What is the data register address for GPIO pin 21 Cortex ARM M0?
You should probably just use the function provided in the header files.

#include &lt;nrf_gpio.h&gt;
nrf_gpio_cfg_output()
nrf_gpio_pin_set()
nrf_gpio_pin_clear()


If you want to see how the registers are defined, read the definitions of these functions in the header files.

There are some examples in the SDK, which I've ranted about before: http://markrages.tumblr.com/post/48981903493  Maybe the examples are better now.