If you can do everything in 16 registers, you don't need RAM. You have to get in the mindset of a control processor application, doing things like reading butto

If you can do everything in 16 registers, you don't need RAM. You have to get in the mindset of a control processor application, doing things like reading buttons on a piece of HP test equipment or sending bytes over the network. You're probably just dealing with a couple of values at a time and don't need RAM. This is a completely different set of applications from a typical computer. As far as addressing modes, the Nanoprocessor has two modes for accessing registers. The first is direct, e.g. store the accumulator in register #3. The second is indexed through R0. Store the accumulator in register # (3 OR R0). This lets you do table lookups (in a small table). Adding to R0 would make more sense, but the Nanoprocessor doesn't have an adder so they use OR. So it makes sense to have multiple addressing modes even without RAM, but there's not a whole lot you can do with them.