In short: amateurs will try to run Doom on anything, but the new Commodore 64 conversion is unique. The effort uses a Raspberry-based app to run the game surprisingly well, since none of the hardware can work alone. The mod includes instructions, so Commodore owners can set up a demo version and run other Raspberry enhanced software for themselves.
Modder “frntc” recently introduced an expansion cartridge for the Commodore 64, which allows the iconic home computer and its relatives to work with programs once considered unimaginable for an 8-bit platform. What better way to test a device than to make it play Doom?
Technically, there is already a Linux-based port of Commodore Doom. Despite the necessary SuperCPU upgrade, it works with degraded colors and barely playable frame rate due to the complete lack of optimization for old hardware.
A comparison of the version on the frntc’s RAD expansion unit shows the difference at night and during the day. The amateur system uses a copy of Doom, which is similar to the low-detail mode of the DOS version, running at 320×200 resolution. Created for PAL CRT, it runs at 50 frames per second — significantly faster than the original game loaded under DOS.
Based on the Raspberry Pi, the expansion cartridge does most of the work, bypassing the Commodore CPU in favor of the ARM RP processor. The C64 converts the frame buffer to output an image to the display, processes input using the keyboard and mouse, and transmits an impressive 22050 Hz audio sample through its SID chip.
Expansion chips that improve the basic capabilities of the platform were quite common for computers and game consoles in the 80s and 90s. The Super NES version of Doom uses the SuperFX chip, which was best known for allowing the console to display 3D polygons in the original Star Fox.
Curious users can buy or assemble RAD cartridges using FRNTC instructions. The device, which supports Commodore 64 and 128, is available in two interchangeable versions: one is based on the dimensions of Raspberry Pi 3A+/3B+, and the other is modeled after Raspberry Pi Zero 2. Some components require a 3D printer to be assembled. Retro-store.de (German) offers ready-made blocks at a price of about 20 euros.
A version of Doom for RAD is also available on GitHub frntc with simple instructions for starting the game. This requires a shareware version of Doom on doom1.wad.