The register r0 through r12 are general purpose. Naturally, it easiest to start with r0. As Jivings pointed out, The manual list each address, and states:

Physical addresses range from 0x20000000 to 0x20FFFFFF for peripherals.

0x20200000 happens to be the peripheral address for the OK led

The register r0 through r12 are general purpose. Naturally, it's easiest to start with r0. As Jivings pointed out, The manual list each address, and states:

Physical addresses range from 0x20000000 to 0x20FFFFFF for peripherals.