UBootEnvVariables < DULG

%SECTION0{name=UBootEnvVariables}% U-Boot Environment Variables

The U-Boot environment is a block of memory that is kept on
persistent storage and copied to RAM when U-Boot starts. It is used
to store environment variables which can be used to configure the
system. The environment is protected by a CRC32 checksum.

This section lists the most important environment variables, some of
which have a special meaning to U-Boot. You can use these variables
to configure the behaviour of U-Boot to your liking.

   * ==autoload==: if set to "no" (or any string beginning with 'n'), the
     =rarpb=, =bootp= or =dhcp= commands will perform _only_ a
     configuration lookup from the BOOTP / DHCP server, but not try to
     load any image using TFTP.

   * ==autostart==: if set to "yes", an image loaded using the =rarpb=,
     =bootp=, =dhcp=, =tftp=, =disk=, or =docb= commands will be
     automatically started (by internally calling the =bootm= command).

   * ==baudrate==: a decimal number that selects the console baudrate (in
     bps).  Only a predefined list of baudrate settings is available.
     %BR%
     When you change the baudrate (using the "setenv baudrate ..."
     command), U-Boot will switch the baudrate of the console terminal
     and wait for a newline which must be entered with the _new_ speed
     setting. This is to make sure you can actually type at the new
     speed. If this fails, you have to reset the board (which will
     operate at the old speed since you were not able to =saveenv= the
     new settings.)
     %BR%
     If no "baudrate" variable is defined, the default baudrate of 
     %CONSOLE_BAUDRATE% is used.

   * ==bootargs==: The contents of this variable are passed to the Linux
     kernel as boot arguments (aka "command line").

   * ==bootcmd==: This variable defines a command string that is
     automatically executed when the initial countdown is not
     interrupted.
     %BR%
     This command is only executed when the variable
     =bootdelay= is also defined!

   * ==bootdelay==: After reset, U-Boot will wait this number of seconds
     before it executes the contents of the
     =bootcmd= variable. During this time a countdown is printed,
     which can be interrupted by pressing any key.
     %BR%
     Set this variable to =0= boot without delay. Be careful:
     depending on the contents of your =bootcmd= variable, this can
     prevent you from entering interactive commands again forever!
     %BR%
     Set this variable to =-1= to disable autoboot.

   * ==bootfile==: name of the default image to load with TFTP

   * ==cpuclk==: (Only with MPC859 / MPC866 / MPC885 processors) On some processors,
     the CPU clock frequency can be adjusted by the user (for example to
     optimize performance versus power dissipation). On such systems the
     =cpuclk= variable can be set to the desired CPU clock value, in MHz. If the
     =cpuclk= variable exists and its value is within the
     compile-time defined limits (=CONFIG_SYS_8xx_CPUCLK_MIN= and =CONFIG_SYS_8xx_CPUCLK_MAX=
     = minimum resp. maximum allowed CPU clock), then the specified value is used.
     Otherwise, the default CPU clock value of =CONFIG_8xx_CPUCLK_DEFAULT= is used.

   * ==ethaddr==: Ethernet MAC address for first/only ethernet interface
     (<nop>= =eth0= in Linux).
     %BR%
     This variable can be set only once (usually during manufacturing
     of the board). U-Boot refuses to delete or overwrite this
     variable once it has been set.

   * ==eth1addr==: Ethernet MAC address for second ethernet interface
     (<nop>= =eth1= in Linux).

   * ==eth2addr==: Ethernet MAC address for third ethernet interface
     (<nop>= =eth2= in Linux).
     %BR%
     ...

   * ==initrd_high==: used to restrict positioning of initrd ramdisk images:
     %BR%
     If this variable is not set, initrd images will be copied to the
     highest possible address in RAM; this is usually what you want
     since it allows for maximum initrd size. If for some reason you
     want to make sure that the initrd image is loaded below the
     CFG_BOOTMAPSZ limit, you can set this environment variable to a
     value of "no" or "off" or "0". Alternatively, you can set it to a
     maximum upper address to use (U-Boot will still check that it
     does not overwrite the U-Boot stack and data).
     %BR%
     For instance, when you have a system with 16 MB RAM, and want to
     reserve 4 MB from use by Linux, you can do this by adding
     "mem<nop>=12M" to the value of the "bootargs" variable. However, now
     you must make sure that the initrd image is placed in the first
     12 MB as well - this can be done with
     %BR%
     <pre>
%UBOOT_PROMPT% setenv initrd_high 00c00000
     </pre>
     Setting ==initrd_high== to the highest possible address in your system (0xFFFFFFFF)
     prevents U-Boot from copying the image to RAM at all.
     This allows for faster boot times, but requires a Linux kernel
     with zero-copy ramdisk support.

   * ==ipaddr==: IP address; needed for =tftp= command

   * ==loadaddr==: Default load address for commands like =tftp= or
     =loads=.

   * ==loads_echo==: If set to 1, all characters received during a
     serial download (using the =loads= command) are echoed back. This
     might be needed by some terminal emulations (like =cu=), but
     may as well just take time on others.

   * ==mtdparts==: This variable (usually defined using the
     [[UBootCmdGroupFlash#UBootCmdFlMtdparts][mtdparts]] command) allows to share a common MTD
     partition scheme between U-Boot and the Linux kernel.

   * ==pram==: If the "Protected RAM" feature is enabled in your board's
     configuration, this variable can be defined to enable the
     reservation of such "protected RAM", i. e. RAM which is not
     overwritten by U-Boot. Define this variable to hold the number of
     kB you want to reserve for pRAM. Note that the board info
     structure will still show the full amount of RAM. If pRAM is
     reserved, a new environment variable "mem" will automatically be
     defined to hold the amount of remaining RAM in a form that can be
     passed as boot argument to Linux, for instance like that:
     <pre>
%UBOOT_PROMPT% setenv bootargs ${bootargs} mem=\${mem}
%UBOOT_PROMPT% saveenv
     </pre>
     This way you can tell Linux not to use this memory, either, which
     results in a memory region that will not be affected by reboots.

   * ==serverip==: TFTP server IP address; needed for =tftp= command.

   * ==serial#==: contains hardware identification information such as
     type string and/or serial number.
     %BR%
     This variable can be set only once (usually during manufacturing
     of the board). U-Boot refuses to delete or overwrite this
     variable once it hass been set.

   * ==silent==: If the configuration option ==CONFIG_SILENT_CONSOLE== has been enabled
     for your board, setting this variable to any value will suppress all console
     messages. Please see =doc/README.silent= for details.

   * ==verify==: If set to =n= or =no= disables the checksum calculation
     over the complete image in the =bootm= command to trade speed for
     safety in the boot process.  Note that the header
     checksum is still verified.

The following environment variables may be used and automatically
updated by the network boot commands (=bootp=, =dhcp=, or =tftp=),
depending the information provided by your boot server:

   * ==bootfile==: see above
   * ==dnsip==: IP address of your Domain Name Server
   * ==gatewayip==: IP address of the Gateway (Router) to use
   * ==hostname==: Target hostname
   * ==ipaddr==: see above
   * ==netmask==: Subnet Mask
   * ==rootpath==: Pathname of the root filesystem on the NFS server
   * ==serverip==: see above
   * ==filesize==: Size (as hex number in bytes) of the file downloaded
     using the last =bootp=, =dhcp=, or =tftp= command.
5.9.10. Miscellaneous Commands		1. Abstract		5.11. U-Boot Scripting Capabilities
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