ishaqe ahamed wrote:

what is the purpose of u-boot?????????????????

To initialize the hardware and run (boot) the operating system of your choice. The board's CPU runs u-boot code from the release of reset until the CPU jumps to the OS code (often linux).

The above is only a fraction of what u-boot _can_ do, but is the primary (original?) reason for its existence.

Lots more background: http://www.denx.de/wiki/DULG/Manual

Best regards, gvb