Thank you very much for explanation both graeme and FengHua.

I think i have got best answer from you Graeme from entire web.

Thanks Tejas

On Tue, Dec 3, 2013 at 4:03 AM, Graeme Russ graeme.russ@gmail.com wrote:

Hi,

On Tue, Dec 3, 2013 at 1:18 AM, FengHua fenghua@phytium.com.cn wrote:

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Hi,

I can not understand why u-boot relocate it self to RAM from flash? Why

is

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it not executing from flash? Can anybody explain.

First, flash is slow. Second, there are many data structures or variables which should be writable. So, we should copy u-boot to RAM and relocate it.

I'll expand a bit...

In theory the writeable could be relocated into RAM, but this would require the relocation address to be hard-coded. Optimally, U-Boot places itself in a memory location to provide the maximum contiguous memory space to load the operating system kernel and supporting data (RAM disks, FDTs, etc).

On most systems now, relocation is done by calculating the delta between the ROM address (TEXT_BASE) and the relocated address in RAM. The relocation code first copies the entire U-Boot image to RAM (apart from a few structures that are used only during the relocation calculation). It then scans the relocation entries (*.rel sections) to find where in the U-Boot executable all the references into the data sections. At each reference point in the executable, the relocation code modifies the reference by the relocation offset. None of this can happen if the U-Boot executable is in Flash.

The other advantage of relocating U-Boot to RAM is that it makes it possible to flash a new U-Boot image from U-Boot. If you try to do this while U-Boot is running from flash, it will crash as you are overwriting the executable code while it's executing.

Regards,

Graeme