That's something I was also curious about and it turns out Arduinos use the Harvard architecture. You might say Arduinos are not really "computers" but after a bit of googling I found an Apple II emulator running on an Arduino and, well, an Apple II is generally accepted to be a computer :)
if i remember it correctly the main difference of the "harvard"-architecture was: it uses its own data and program/instruction buses ...
* https://en.wikipedia.org/wiki/Harvard_architecture
i think texas instruments 320x0 signal-processors used this architecture ... back in, you guessed it!, the 1980ties.
* https://en.wikipedia.org/wiki/TMS320
ah, they use a modified harvard architecture :))
* https://en.wikipedia.org/wiki/Modified_Harvard_architecture
cheers!