Yes, that's a Game Boy Color cartridge sporting the Wolfenstein 3D logo. It's not just cosmetic, because a modder named Anders Granlund has built a playable version of the classic FPS for the ancient handheld console. To give you an idea of the degree of difficulty, Granlund designed and built a custom ARM-powered board to power the graphics, and built it into the cartridge. The final result is playable on any Game Boy Color.
Wolfenstein 3D and its '90s brethren Doom, have been modded for a number of unlikely devices, including a TI calculator and Canon printer. And in fact, Wolfenstein 3D was actually ported to the Game Boy Advance directly from the MS-DOS version.
However, this mod is on another level. Granlund used a breadboard to program the EEprom himself, then ordered a custom board, complete with pinouts. Using those, he added an NXP graphics processor, equipped with an ARM Cortex-M0 running at 48MHz. That chip is obviously more powerful than the Intel 386 CPU that originally ran Wolfestein on MS-DOS.
Once all that worked, Granlund ordered another custom board combining his original design and the NXP processor. After more tweaks, he ordered and received the final Rev. C cartridge, saying "everything works as expected and ... I didn't need any bodge wires this time around." The whole thing is, of course, powered strictly with the Game Boy Color's batteries.
The result is a game that plays amazingly smoothly on the 160 x 144 pixel screen, as you can see in the video above. A Reddit user perhaps sums it up best: "This is hella impressive. I mean, I'm absolutely in awe of the technical skill required to do this."
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