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How can I add them?
I have DOOM.WAD, DOOM2.WAD and SIGIL_v1_21.wad but these are not shown by DIG.
I'd also like to know if there's a solution to this.
Not sure if there's currently a way to do this through retroarch right now. My current solution is to use the Dreamcast homebrew port which I found a couple of CDI format images out in the wilds of the world wide web which both come with a multitude of compatible wad packs.