In a feat of retro-engineering that blends biology with computing, Linux developer Matthew Garrett has successfully infected a classic Commodore Amiga with "Cordoomceps"—a parasitic Linux system that hijacks the vintage hardware solely to run id Software’s 1993 shooter Doom. The project exploits the PiStorm, an adapter board designed by Claude Schwarz that bridges a Raspberry Pi’s GPIO to the Motorola 68000 bus in Amiga motherboards.

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The PiStorm CPLD board, acting as the 'fungal' link between Raspberry Pi and Amiga hardware. (Credit: The Register)

Typically, PiStorm accelerates aging Amigas by emulating the 68000 CPU while adding modern peripherals. Garrett, however, weaponized it differently: "We can run whatever we want on the Pi and then access Amiga hardware. And, obviously, the thing we want to run is Doom." His goal? To bypass the Amiga OS entirely and boot directly into the game—effectively turning the machine into a Doom-dedicated appliance.

The execution required surgical precision. First, Garrett leveraged ADoom (a 1997 Amiga port) and Chocolate Doom for compatibility. But critical hurdles emerged:
1. Kickstart Dependency: The Amiga’s ROM-based Kickstart firmware had to initialize under 68000 emulation first. Skipping this left hardware in "a weird state."
2. Memory Lock: The system couldn’t write to RAM until executing code but couldn’t execute code without knowing where the code resided—a circular dilemma.

Garrett’s breakthrough came from exploiting an obscure hardware feature: overlaying ROM contents onto the zeroth address. "Poorly documented," he noted, "because it’s not something you need to care about if you execute Kickstart... and I’m only in this position because I’ve made poor life choices." Pulsing the reset line then unlocked the Amiga’s RAM, allowing the Raspberry Pi to seize control.

The result? A visually glitchy but functional Doom session, rendered on the Amiga’s native display—no sound yet, but a proof-of-concept that redefines retro hacking. "Further updates," Garrett warns, "will likely make things even worse."

This project underscores the enduring allure of Doom as a porting benchmark and showcases how modern SBCs can breathe unnervingly specific new life into legacy systems. For Garrett, the Amiga isn’t just emulated—it’s possessed.

Source: Matthew Garrett's write-up | Project Code (GitLab) | Original reporting via The Register