You will find an example in this thread, reply #1 :
and chap. 31 of Sam3x datasheet is your best friend for PIO programming.
Furthermore, in the folder ...\Arduino15\packages\arduino\hardware\sam\1.6.9\system\CMSIS\Device\ATMEL\sam3xa you can find all register adress defines and also an HTML documentation for every register! It's really great I use it alot!
If you are interested you can use pointers too! But it’s necessary to refer and understand the datasheet first
// Example for just to give you an Idea. Don’t use this as it is
// This is to Write data to the ADC Channel Enable Register
// Memory Address = 0x400C0010
// Register Name = ADC_CHER
// Analog channels that I want to enable = 0,1,2,9
// Page no. 1338 on Datasheet
uint32_t * ADC_CHER = (uint32_t*)0x400C0010 //You can do this declaration on void setup or outside
// Writing values to the register
*ADC_CHER = 0x00000207; //Look in the page 1338 on data sheet.
This method will increase the speed of your code(According to my knowledge)
But be very careful when you are addressing registers