In embedded system, differently in general PC, general firmware like CMOS does not exist.
So to boot embedded system for the first time, we have to make bootloader which adjusted well to target board.

Bootloader plays a very important part in embedded system. We explain the roles of bootloader simply below.

Copy kernel to RAM from flash memory, and execute kernel.
Initialize hardware.
Bootloader have the function that writing data to flash memory.
(Downloading kernel or Ram disk by serial port or other network hardware, data is stored in RAM. But RAM lost all data downloaded if you cut power supply, so to avoid this work you have to store to flash memory.)
It provides interface to send commands to target board or to inform users state of target board.

短了一些，不过几个要点都说到了。 看看E文的也蛮不错。