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Dec., 2014
Chapter 4. U-Boot Reference Manual
37
Protected variables
Several variables are of great relevance for the system and are stored in a
protected section of NVRAM.
Some of these protected variables are, for example, the serial number of
the module and the MAC addresses of the network interfaces, which are
programmed during production and normally should not be changed.
4.12 Boot commands
U-Boot runs code placed in RAM, although it also can read from other
media. The boot process normally takes place in two steps:
◆
Reading the OS image from media (Ethernet, flash, USB) into RAM
◆
Jumping to the first instruction of the image in RAM
4.12.1 From Ethernet
The most common way to boot an image during development is by
transferring it using TFTP over the Ethernet interface. You do this with the
tftpboot command, passing:
◆
The address of RAM in which to place the image
◆
The image file name
The TFTP transfer takes place between the serverip address (host) and
the ipaddr address (target). The host must be running a TFTP server and
have bootfilename archive placed in the TFTP-exposed directory.
For Linux kernel images, if the autostart variable is set to yes, this
command directly boots the kernel after downloading it.
4.12.2 From USB
Another way to boot an image is by reading it from a USB flash storage
device. The USB disk must be formatted in FAT file system.
To read an image from a USB flash disk, enter:
4.11.3.2 Common system variables