EmbLogic's Blog

The Universal Boot Loader (“Das U-Boot”)

The Universal Boot Loader (“Das U-Boot”)
Table of contents:
1. Abstract
? 1.1. Introduction
? 1.2. History
? 1.3. Supported Hardware
? 1.4. Design Principles
? 1.5. User Interface
? 1.6. Basic Command Set
1.7. Advanced Commands
? 1.7.1. Logbuffer Manipulation Commands
? 1.7.2. Bedbug Embedded Debugger Commands
? 1.7.3. POST – Hardware Diagnose Commands
? 1.8. Environment Variables
? 1.9. Boot Options
? 1.10. Command Interpreters
? 1.11. Standalone Programs
1.12. Special Features
? 1.12.1. Bitmap and Splash Screen Support
? 1.12.2. Boot Count Limit
? 1.12.3. Keyboard Support
? 1.12.4. Automatic Updates

1.1. Introduction
• The “Universal Bootloader” (“Das U-Boot”) is a monitor program.
• Free Software: full source code under GPL
• hosted on SourceForge: http://sourceforge.net/projects/u-boot
• production quality: used as default boot loader by several board vendors
• portable and easy to port and to debug
• many supported architectures: PPC, ARM, MIPS, x86, m68k, NIOS, Microblaze
• more than 216 boards supported by public source tree
• many, many features
1.1. Introduction 1
1.2. History
• Oct 22, 1999: fadsrom – Dan Malek => PPCBoot rev. 1.1
• Dec 18, 1999: 8xxrom-0.3.0 – Magnus Damm, Raphael Bossek => PPCBoot rev. 1.2
• Jul 07, 2000: Wolfgang Denk => PPCBoot rev. 1.3
Jul 19, 2000: Wolfgang Denk => PPCBoot-0.4.1
first public version of PPCBoot

Siemens PSE, Vienna: Development of a Bluetooth LAN Access Point with a MPC850 Processor that
needed to be able to boot over Ethernet => first commercial sponsor

Aug 08, 2000: PPCBoot rev. 1.4 = PPCBoot-0.4.2
(only PPC, only MPC8xx, 4 boards)

• Oct 01, 2000: added network support => PPCBoot-0.4.4
• Oct 01, 2000: Stefan Roese: add support for IBM PPC401/403/405GP processors => PPCBoot-0.5.1
• Nov 16, 2000: Murray Jensen: add support for MPC8260 => PPCBoot-0.6.2
• Nov 20, 2000: Rob Taylor: add support for MPC8240 => PPCBoot-0.6.3
• End 2000: PPCBoot-0.7.1 (MPC8xx, MPC8240, MPC8260, PPC401/403/405GP; 27 boards)
• End 2001: PPCBoot-1.1.3 (MPC8xx, MPC8240, MPC8260, 7xx, 74xx, IBM 4xx, 63 boards)
• Mar 2002: SYSGO: split ARMBoot project, separate (incompatible) source tree
• Jul 2002: begin merging with ARMBoot tree
Nov 2002: PPCBoot-2.0.0 (last release of PPCBoot)
(PPC: 8xx, 824x, 826x, 7xx, 74xx, 4xx; ARM: StrongARM, ARM7, ARM9, XScale; >106 boards)
=> Start U-Boot project: PPCBoot-2.0.0 = U-Boot-0.1.0

• Nov 2002: x86 support
• Mar 2003: MIPS32
• Apr 2003: MIPS64
• Oct 2003: Altera NIOS-32
• Dec 2003: Coldfire
• Apr 2004: Microblaze
today (31 May 2004): U-Boot-1.1.2
(PPC: 5xx, 5xxx, 8xx, 824x, 826x, 85xx, 7xx, 74xx, 4xx;
ARM: StrongARM, ARM720T, ARM92xT, S3C44B0, AT91RM9200, XScale;
x86: SC520; m68k: Coldfire; MIPS32: 4Kc, Au1x00; MIPS64: 5Kc; NIOS32; Microblaze;
>216 boards in public tree; many more not submitted back

• several board manufacturers use U-Boot as default firmware on some or all of their boards
1.3. Supported Hardware
Architecture Processor Number of Boards
PPC 5xx 2
5xxx 6
8xx 71
824x 15
826x 26
85xx 3
1.3. Supported Hardware 2
7xx/74xx 11
4xx 38
ARM StrongARM 5
ARM720T 3
ARM92xT 11
S3C44B0 1
AT91RM9200 1
XScale 8
x86 SC520 2
m68k Coldfire 2
MIPS32 4Kc 2
Au1x00 3
MIPS64 5Kc 1
NIOS32 3
Microblaze 1
Blackfin BF5xx 18
1.4. Design Principles
• easy to port to new architectures, new processors, and new boards
• easy to debug: serial console output as soon as possible
• features and commands configurable
• as small as possible
• as reliable as possible
1.5. User Interface
U-Boot uses a simple command line interface (CLI), usually over a serial console port.
Two different command interpreters are available:
• simple CLI
• Bourne compatible shell (HUSH shell from Busybox)
Configuration parameters and commands / command sequences (scripts !) can be stored in “environment
variables” which can be saved to non-volatile storage (flash, EEPROM, NVRAM, etc.)
1.5. User Interface 3
1.6. Basic Command Set
Information Commands
? bdinfo – print Board Info structure
? coninfo – print console devices and informations
? flinfo – print FLASH memory information
? iminfo – print header information for application image
? imls – list all images found in flash
? help – print online help

Memory Commands
? base – print or set address offset
? crc32 – checksum calculation
? cmp – memory compare
? cp – memory copy
? md – memory display
? mm – memory modify (auto-incrementing)
? mtest – simple RAM test
? mw – memory write (fill)
? nm – memory modify (constant address)
? loop – infinite loop on address range

Flash Memory Commands
? cp – memory copy (program flash)
? flinfo – print FLASH memory information
? erase – erase FLASH memory
? protect – enable or disable FLASH write protection

Execution Control Commands
? autoscr – run script from memory
? bootm – boot application image from memory
? bootelf – Boot from an ELF image in memory
? bootvx – Boot vxWorks from an ELF image
? go – start application at address ‘addr’

Network Commands
? bootp – boot image via network using BOOTP/TFTP protocol
? cdp – Perform Cisco Discovery Protocol network configuration
? dhcp – invoke DHCP client to obtain IP/boot params
? loadb – load binary file over serial line (kermit mode)
? loads – load S-Record file over serial line
? nfs – boot image via network using NFS protocol
? ping – send ICMP ECHO_REQUEST to network host
? rarpboot- boot image via network using RARP/TFTP protocol
? tftpboot- boot image via network using TFTP protocol

Environment Variables Commands
? printenv- print environment variables
? saveenv – save environment variables to persistent storage
? askenv – get environment variables from stdin
? setenv – set environment variables
? run – run commands in an environment variable
? bootd – boot default, i.e., run ‘bootcmd’

Filesystem Support (FAT, cramfs, JFFS2, Reiser)
? chpart – change active partition

1.6. Basic Command Set 4
? fsinfo – print information about filesystems
? fsload – load binary file from a filesystem image
? ls – list files in a directory (default /)
? fatinfo – print information about filesystem
? fatls – list files in a directory (default /)
? fatload – load binary file from a dos filesystem
? nand – NAND flash sub-system
? reiserls- list files in a directory (default /)
? reiserload- load binary file from a Reiser filesystem
Special Commands
? i2c – I2C sub-system
? doc – Disk-On-Chip sub-system
? dtt – Digital Thermometer and Themostat
? eeprom – EEPROM sub-syste
? fpga – FPGA sub-system
? ide – IDE sub-system
? kgdb – enter gdb remote debug mode
? diskboot- boot from IDE device
? icache – enable or disable instruction cache
? dcache – enable or disable data cache
? diag – perform board diagnostics (POST code)
? log – manipulate logbuffer
? pci – list and access PCI Configuraton Space
? regdump – register dump commands
? usb – USB sub-system
? sspi – SPI utility commands

Miscellaneous Commands
? bmp – manipulate BMP image data
? date – get/set/reset date & time
? echo – echo args to console
? exit – exit script
? kbd – read keyboard status
? in – read data from an IO port
? out – write datum to IO port
? reset – Perform RESET of the CPU
? sleep – delay execution for some time
? test – minimal test like /bin/sh
? version – print monitor version
? wd – check and set watchdog
? ? – alias for ‘help’

1.7. Advanced Commands
Some of the following commands depend on certain hardware features and may not be available on all boards.
1.7. Advanced Commands 5
1.7.1. Logbuffer Manipulation Commands
Use a reserved area of system memory as log buffer;
can be re-used by syslogd in Linux;
content will survive reset / warm boot
• log info – show pointer details
• log log reset – clear contents
• log log show – show contents
• log log append – append to the logbuffer
• setenv stdout log – redirect standard output to log buffer
Used for example to pass POST results to Linux application code or for post-mortem checking of the Linux
system logs.
1.7.2. Bedbug Embedded Debugger Commands
• ds – disassemble memory
• as – assemble memory
• break – set or clear a breakpoint
• continue – continue from a breakpoint
• step – single step execution.
• next – single step execution, stepping over subroutines.
• where – Print the running stack.
• rdump – Show registers.
1.7.3. POST – Hardware Diagnose Commands
• cache – Cache test
• watchdog – Watchdog timer test
• i2c – I2C test
• rtc – RTC test
• memory – Memory test
• cpu – CPU test
• uart – UART test
• ethernet – ETHERNET test
• spi – SPI test
• usb – USB test
• spr – Special register test
• sysmon – SYSMON test
• dsp – DSP test
1.8. Environment Variables
Environment Variables (EV) can be used to configure the system and to store parameters for commands, and
1.8. Environment Variables 6
even commands and sequences of commands (simple scripts).
Board Configuration
baudrate, ethaddr, serial#, cpuclk

Startup Behaviour
bootdelay, bootcmd

Network Parameters
ipaddr, serverip, gatewayip, dnsip, netmask, hostname, rootpath, bootfile

autoload (network lookup without download); autostart (start downloaded image); pram
(reserve “protected RAM” area); silent (supress console messages); verify (disable CRC32
checks); …

User Defined
=> setenv name word1 word2 word3
=> saveenv

The real power of EVs results from the fact that Unix shell like variable expansion is available. For example:
=> setenv ipaddr
=> setenv serverip
=> setenv netdev eth0
=> setenv hostname testbox
=> setenv rootpath /opt/eldk/ppc_8xx
=> setenv ramargs setenv bootargs root=/dev/ram rw
=> setenv nfsargs ‘setenv bootargs root=/dev/nfs rw nfsroot=${serverip}:${rootpath}’
=> setenv addip ‘setenv bootargs ${bootargs} ip=${ipaddr}:${serverip}:${gatewayip}:${netmask}:${hostname}:${netdev}:off’
=> setenv kernel_addr 40040000
=> setenv ramdisk_addr 40100000
=> setenv flash_ram ‘run ramargs addip;bootm ${kernel_addr} ${ramdisk_addr}’
=> setenv flash_nfs ‘run nfsargs addip;bootm ${kernel_addr}’
=> setenv net_nfs ‘tftp 200000 ${bootfile};run nfsargs addip;bootm’
=> setenv net_ram ‘tftp 200000 ${bootfile};run ramargs addip;bootm 200000 ${ramdisk_addr}’
Boot Kernel Image in flash with ramdisk in flash:
=> run flash_ram
Boot Kernel Image in flash with root filesystem over NFS:
=> run flash_nfs
Download Kernel Image over network and use root filesystem over NFS:
=> run net_nfs
Download Kernel Image over network with ramdisk in flash:
=> run net_ram
Step by step:
=> run flash_nfs
1.8. Environment Variables 7
Step Action Result in bootargs
setenv bootargs root=/dev/nfs rw
root=/dev/nfs rw
run addip
setenv bootargs ${bootargs}
root=/dev/nfs rw
40040000 boot Linux kernel
Example: simple recovery strategy:
=> setenv bootcmd ‘run flash_ram; setenv kernel_addr ${alt_kernel}; run flash_ram; run net_ram’
1.9. Boot Options
U-Boot supports many different ways to load and boot an image.
Basic command:
“bootm” – Boot Image in memory (RAM, flash)
Header + Payload
? Creation Timestamp
? Data Load Address
? Entry Point Address
? Data CRC Checksum
? Operating System
? CPU architecture
? Image Type
? Compression Type
? Image Name
? test CPU architecture and OS
? test checksum (optional)
? if compressed, uncompress
? copy to load address
? prepare boot arguments
? start at entry point
Load image in memory:
? Serial Port: “loads” (S-Record), “loadb” (Kermit binary protocol)
1.9. Boot Options 8
? Ethernet: “tftp”, “bootp”, “dhcp”, “nfs”, …
? Harddisk, CDROM: “ide read”
? CompactFlash card etc.: “ide read”
? USB Mass Storage Device: “usb read”
? SCSI Disk and CDROM: “scsi read”
? NAND flash with JFFS2 filesystem: “nboot”
? Disk on Chip: “doc read”
? PCI Bus: copy
? …
Supported Filesystems (read-only):
? Reiser
1.10. Command Interpreters
Two command line interfaces:
Simple (old) command interpreter:
? sequential statements
? statements separated by newline or ‘;’
no conditional execution except simple builtin rules:
“run cmd1; run cmd2; run cmd3″
will always run all three commands
“run cmd1 cmd2 cmd3″
will stop when a command fails
? “scripts” (canned sequences of commands) avaialble using “autoscr” command

Hush Shell (from Busybox, see http://www.busybox.net/):
? Bourne Shell compatible
(local) shell variables (“name=val”), (global) environment variables (“setenv name
? Conditionals: “if … then … else … fi”
Control loops: “for … do … done”, “while … do done”, “until …
do … done”
? Control operators: && and || (AND and OR lists: “command1 && command2″)
? real shell scripts
? no functions
? no command substitution
? no backquotes

1.11. Standalone Programs
1.11. Standalone Programs 9
U-Boot can dynamically load independent software modules, called “standalone programs”. Standalone
programs have a standard C calling environment, and can use standard services like printf(), malloc(),
Used for:
• special test software that is used in bring-up but shall not be included with customer release
• software that is needed only occasionally
• code that performs special actions that were not foreseen (software updates)
• code that shall not be made available under GPL
int hello_world (int argc, char *argv[])
int i;
printf (“Example expects ABI version %d\n”, XF_VERSION);
printf (“Actual U-Boot ABI version %d\n”, (int)get_version());
printf (“Hello World\n”);
printf (“argc = %d\n”, argc);
for (i=0; i<=argc; ++i) {
printf ("argv[%d] = \"%s\"\n",
argv[i] ? argv[i] : "”);
return (0);
=> tftp 40000 /tftpboot/hello_world.bin

=> go 40004 Hello World! This is a test.
## Starting application at 0×00040004 …
Hello World
argc = 7
argv[0] = “40004″
argv[1] = “Hello”
argv[2] = “World!”
argv[3] = “This”
argv[4] = “is”
argv[5] = “a”
argv[6] = “test.”
argv[7] = “”
## Application terminated, rc = 0×0
1.12. Special Features
• Bitmap and Splash Screen Support
• Boot Count Limit
• Keyboard Support
1.12. Special Features 10
• Automatic Updates
1.12.1. Bitmap and Splash Screen Support
Problem: booting Linux and starting a GUI takes a couple of seconds, but the user expects to see something
“immediately” after power-on.
Solution: display a static splash screen as soon as possible; available commands:
• bmp info – print Bitmap info
• bmp display – display bitmap image on screen
• setenv splashimage addr – display spash screen image at address addr
1.12.2. Boot Count Limit
The Open Source Development Labs Carrier Grade Linux Requirements Definition says:
CGL shall provide support for detecting a repeating reboot cycle due to recurring failures and will go to an
offline state if this occurs.
U-Boot allows to run an arbitrary command in such a case:
• bootcount (EV) – number of reboots since power-on
• bootlimit (EV) – maximum number of reboot cycles
• altbootcmd (EV) – alternate boot action
1.12.3. Keyboard Support
Problem: make system behaviour dependent on keys pressed at power-on
Solution: in U-Boot you can define one or more keys or key combinations and commands which are executed
when these keys are pressed at power-on:
“magic_keys” (EV) – List of characters for keys; for example:
=> setenv magic_keys 0123CB*

“key_magic_?_” (EV) – Key code or list of key codes for this action; scanned in the order as
listed in magic_keys; for example:
=> setenv key_magic0 3a+3b
=> setenv key_magic1 3c
=> setenv key_magic2 4a
=> setenv key_magic3 4c+51
=> setenv key_magicC 3a+4a+5a
=> setenv key_magicB 52+53
=> setenv key_magic* 55+56

• “key_cmd?” (EV) – Action to be performed when corresponding key(s) are pressed; for example:
1.12.3. Keyboard Support 11
=> setenv key_cmd0 setenv bootdelay 10
=> setenv key_cmd1 setenv addcons ‘console=ttyS0,${baudrate}’
=> setenv key_cmd2 setenv memtest on
=> setenv key_cmd3 setenv bootcmd run old_version
=> setenv key_cmdC setenv bootcmd run recovery
=> setenv key_cmdB setenv bootcmd run usb_update
=> setenv key_cmd* setenv bootcmd run demo_mode
Example: after a software update, the user can select to boot the old software version (“run old_version”) by
holding the keys with keycodes “4c” and “51″ at power-on.
1.12.4. Automatic Updates
Distribute Software Updates to customers
use cheap standard media like USB memory sticks
• Keep BOM (Versions, Timestamps etc.) in persistent memory (EEPROM etc.)
• When booting, check whether a USB memory stick is plugged in.
If one is found:
If prepare.img is found load it into memory.
If it is valid then run it (always).
If preinst.img is found load it into memory.
If it is valid then run it. Update the EEPROM.
If firmware.img is found load it into memory.
If it is valid, burn it into FLASH and update the EEPROM.
If kernel.img is found load it into memory.
If it is valid, burn it into FLASH and update the EEPROM.
If app.img is found load it into memory.
If it is valid, burn it into FLASH and update the EEPROM.
If disk.img is found load it into memory.
If it is valid, burn it into FLASH and update the EEPROM.
If postinst.img is found load it into memory.
If it is valid then run it. Update the EEPROM.

1.13. Resources, Summary
• The U-Boot project is hosted at Sourceforge: http://sourceforge.net/projects/u-boot
• Tarballs can be found either at sourceforge.net or at the DENX ftp server.
• There is a pretty active u-boot-users mailing list.
The Mailing list archive can be viewed at sourceforge.net (if it works – it is regularly broken, so don’t
get discouraged and try again later).

The DENX U-Boot and Linux Guide is a Wiki based documentation documenting U-Boot and its
interaction with Linux. It can be viewed (and improved) at www.denx.de. The whole DULG web

1.13. Resources, Summary 12
packed into a single HTML page or a PDF file is also available (TQM8xxL version, i.e. PowerPC
• The current README file can be viewed through viewcvs at sourceforge.net
Sample configuration files for the Abatron BDI2000 debugger are available at the FTP Server at

• Even more useful links can be found in the DENX Training wiki documentation at www.denx.de.
• U-Boot is a very active community project.
It lives from the spritit of Free Software. It would be impossible without the numberless contributions
of other developers.

1.13. Resources, Summary 13

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>