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With a Switch 2 looming, Nintendo is sending out the original with a bang
(www.theverge.com)
This is a most excellent place for technology news and articles.
The 3DS had an ARM9 processor on the board which is what the DS line used so that it could play DS games natively.
The DSi actually has an ARM7 coprocessor (GBA CPU) just like the DS and DS Lite but lacks the physical cartridge slot. You can play GBA games natively on a hacked DSi.
Actually, the 3DS has an ARM7 core too so it can play GBA games natively as well. Either the Nintendo Ambassador titles, or whatever you want if you've installed CFW.