[U-Boot] Need answers of basic questions regarding u-boot

[Jerry Van Baren]

On 08/07/2011 02:55 PM, Rakesh Modi wrote:
> Hi,
> I am new to linux and u-boot. Please answer my following questions.
> 1) what is u-boot? why it require?

It initializes and configures the hardware (processor, RAM, 
peripherals), it loads linux (or other OS) from storage (flash, hard 
drive, ethernet, other) into RAM and then passes control to linux.

While it isn't required, it is very convenient to have it.  The other 
features of u-boot are that u-boot can be used to load new linux images 
into your storage device.  The general principle is that u-boot is not 
often replaced on a board, but the linux image is updated and upgraded 
regularly.

> 2) why we can not load linux kernel directly without use of u-boot?

You can, but you need to do the hardware initialization and 
configuration in the linux start up code.  You will also have to create 
a method to and mechanism for linux to upgrade itself, preferably in a 
"failure resistant" way so that you don't "brick" your board 
inadvertently by having a failure when upgrading linux.

U-boot has solved these problems already for you.

> 3) How does any processor sequence starts when power is on?

That is described in the processor hardware reference manual.  It is 
different for each processor, but in general the processor starts 
fetching and executing code from a known location - sometimes from a 
fixed memory address, sometimes it fetches a pointer from a fixed memory 
address and starts executing from the address pointed to.

Best regards,
gvb