Comment by mid-kid

gah, you're right, I was thinking of memory protection (as in, marking the relevant regions as read-write and read-execute) when I wrote MMU.

It's of course optional, and you can ignore it for trivial examples, but most games and SDKs will tweak it all the time when loading additional code modules from the cartridge.

It's just another way in which the DS is more complex to use properly without an SDK to do this for you - there's just more to think about. At least compared to how the GBA lacks all of this and the entire cartridge is mapped into memory at all times.