R4 cartridge

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The R4 (also known as Revolution for DS) is an unlicensed flash cartridge for the Nintendo DS handheld system. It allows ROMs and homebrew to be booted on the Nintendo DS handheld system from a microSD card. This allows the user to run homebrew applications, to store multiple games and MP3 music files on a single memory card, and to play games that have been backed up by the user.

The R4 flashcard's original developer stopped production, however, due to the popularity of the original R4 there are many other Nintendo DS flashcards using the R4 name despite the lack of any connection to the original. Some of these cartridges can also store a 3DS boot image; a 3DS can in turn be made to boot from the cartridge by way of an undocumented button combination recognized by the system's bootloader. Since the 3DS's secure boot signature validation has been broken, this provides a useful means of installing custom firmware on a 3DS.^[1]