Back to the NES

This weekend would've been great - things warmed up outside, the sun came out (and I had to look on Wikipedia to see what the glowing thing in the sky was, it's been so long), the beach bars were having their grand opening for the year 10 minutes walk away...

...and fate rewards me with a heavy cold that felt like the not-at-all fun bits of being drunk and a 3-day hangover combined into one handy package. Fantastic.

So like any normal geek who can count to 1023 on his fingers I grabbed a Lemsip, curled up on the sofa and started investigating Unreal Engine 4 and getting something out of it that didn't make my old backup PC and netbook scream in pain. The requisite hacks I found online required multiple recompilations which took a not insignificant amount of time.

So in the meantime I turned to WUDSN and the Nintendo NES toolchain. I also remembered discussing ideas way back with someone about an old arcade game where you pushed eggs around to free the birds trapped within whilst avoiding the enemies patrolling the screen. As a test of doing an old style single-screen game (Like the original NES classics, such as balloon flight) I started to implement the basis of something similar on the NES:

What I currently have is a hardcoded test level, which is somewhere close to the final (decompressed) format I want to use, a player sprite composed of 2 16x8 hardware sprites, all the hardware initialization and RAM clearup code and Shiru's Famitone sound player handling the sound. The collision detection is in, although my (deliberate) non-handling of diagonal joypad presses doesn't feel as fluid as I hoped when moving around, so I'm going to go back and redo that.

On the Actual hardware front the initialization, bankswitching (for the 64k cartridge image) and NMI-based Picture Processing Unit (PPU) update code means that the game works properly on hardware. It should also be able to determine if it's running on PAL/NTSC and adjust movement speeds and music pitch accordingly.

All the game logic is in 6502 assembly, however the collision routines and logic all happens in game-space (as opposed to screen coordinates) and only references the machine-agnostic game data with things being transferred across to the hardware as needed so hopefully if this thing gets finished I can port it across to any other 6502-based machines afterwards.

Seeing as I got this far in a lazy day I figure I might as well keep on with it and see how it goes. Hopefully documenting it as I go will motivate me.

Next task - handling the egg pushes. Pushing an egg is easy - it moves until it hits a wall, then bounces back to it's original position with 1 hit of damage taken. The harder part is if it hits another egg, in which case it causes a chain reaction until the last egg in the chain hits the wall and then everything bounces back into place in reverse order. This would be easy enough to do in C, but in 8-bit assembly with 3 registers and a clockspeed of under 2MHz this is going to take a bit of thinking about how to do it for the best.