Both of the motion sensors on the micro:bit, the magnetometer and the accelerometer, are packaged in a single component: the LSM303AGR integrated circuit. These two sensors can be accessed via an I2C bus. Each sensor behaves like an I2C target and has a different address.

Each sensor has its own memory where it stores the results of sensing its environment. Our interaction with these sensors will mainly involve reading their memory.

The memory of these sensors is modeled as byte addressable registers. These sensors can be configured too; that's done by writing to their registers. So, in a sense, these sensors are very similar to the peripherals inside the microcontroller. The difference is that their registers are not mapped into the microcontrollers' memory. Instead, their registers have to be accessed via the I2C bus.

The main source of information about the LSM303AGR is its Data Sheet. Read through it to see how one can read the sensors' registers. That part is in:

Section 6.1.1 I2C Operation - Page 38 - LSM303AGR Data Sheet

The other part of the documentation relevant to this book is the description of the registers. That part is in:

Section 8 Register description - Page 46 - LSM303AGR Data Sheet