进入uboot时，在命令行上敲“？” ，回车就会打印出在uboot里可用的命令：
#?
? - alias for 'help'
base - print or set address offset
boot - boot default, i.e., run 'bootcmd'
bootd - boot default, i.e., run 'bootcmd'
bootelf - Boot from an ELF image in memory
bootm - boot application image from memory
bootvx - Boot vxWorks from an ELF image
checkcrash- check ATAGS from crash and dump to file
chpart - change active partition
cmp - memory compare
cp - memory copy
crc32 - checksum calculation
echo - echo args to console
efuse - opt efuse data
env - environment handling commands
exit - exit script
ext4load- load binary file from a Ext2 filesystem
ext4ls - list files in a directory (default /)
false - do nothing, unsuccessfully
fastboot- fastboot
fatinfo - print information about filesystem
fatload - load binary file from a dos filesystem
fatls - list files in a directory (default /)
fatwrite- write file into a dos filesystem
go - start application at address 'addr'
gpio - input/set/clear/toggle gpio pins
help - print command description/usage
i2c - I2C sub-system
jtag - jtag system
loop - infinite loop on address range
md - memory display
mm - memory modify (auto-incrementing address)
mmc - MMC sub system
mmcinfo - display MMC info
mtdparts- define flash/nand partitions
mtest - simple RAM read/write test
mw - memory write (fill)
nand - NAND sub-system
nboot - boot from NAND device
nm - memory modify (constant address)
nu7 - nu7 sub system
pmic - pmic interface
printenv- print environment variables
reset - Perform RESET of the CPU
run - run commands in an environment variable
saveenv - save environment variables to persistent storage
setenv - set environment variables
showvar - print local hushshell variables
source - run script from memory
test - minimal test like /bin/sh
true - do nothing, successfully
ubi - ubi commands
ubifsload- load file from an UBIFS filesystem
ubifsls - list files in a directory
ubifsmount- mount UBIFS volume
ubifsumount- unmount UBIFS volume
usb - USB sub-system
usbboot - boot from USB device
version - print monitor, compiler and linker version
我们想要查看其中某个命令的用法时，在命令行上敲入“命令名 ？”即可，如“mmc ?”:
#mmc ？
mmc - MMC sub system
Usage:
mmc read addr blk# cnt
mmc write addr blk# cnt
mmc erase blk# cnt
mmc rescan
mmc part - lists available partition on current mmc device
mmc dev [dev] [part] - show or set current mmc device [partition]
mmc list - lists available devices
mmc read用来读取mmc内容到内存，
mmc write用来写入内存内容到mmc中
具体用法,
mmc read <device num> addr blk# cnt [partition]
mmc write <device num> addr blk# cnt [partition]
mmc list
device num是mmc的设备号，可以通过mmc list查询
addr是内存地址
blk# 是mmc内的块号
cnt 块数目
使用范例：
mmc write 2 0x90000000 0x600 0x10
把内存0x90000000开始，长度为16x512大小的数据，写入到第2个mmc设备块偏移位置为0x600处
mmc read 2 0x90000000 0x600 0x10
把第二个mmc设备块偏移为0x600处开始，长度为16x512大小的数据，读出到内存0x90000000处
从外置mmc卡load uImage文件： #mmc list SD/MMC: 0 SD/MMC: 1 #mmc dev 0 mmc0 is current device #fatload mmc 0:1 0x82007fc0 uImage #pri #run default_bootargs
只设pmic vdd: pmic vdd 1250000 设置bootargs #setenv bootargs console=ttyS0,115200 root=/dev/mmcblk1p2 rw rootwait gpu=384M mem=792M video=nusmartfb:${resolution}-${dispformat} 706_hw_ver=2 init=/init quiet no_console_suspend 设置default_bootargs #setenv default_bootargs console=ttyS0,115200 root=/dev/mmcblk1p2 rw rootwait gpu=384M mem=792M video=nusmartfb:${resolution}-${dispformat} 706_hw_ver=2 init=/init quiet no_console_suspend #ext4load mmc 1:2 0x82007fc0 uImage
#bootm