U-Boot Commands: Bootloader for Embedded Systems

What is U-Boot?

U-Boot (Das U-Boot) is the most widely used bootloader for embedded systems, especially ARM-based boards. It initializes hardware and boots the operating system.

Key responsibilities:

• Hardware initialization (RAM, clocks, peripherals)
• Loading kernel and device tree to memory
• Passing boot arguments to the kernel
• Providing recovery mechanisms

Basic Commands

# Get help
help
help bootm

# Print environment variables
printenv

# Print specific variable
printenv bootcmd

# Set environment variable
setenv myvar "my value"

# Save environment to flash
saveenv

# Reset the board
reset

# Show board info
bdinfo

# Show memory info
md 0x80000000 100

Environment Variables

Common Variables

# Boot command (executed automatically after bootdelay)
setenv bootcmd 'run nfsboot'

# Delay before auto-boot (seconds)
setenv bootdelay 3

# Kernel arguments
setenv bootargs 'console=ttyS0,115200 root=/dev/mmcblk0p2 rootwait'

# IP configuration
setenv ipaddr 192.168.1.100
setenv serverip 192.168.1.50
setenv netmask 255.255.255.0
setenv gatewayip 192.168.1.1

# MAC address
setenv ethaddr 00:11:22:33:44:55

# Load addresses
setenv kernel_addr 0x82000000
setenv fdt_addr 0x83000000
setenv ramdisk_addr 0x84000000

Save and Load Environment

# Save current environment to storage
saveenv

# Load saved environment
loadenv

# Reset to default environment
env default -a
saveenv

Memory Operations

# Display memory (hex dump)
md 0x80000000 100

# Display as words (32-bit)
md.l 0x80000000 10

# Display as bytes
md.b 0x80000000 100

# Write to memory
mw 0x80000000 0xDEADBEEF

# Copy memory
cp 0x80000000 0x81000000 0x1000

# Compare memory
cmp 0x80000000 0x81000000 0x1000

# Fill memory with pattern
mtest 0x80000000 0x81000000

TFTP Boot (Network Boot)

TFTP is essential for development—load kernel and rootfs over the network.

Setup

# Configure network
setenv ipaddr 192.168.1.100
setenv serverip 192.168.1.50
setenv netmask 255.255.255.0

# Test network
ping 192.168.1.50

Load via TFTP

# Load kernel image
tftp 0x82000000 zImage

# Load device tree
tftp 0x83000000 myboard.dtb

# Load into variable address
tftp ${kernel_addr} zImage

Complete TFTP Boot Script

# Set up TFTP boot command
setenv tftpboot 'setenv bootargs console=ttyS0,115200 root=/dev/nfs nfsroot=${serverip}:/srv/nfs/rootfs,nfsvers=3 rw ip=dhcp; tftp ${kernel_addr} zImage; tftp ${fdt_addr} myboard.dtb; bootz ${kernel_addr} - ${fdt_addr}'

# Run TFTP boot
run tftpboot

SD Card / eMMC Operations

# List MMC devices
mmc list

# Select MMC device
mmc dev 0

# Show MMC info
mmc info

# Read partition table
mmc part

# Rescan for new cards
mmc rescan

Load from FAT Partition

# List files on FAT partition (device 0, partition 1)
fatls mmc 0:1

# Load file from FAT
fatload mmc 0:1 0x82000000 zImage
fatload mmc 0:1 0x83000000 myboard.dtb

Load from ext4 Partition

# List files on ext4 partition
ext4ls mmc 0:2

# Load file from ext4
ext4load mmc 0:2 0x82000000 boot/zImage

Complete SD Boot Script

# Set MMC boot command
setenv mmcboot 'setenv bootargs console=ttyS0,115200 root=/dev/mmcblk0p2 rootwait rw; mmc dev 0; fatload mmc 0:1 ${kernel_addr} zImage; fatload mmc 0:1 ${fdt_addr} myboard.dtb; bootz ${kernel_addr} - ${fdt_addr}'

# Save and run
saveenv
run mmcboot

NAND Flash Operations

# Show NAND info
nand info

# Read from NAND to RAM
nand read 0x82000000 0x100000 0x500000
# (dest_addr, nand_offset, size)

# Write from RAM to NAND
nand write 0x82000000 0x100000 0x500000

# Erase NAND region
nand erase 0x100000 0x500000

# Mark bad block
nand markbad 0x100000

UBI for NAND

# Attach UBI
ubi part ubi

# List UBI volumes
ubi info layout

# Load from UBI volume
ubi read 0x82000000 kernel

# Write to UBI volume
ubi write 0x82000000 kernel ${filesize}

Flashing Firmware

Flash Kernel via TFTP + NAND

# Download kernel
tftp 0x82000000 zImage

# Erase kernel partition
nand erase 0x100000 0x500000

# Write kernel to NAND
nand write 0x82000000 0x100000 ${filesize}

Flash UBI Image

# Download UBI image
tftp 0x82000000 rootfs.ubi

# Erase rootfs partition
nand erase 0x600000 0x1FA00000

# Flash UBI
nand write.trimffs 0x82000000 0x600000 ${filesize}

# Or for UBI update:
ubi part ubi
ubi write 0x82000000 rootfs ${filesize}

Flash eMMC

# Download image
tftp 0x82000000 sdcard.img

# Write to eMMC (raw)
mmc dev 0
mmc write 0x82000000 0 ${filesize}

Boot Commands

# Boot zImage (ARM)
bootz ${kernel_addr} - ${fdt_addr}

# Boot uImage (legacy)
bootm ${kernel_addr} - ${fdt_addr}

# Boot with ramdisk
bootz ${kernel_addr} ${ramdisk_addr} ${fdt_addr}

# Boot from command line
boot

# Boot from specific source
run mmcboot
run nfsboot
run tftpboot

Boot Scripts

# Create multi-line boot script
setenv nfsboot 'setenv bootargs console=ttyS0,115200 root=/dev/nfs nfsroot=${serverip}:/srv/nfs/rootfs,nfsvers=3,tcp rw ip=${ipaddr}::${gatewayip}:${netmask}::eth0:off; tftp ${kernel_addr} zImage; tftp ${fdt_addr} am335x-board.dtb; bootz ${kernel_addr} - ${fdt_addr}'

# Set as default boot command
setenv bootcmd 'run nfsboot'
saveenv

# Run a saved script
run nfsboot

Fallback Boot Logic

# Try SD first, fall back to NAND
setenv bootcmd 'if mmc dev 0; then run mmcboot; else run nandboot; fi'
saveenv

Debugging

# Show version
version

# Show board info
bdinfo

# Show clock configuration
clk dump

# Test memory
mtest 0x80000000 0x81000000

# Check CRC of loaded image
crc32 0x82000000 ${filesize}

# Verify image header
iminfo 0x82000000

# Read device tree info
fdt addr 0x83000000
fdt print

Debug GPIO/Registers

# Read hardware register
md.l 0x44E07000 1

# Write hardware register
mw.l 0x44E07000 0x00000001

# GPIO operations (if supported)
gpio status
gpio set 10
gpio clear 10

Recovery Scenarios

Boot Stops at U-Boot

# Check bootargs
printenv bootargs

# Check bootcmd
printenv bootcmd

# Manually boot
tftp 0x82000000 zImage
tftp 0x83000000 board.dtb
setenv bootargs 'console=ttyS0,115200 root=/dev/mmcblk0p2 rootwait'
bootz 0x82000000 - 0x83000000

Reset to Defaults

# Reset environment to defaults
env default -a

# Save defaults
saveenv

# Or don't save - just boot with defaults this once

Quick Reference

Resources

• U-Boot Documentation
• U-Boot Source Code

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