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That said, the emulation community has traditionally treated BIOS files as “abandonware” because Sega no longer manufactures Dreamcast hardware. While Sega is lenient, it’s still technically piracy. For this guide, we focus only on how they work—not where to obtain them. sega dreamcast bios files work
There are generally two ways emulators handle this: How Sega Dreamcast BIOS files work — concise