In message 001901c7f679$40e08480$6401a8c0@hy you wrote:

how to get the kernel or filesystem from long-distance host(not in LAN ) .

U-Boot provides TFTP and NFS for download. For anything else you should use an operating system.

I want to migrate a ftp tool into U-boot at one time.but it seems so hard and I' m not good at program in U-boot.

Foget it. FTP needs TCP/IP, and we will not have this in U-Boot.

maybe.I can get the kernel or filesystem before boot the system.and write the data into RAM.

This can be done. See kexec.

how do I copy the RAM data to ROM(nand flash).can I write the data in linux system if can not do this .

You can use standard commands both in U-Boot and Linux to do this. Please read the docs.

I need shutdown the OS and run a nand flash write program.

Why do you need to shutdown the OS for that purpose? Linux can write to NAND flash just fine.

well ,where is the process counter when the linux system shutdown.

If you perform a reboot, the system will usually start through the reset entry point.

and how to call the nand flash writer.

In Linux? See the MTD system. Un U-Boot? See the nand commands.

Please be aware that in general there is no guarantee that memory contents set up in Linux will survive a reset.

Best regards,

Wolfgang Denk