure out how exactly to generate a valid WAV file. For the release version, I ended up going via an HTP file (a byte-level representation of the cassette tape contents) which is used by some of the pre-existing emulators. There's an HTP to WAV converter completing the pipeline.

There's not much to say my Snake. I tried to give it a bit of an arcade machine flair, with an animated attract screen and some simple screen transitions between levels. One of the latter was inspired by Wolfenstein 3D's death transition effect: since the textual video mode has 40×25 characters, a 10-bit maximal LFSR can be used as a computationally cheap way of replacing every character in a seemingly random (yet full-screen-covering) way.

For my second entry, I went with 2048. Shortly after finishing Snake, thanks to Gábor Képes I had the opportunity to try a real HomeLab-2 machine. Feeling how unresponsive the original keyboard is convinced me that real-time games are not the way to go.

The challenge with a 2048-like game on a platform like this is that you have to update a large portion of the screen as the tiles slide around. Having an assembler that is an EDSL made it a breeze to try various speedcoding techniques, but with lots of tiles sliding around, I just couldn't get it to fit within the frame budget, which led to annoying flickering as a half-finished frame would get redrawn before the video refreshing interrupt handler eventually returned to finish the rest of the frame. So I ended up using double buffering, which technically makes each frame longer to draw, but avoids inconsistent visible frames.

Since the original 2048 is a product of modern times, I decided to pair my implementation with a more clean design: just like a phone app, it boots straight into the game with no intro screen, with all controls shown right on the main screen.

Between these two games, and all the other fun stuff one doesn when visiting home for a month, September flew by. As October approached, I stumbled upon this year's RetroChallenge announcement and realized the potential for synergy between doing something cool in a last hurrah before the HomeLab-2 game jam deadline and also blogging about it for RetroChallenge. But this meant less than two weeks to produce a magnum opus. Which is why this blogpost series became retrospective — there was no way to finish my third game on time while also writing about it.

But what even was that third and final game idea? Let's find out in the next post.