The F4’s 70 GPIO pins can run via internal peripherals ... so things worked just fine. Simple Game Boy cartridges like Tetris were able to directly map a ROM device into the Game Boys memory ...

[PJ Evans] had a ruined game cartridge lying around ... When he came across a couple of 9-pin D-sub joystick ports, he had an idea. He realized his Rasperry Pi Zero could fit inside the cartridge.