Q: the original Super Mario underground theme is literally only a 12 second loop. How did they get away with this?
A: they chopped off some empty spaces in the first 4 bars, which shortens it from what it would have been if the bars had been complete (like in the later versions with drums). and also, the levels are not very long. also, perhaps going so crazy on the last line probably makes up for the shortness. it'd probably take way more than 12 seconds for a person just to process all those crazy note choices in that final line. But i think the more important issue here is not so much 'how' but 'why' was it necessary, and the answer just comes down to technical limitations. surely they must have been working on an unimaginably tight computational budget, compared to today's standards. make every byte count. and when you truly have firm limitations, that's when the miracles just happen. because they have to