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97 lines
6.0 KiB
Markdown
97 lines
6.0 KiB
Markdown
---
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title: "A game(boy jam) story"
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description: ""
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date: 2024-08-15
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tags: ["gameboy", "asm", "school", "gamejam"]
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draft: true
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---
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I recently mentioned on Twitter that I made a gameboy game with some friends during my first year of college. Since there's some cool stuff to showcase, here's a blog about it!
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## Context
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This is some context to explain why we decided to make a gameboy game and a bit of background, if you don't care feel free to directly skip to [the first paragraph](#space-shooter).
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### Introduction
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Quickly after starting college, I made some friends in my year group and some who were already in the 3rd year of college. All courses in the first year of my college were in C and the second year contained lot of C, C++ and one assembly course. It was common for students to have low-level side projects. My friends in 3rd year were doing a gameboy emulator.
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It looked incredibly fun, they would barge in the room with their laptop in hand like "LOOK IT WORKS" and you would see a Pikachu in the screen, saying a BAURHAU because the sound system was broken.
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Naturally, with friends of our year we decided to also make an emulator: a SNES one. This project is completely unrelated to this story, but that's how we learned how assembly & retro games worked. While this was not the same CPU, we made a disassembly and implemented instructions, learned about DMA & some hacks that games abused to run on limited hardware. Btw if you're interested, checkout [Retro Game Mechanics Explained](https://www.youtube.com/@RGMechEx) on youtube, especially his [SNES series](https://www.youtube.com/playlist?list=PLHQ0utQyFw5KCcj1ljIhExH_lvGwfn6GV) which is phenomenal.
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### Why a gameboy game?
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We had a teacher that pushed us to do as much crazy stuff as possible (that's how I ended up making a xml parser in C for a game project...) Since the game jam war organized by the school, he started teasing our friends saying stuff like "for the game jam, you're going to make a gameboy game, right?". Of course, he was not serious, but the idea was already growing in our heads. We decided to make this game with our group of 6 (4 3rd year that did the gameboy emulator and me & a friend who did the snes emulator). One of us thought this would be a huge failure and refused to work on the asm side. He instead thought it would be cool to make a real cartridge that could run roms in a real hardware.
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## Space shooter
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### Initial goal
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The game jam started, we had a weekend to make our game. The theme was announced a Friday at 18h, and we needed to make a keynote/presentation of the game on the Monday morning (even if we failed). We had access to everything in the school 24/24, even the electronics stuff. The theme for this jam was `Space`.
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Since we wanted to make the simplest game possible, it didn't take us long (approximately 10ms) to settle on the game idea: a space shooter.
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> On a side note, I don't think we ever decided something this quickly :3
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We wrote on a whiteboard what we wanted to do and started working. At the end of the weekend, the board looked like this:
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Here's the same board but in text & English:
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| Must | Should | Could |
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| :------------------------------------ | :----------------------------- | :-------- |
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| {{< icon "check" >}} Move | {{< icon "check" >}} Game over | ~multi~ |
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| {{< icon "check" >}} Obstacle | {{< icon "check" >}} Music | ~Colors~ |
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| {{< icon "check" >}} Shoot | Enemies can shoot | Power ups |
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| {{< icon "check" >}} Moving obstacles | Lifes | |
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| {{< icon "check" >}} SFX | ~cant read sorry~ | |
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| {{< icon "check" >}} Get hit | Flash | |
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| {{< icon "check" >}} Simple menues | {{< icon "check" >}} Boss | |
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| | {{< icon "check" >}} Intro | |
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| | {{< icon "check" >}} Score | |
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Turns out we overestimated the difficulty of the thing, we made a playable POC before going home (at 2am).
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It might sound complicated, but you don't have much to understand in order to write code. We have less than 256 instructions. Once you dedup via parameters (because `increment register a` is a different instruction than `increment register b`), you end up with only 44 instruction types (and that's counting the 13 bitshift instructions & the 7 interrupt instruction [like `stop` that stops the CPU]).
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The game boy doesn't have an OS. This means you don't have to deal with syscalls or to share memory. Allocating memory become easier:
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- You look at `constant.asm` which can look like (note that `$` means hexadecimal):
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```asm
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; Game registers
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DEF FRAME_COUNTER EQU $C000
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DEF HARDWARE_TYPE EQU $C001
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DEF RANDOM_REGISTER EQU $C002
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DEF PLAYER_STRUCT EQU $C003
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```
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- You find an empty address (like `$C005` for example, because `PLAYER_STRUCT` takes 2 bytes).
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- You shoot in the room "Is `$C005` taken?", if nobody answers add a line in `constant.asm` like
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```asm
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DEF BOSS_DEATH_COUNTER EQU $C005
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```
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congratulation you just alloced memory (forever). Until you merge your code, you must inform others that your address is used.
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Jokes aside, this means you can write at any arbitrary addresses so your struct or variables are just magic addresses.
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This logic is applied everywhere, since everything is memory mapped.
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There's no GPU or fancy graphic system, the gameboy (and most consoles of the time) had a PPU instead: a `Picture Processing Unit`. This thing can display sprite, background and even has palettes/colors if you are on a gameboy color.
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### Cool stuff
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## Break the target
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### Oh shit
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### But wait
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## Conclusion
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