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@Normmatt has created a way to run B9S .firm files from bootrom via a DSi Flashcard and a magnet! This works on every 3DS on any firmware version.
For installation without a PC, user @TheCyberQuake has created a pack which will automatically install B9S and copy over essential starter homebrew from the flashcard's SD to the 3DS's. This will mainly be used for PC-less B9S installations. If you have a PC with you, use 3ds.guide. Read more here: https://gbatemp.net/threads/481141/
How does this work?
This works because of a flaw in the bootrom. Before the bootrom boots the NAND, it checks to see if Start+Select+X is held down, and if the shell is closed. If these requirements are met, it will boot an NDS cartridge from the bootrom. This gives that cartridge bootrom access. You might be wondering how you'd hold down buttons while the shell is closed, and why you need a magnet. If you put a magnet in a specific spot on the 3DS, it will go into sleep mode. Using this, you can boot the NDS cartridge with the buttons held down while in sleep mode! Using a reflashable flashcard, you can boot B9SInstaller using the flashcard, and easily install it on your 3DS.
https://gbatemp.net/threads/ntrboot-released.472585/
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