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The Arduino® bootloader is a small piece of software burned into a chip in all Arduino® boards. Its purpose is to facilitate easy and rapid reprogramming. The Arduino® bootloader is open source, meaning that it can be freely copied, modified, and redistributed, and is only useful on Arduino® systems.
Bootloaders are small pieces of software that run when a computer is first turned on. Most desktop computer use a bootloader to load the computer's operating system into memory or to start the operating system along the path of loading itself. On machines configured to boot multiple operating systems, a bootloader also serves to ask the user to select which operating system to use. Despite their relative simplicity, Arduino® devices are essentially very small general purpose computers and require a bootloader when turned on.
The default Arduino® bootloader runs through a sequence to determine what software to load into short term memory. If the board is attached to a computer by USB cable, Xbee® radio, or serial connection, the bootloader will attempt to download new software from the computer. Arduino® programs are also referred to as Arduino® sketches. Once the sketch is downloaded, it will be written into long-term memory and then loaded back into short term memory to run. If the board is not connected to a computer or if it fails to download new software, it will load the most recently written program from long term memory.
An Arduino® bootloader is especially useful because it eliminates the need for an external programming device, which is usually required to change the software on similar devices. Arduino® inventions usually run their software directly off the chips without the advantage of magnetic storage devices. This approach is faster and cheaper than using magnetic storage. The major disadvantage of a bootloader is that it uses up some of the limited space on chips, minimizing the space available for software.
Some users have been frustrated with the limitations of the Arduino® bootloader and resort to using an external programming device to remove the bootloader and manually add new programs. Although this technique increases the available space and somewhat increases the execution speed of software, it is fairly time consuming and not recommended for most users. Other users have written alternative Arduino® bootloaders, some of which are open source and available for free online.