Super Mario Bros. cannot be played on the Xbox, because it is a Nintendo series. Nintendo would never allow its series to be released on the Xbox (or the PlayStation, for that matter).
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Because Mario belongs to Nintendo and Nintendo make their own consoles, it is very unlikely that Nintendo will allow Microsoft to make a Mario game for the Xbox.
Yes. Your xbox may have to be modded before allowing unrestricted download. -- No, but you can install mods on your xbox.
NO.
You can download it in the video marketplace.
No emulator has been released for Xbox 360