If it boot on gpu, then everything is explained ?
"don't take any risk (see what I did there ?) with the booting chip" on top of "things must not be peripherical dependant" which combos in "booting chip must not be peripherical depending"
This booting on gpu is surprising, but not totally from outer space when you considering pi's ARM nature :) @Substring
I think it's possible to achieve what your usual computer does. There's no impossibility, but a lot to learn before doing that : to do this right, you must take this before kernel. I would have to document myself on that before coming with questions. IRQ is already little known territory to me, but ARM board IRQ may be very much alien space in my Windows guy's eyes : I learnt how from a pile of sand making something this is driven by hardware or asm drivers 20 years ago, there should be remains :)
If you want the exact mapping of the N64, you need to edit yourself the buttons and switch to a custom config. What you need is :
edit the files in /recalbox/share/configs/mupen (or mupen64 or mupen64plus, i never remember)
add in the recalbox.conf and add at the bottom (or wheverever else) n64.configfile=dummy so that the files you edited won't be overwritten
This will be slightly easier in Recalbox 4.1
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