I'm a high level app developer (C#, python) and I'd like to get my hands dirty with low level embedded application development. Cambridge University has posted a series of tutorials teaching you how to create an operating system for the Raspberry Pi called Baking Pi. I've intended to post my question regarding this tutorial here and update the answers as I go along. Maybe even post some points that I figured out myself.

So I've been following along with lesson 1 and I'm confused already. Here is the page link.

The first part of the tutorial say we need to (I assume) turn on the GPIO controller by writing the value 0x20200000 to register r0. I cannot find such relevant information anywhere in the data sheet. (link). I've looked at the GPIO section but I didn't see any indication of 16 registers or r0. Where is this information?