Growing up with Combat, Breakout, Space Invaders, and Asteroids in the late ’70s and early ’80s, I’ve always had a soft spot for the Atari VCS. After attending the Central Ohio Retro Gaming Society convention in 2022 and hearing industry veterans talk about developing for the platform, I decided to learn 6502 assembly myself. The ultimate goal is a homebrew game — complete with burned EPROM, cartridge, and label art. The Christmas demo is the first real milestone on that path.
The idea#
I want to know enough 6502 assembly to create a complete homebrew Atari VCS game from scratch — code, cartridge, and labeling. I enrolled in a Pikuma course on Atari 2600 assembly programming to get started, and the Christmas demo was an approachable target given how far I’d gotten in the coursework.
What I did#
The toolchain is DASM assembler and Stella emulator for testing, with 8bitworkshop for interactive development in the browser. Between the course material, sample code, and advice from an assembly wizard at the local meetup, I put together a Christmas tree demo. Getting it to render correctly on screen was the real challenge — more on that below.
What surprised me#
I went in eyes wide open to the concept of “racing the beam” — on the Atari VCS, there’s no frame buffer. Your code runs in lockstep with the television’s electron beam as it draws each scanline. You’re not rendering to memory and displaying it; you’re feeding pixel data to the TV in real time, and if your code falls behind the beam, you get garbage on screen. After many iterations where the code looked logically perfect but the tree rendered as a jumbled mess, my assembly expert friend spotted it immediately: timing problem. Storing the tree pattern in a data structure and reading it back was just too slow for the scanline timing. The code was correct — it just couldn’t keep up with the beam.
Result#
For last Christmas, I wanted the demo running on a real Atari. I haven’t yet nailed down the EPROM programming workflow, so for now I copied the binary to an SD card running through a Harmony cartridge to get it on real hardware. It’s a bit of a cheat, but it’s a stepping stone toward the ultimate goal of burning the ROM onto an actual EPROM chip. I’ll be back to elaborate on the EPROM side of things once I have the whole story. But for now, I’ll relish in the satisfaction of seeing my code running on the VCS and showing on the Trinitron!
Takeaways#
- “Racing the beam” isn’t just a concept — until your logically correct code fails because of scanline timing, you don’t truly feel it
- The Harmony cartridge is a great intermediate step for testing on real hardware while the EPROM workflow is still in progress
- Having access to an experienced assembly programmer saved hours of frustration — some bugs only make sense to someone who’s seen them before
- 8bitworkshop’s interactive browser environment is invaluable for quick iteration
What’s next#
The demo needs some embellishments — it should play a carol and have twinkling lights or other holiday touches. Beyond that, I’m working toward burning the ROM onto a real EPROM and eventually building a complete homebrew game with a proper cartridge and label art.