Starting from the IDE version 1.6.2, only AVR Arduino boards are installed by default. Some Arduino boards require an additional Core to be installed
One example is the Arduino Due, which uses an ARM/SAM microcontroller. In order for the Arduino IDE to be able to program an Arduino Due, you need to install the SAM Core using the Boards Manager
In this example we will install the core required by the Arduino Due board.
Click on Tools menu and then Boards > Boards Manager.
The board manager will open and you will see a list of installed and available boards.
Select the SAM core, choose the version in the drop-down menu and click on install.
The download time will depend on your connection speed.
After installation is complete an Installed tag appears next to the core name. You can close the Board Manager.
Now you can find the new board in the Board menu.
The text of the Arduino getting started guide is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the guide are released into the public domain.