SidecarTridge Multi-device GPU Demo
Source code CHANGELOG v1.0.1dev
This micro-firmware app for the SidecarTridge Multi-device showcases the RP2040’s capabilities in the Raspberry Pi Pico W acting as a graphics co-processor.
Table of contents
⚠️ Attention
This demo is designed only for low-resolution monitors. It will not work in high-resolution modes.
🚀 Installation
To install the GPU Demo app on your SidecarTridge Multi-device:
- Launch the Booster App on your SidecarTridge.
- Open the Booster web interface in your browser.
- Go to the Apps tab and select GPU Demo from the list.
- Click Download to install the app to your SidecarTridge’s microSD card.
- Once installed, select the app and click Launch.
After installation, the GPU Demo will start automatically every time your Atari is powered on.
🕹️ Usage
On startup, the app shows a brown wall and torches. Then:
- The demo begins rendering 48×48 sprites with a 16-color palette over a background of 64×64 tiles (also 16 colors).
- Every 3 seconds, a new sprite appears — up to ~37–38 sprites (limited by time needed to maintain 50 FPS).
Controls:
- Any other key - exits the emulator and boots into the Atari GEM desktop.
- ESC — return to the Booster App.
⏏️ Exiting to GEMDOS or Booster
- ESC → Launch Booster app.
- Any other key → Exit emulator, boot Atari GEM desktop.
- SELECT button on Multi-device → Reboot demo (Atari ST reset must be done manually).
- Hold SELECT for 10 seconds → Factory reset the Multi-device.
♻️ Reset Behavior
- Pressing the Atari reset button alone will not restart the demo. To restart: press SELECT first, then reset the computer.
🔄 Power Cycling
After a power cycle, the demo auto-launches.
🛠️ Under the hood
This demo is a re-imagining of the RP2040 VGA 6-bit demo for the Atari ST world. Instead of producing VGA signals, the RP2040 fills Atari-friendly framebuffers in its own memory — the same memory used by the ROM emulation logic in the SidecarTridge.
The framebuffers follow the classic Atari ST low-res layout: 32 KB of contiguous memory, four bitplanes. The RP2040 keeps them updated with whatever’s happening in the scene, and the Atari just reads and displays them.
Tiles, sprites, and color conversion
In the original VGA demo, tiles and sprites are stored in RGB6 format: 6 bits per pixel, two bits each for red, green, and blue, allowing 64 colors. The Atari ST can only show 16 colors at a time, so conversion is necessary. A “smart” approach would be to pre-convert all data to the ST’s planar format. This demo takes the opposite route: it converts on-the-fly.
Why? Because the goal isn’t raw performance — it’s to explore strategies for live conversion from chunky RGB6 to the ST’s planar format, which might be useful for future work. Converting a whole screen of background tiles takes around 8 ms, which would be too slow for some uses but serves fine here.
Transparency
Sprites use a simple color-key transparency: one palette color is treated as invisible, and any pixel matching it isn’t drawn. This, too, is handled live during conversion.
Text
Text rendering works the same way — a small 6×8 bitmap font is stored in the RP2040, converted to planar format as needed, and blended into the scene.
ST vs. STE
The core loop is tuned differently for the two machines. On a plain ST, it’s raw 68000 speed, using self-generated MOVEM.L sequences to blast the framebuffer to screen memory in under 20 ms:
MOVEM.L $FA0600, D0-D7/A0-A7
MOVEM.L D0-D7/A0-A7, $70000
...
On an STE, the blitter takes over, copying large chunks in one go and finishing much faster. In the ST version you’ll see a red timing bar at the bottom showing how much copy time is used; on the STE, that bar turns blue and is shorter, thanks to the blitter’s speed.
Double buffering
Two framebuffers live in the RP2040’s RAM and two more in the Atari’s. This is overkill but makes tearing impossible: while one buffer is displayed, the other is being drawn. It could be made leaner, but again, performance tuning wasn’t the main goal here.
What’s next
I don’t plan to push this much further — it’s a proof of concept and a learning project before tackling something far bigger, like getting DOOM running on the Multi-device.
If you want to take it further, you could pre-convert all assets to planar format, shrink the number of framebuffers, or cut down on redundant OR operations when drawing tiles.
📜 License
This project is licensed under the GNU General Public License v3.0.
See the LICENSE file for full terms.
🤝 Contributing
Made with ❤️ by SidecarTridge