The Boot-Loader is a code which executes when a microcontroller is powered ON or reset. It basically sets an environment for the application code to execute. It is the Boot-Loader that sets the hardware and loads the application code from any storage medium or received through external communication and let the application to execute. Thus a Boot-Loader has to perform the following basic function : Initialize the controller peripherals, Initialize the devices in the board, Allow an option for the user to select from the available applications to load, Load the selected application, Let the application code to execute.
Apart from the above mentioned functions some Boot-Loaders can perform many other functions and there are Boot-Loaders which don’t perform all these functions like provide option to select the required application etc. The Boot-Loader codes in microcontrollers are actually very small and simple compared to the Boot-Loaders in advanced devices like PC. In most of the microcontroller the functionality of a Boot-Loader is limited only to set the initial clock and other settings for the microcontroller, load an application binary from the serial port etc.