Atari 800XL Gets 3D Graphics Card Upgrade
Despite its age and hardware limitations, element14's Andy West managed to upgrade an Atari 800XL with a 3D graphics card.
The Atari 800XL was an 8-bit home computer released in 1983. It was a true computer that contained 64K of RAM and a full keyboard. But it also had a cartridge slot and Atari's standard video chips from the era, so it could play games. Rudimentary tech demos aside, the Atari 800XL was incapable of displaying 3D graphics — at least as we know them today. It also wasn't expandable in manner that would become commonplace latter, such as through PCI-E slots. Despite those limitations, element14's Andy West managed to upgrade an Atari 800XL with a 3D graphics card.
With this graphics card, the Atari 800XL can display high-resolution 3D graphics via HDMI. Of course, the HDMI standard was decades away from development when the Atari 800XL came out. Instead, the Atari 800XL outputted a low-resolution composite video signal. That means that West had to cheat to make this happen. In this case, the Atari 800XL isn't processing the 3D graphics or the HDMI video output. A Raspberry Pi Compute Module 3+ handles all that heavy lifting.
The Atari 800XL's job is to tell the Raspberry Pi what to do. The Atari 800XL has a handy PBI (Parallel Bus Interface), which provides an unbuffered connection between the system bus and peripherals. Atari intended the PBI for memory modules, disk drives, and so on. Here, West is using it for communication between the Atari 800XL and the Raspberry Pi. A few level shifter ICs facilitate the connections between the PBI and the Raspberry Pi's GPIO pins.
West setup the Pi to accept data, which defines a 3D model, from the Atari 800XL via the PBI. When the Pi gets that data, it renders the 3D model in Circle and displays it over HDMI. The 3D model data resides on a flash cart so the Atari can pull it into memory. The Atari 800XL also sends controller commands over the PBI to the Pi, so West can rotate and zoom in on the model using an Atari controller.
This may be bit of a cheat, since the Atari 800XL is acting as a glorified storage device and controller for the Raspberry Pi. But this is a trick that developers used in the past. For example, Star Fox on the SNES contained a Super FX graphics processor, because the console didn't have the horsepower to display the game's groundbreaking 3D graphics all on its own. So there is precedent and we’ll let the cheat slide.