User`s manual

Getting Started With Linux on the LPC3250 OEM Board Page 26
Copyright 2009 © Embedded Artists AB Rev A
4 Universal Boot Loader - u-boot
4.1 Introduction
The Universal Boot Loader also known as Das U-boot or u-boot for short is an open-source boot
loader that supports a wide range of different architectures such as ARM, PowerPC, XScale, x86,
MIPS, Coldfire, 68k, and MicroBlaze. Many board configurations have been made available, for each
architecture, by an active community. The u-boot boot loader has actually become the most widely
used boot loader on ARM based systems.
Besides supporting a wide range of architectures the u-boot also supports a wide range of booting
options. Below is a list of some of these booting options:
• From Flash memory (for example NOR or NAND)
• From a USB mass storage device
• From an MMC/SD memory card
• From a harddisk or CDROM
• Using Ethernet: TFTP, BOOTP, DHCP or NFS
• Using a serial connection
A booting option means a location from where the u-boot searches for the kernel image to load. If a
MMC/SD card has been selected the u-boot will initialize the memory card controller and try to read the
image(s) from that device.
4.2 Console / Environment
The u-boot supports a command line interface usually accessed via a terminal application, such as
Tera Term. The terminal application is connected to the serial port associated with the development
board. The command line interface allow you to manually type in boot commands or update the
environment variables that can later be used as boot options.
4.2.1 Commands
It is possible to discover which commands are available by using the help command.
uboot> help
When issuing the help command a list of all the available commands will be presented. These
commands are the ones that have been selected to be supported when configuring the u-boot. If more
information is needed about a specific command type help followed by the name of the command.
The example below gives you more information about the setenv command.
uboot> help setenv
Below is a list of some of the commands used to modify, list and execute variables in the u-boot
environment.
• printenv – This command will print the u-boot environment.
• setenv – This command is used to set the value of a variable. If the variable doesn’t exist
when calling setenv it will be created. If calling setenv on a variable, but not specifying a
value the variable will be deleted.
•
saveenv – This command will save any changes done to the environment and must be
called after setenv has been used in order for the changes to be saved persistently.