* With the help of the U-Boot commands (tftpload, protect, erase and cp) we can download and store kernel images, file system images and U-Boot itself in the target system. * The main disadvantage is that this procedure usually requires to physically access the system, attaching to the serial console through a serial cable and using a PC with a terminal emulator software.* Implement automatic upgrade procedures in u-boot, though possible, is not so easy, due to the limited set of commands provided by the u-boot shell. Moreover, U-Boot usually doesn't support all the available storage devices (for example, on a system with both NOR and NAND flash, it's possible that u-boot supports just the NOR, not allowing to program the NAND flash from the command line).