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Please tell me where can i find hello_world.bin file

In the "examples" directory.

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and where should it be put on the host system to be accessible from u-boot based target system with

tftpboot command.

That depends on how your tftp server was set up. See http://www.denx.de/twiki/bin/view/DULG/SystemSetup#Section_4.6.

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It will be quite helpful for me if you can stepwise describe the procedure

as I

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am unable to understand it from the denx tutorial.

The procedure is already described "stepwise". See http://www.denx.de/twiki/bin/view/DULG/UBootStandalone#Section_5.12.1.

Now, everyone, all together..."Please don't post HTML", that's a good way to get your message ignored.

Regards, -Craig