一般利用一张SD卡就能进行系统的更新，以前一直不知是什么原理，最近了解了下，对了解到的内容做个记录。使用的是AM335X平台，系统是Linux，文件系统是EXT3：
1、首先需要一张分好分区的SD卡（分区方法这里不做详细介绍，一般可使用TI官方提供的create-sdcard.sh脚本可以进行分区）。其中分区1（FAT32）存放MLO、u-boot.img、uImage三个文件；分区2（EXT3）存放需要从SD卡启动的Linux文件系统。
2、AM335X上电后，根据Boot Sequence启动配置（LCD0-LCD15引脚，具体可参考TI官方的335X参考手册），从相应的存储设备启动，这里配置的是从SD卡启动。
3、AM335X上电后首先执行固化在芯片内部ROM中的代码，我们称之为BLO，BLO的功能很强大，能读取SD卡中的第一个分区（必须是FAT格式）的MLO文件，复制到AM335X内部的RAM中吞下，复制的起始地址为0x402f0400，所以编译MLO时，连接地址必须设置为0x402F0400。
4、一般的，MLO的作用是：禁用中断、关看门狗、设置栈、初始化时钟、初始化DDR3、初始化串口、最后复制S卡中boot（FAT32）分区的u-boot.img到DDR3的0x80000000（内存）处运行。MLO的最大体积可以达到109K，对于简单的逻辑程序绰绰有余了。
5、u-boot的功能主要是启动内核：u-boot从SD卡的boot(FAT32)分区复制uImage到内存运行。
6、启动Linux后挂载SD卡的rootfs分区（EXT3）的文件系统
7、文件系统是利用Busybox工具集创建的，首先内核会启动BusyBox里的Init进程。init进程首先主要对/etc/inittab文件进行解析，然后按照它的指示创建各种子进程。
8、inittab文件的内容主要以下几部分。其中系统运行后最先执行的是/etc/rc.d/rcS进程，并且它只执行一次，其它进程只有等它执行完毕后才能运行，u-boot、内核、文件系统的烧写在这里执行。
1 ::sysinit:/etc/rc.d/rcS 2 ::respawn:/sbin/getty 9600 tty1 3 ::ctrlaltdel:/sbin/reboot 4 ::shutdown:/etc/rc.d/rcS stop 5 ::restart:/sbin/init
9、接着继续看到/etc/rc.d/rcS文件，这是一个脚本文件，下面一一分析它做的工作
#!/bin/sh
# minimal startup script, will work with msh (this is best available in
# MMUless format).
mode=${1:-start} #若第一个参数存在，则mode的值为第一个参数；否则mode等于start
# load the configuration information
if [ -x /etc/rc.d/rc.conf ]
then
. /etc/rc.d/rc.conf #不管/etc/rc.d/rc.conf是否可执行，都去执行它，等价于sh /etc/rc.d/rc.conf
fi
if [ $mode = "start" ] #若mode的值为start，
then
services=$cfg_services #那么services等于$cfg_services（这个值在/etc/rc.d/rc.conf中定义："crond mount-proc-sys hostname depmod modules udev filesystems inetd"）
else
services=$cfg_services_r #否则services等于$cfg_services_r （这个值在/etc/rc.d/rc.conf中定义："inetd filesystems udev modules depmod hostname mount-proc-sys crond"）
fi
cfg_services=${2:-$services} #若第二个参数存在，则cfg_services的值为第二个参数；否则cfg_services等于services
# run the configured sequence #按配置的顺序，启动各个进程
for i in $cfg_services
do
if [ -x /etc/rc.d/init.d/$i ] #如果进程是可执行的，那么
then
echo "/etc/rc.d/init.d/"$i $mode #打印进程信息
/etc/rc.d/init.d/$i $mode #挨个执行
fi
done
if [ $# -ge 2 ] #如果参数个数大于等于2
then
exit 0 #退出
fi
if [ $mode = "start" ] #若mode的值为start，
then
# show all kernel log messages
if [ -x /proc/sys/kernel/printk ] #若/proc/sys/kernel/printk可执行
then
#echo 8 > /proc/sys/kernel/printk
echo 0 > /proc/sys/kernel/printk #清空/proc/sys/kernel/printk
fi
# echo set kernel halt time
echo -e "\033[9;0]" > /dev/tty0
#echo Bringing lo up
/sbin/ifconfig lo 127.0.0.1 up 2>/dev/null
/sbin/route add -net 127.0.0.0 netmask 255.0.0.0 gw 127.0.0.1 dev lo 2>/dev/null
#echo eth0 setting #网卡0设置
if [ -x /etc/eth0-setting ]
then
echo "Found /etc/eth0-setting..."
/etc/rc.d/ifconfig-eth0 /etc/eth0-setting #在/etc/eth0-setting文件中配置IP、MASK、GATEWAY信息
fi
#echo eth1 setting #网卡1设置
if [ -x /etc/eth1-setting ]
then
echo "Found /etc/eth1-setting..."
/etc/rc.d/ifconfig-eth1 /etc/eth1-setting #在/etc/eth1-setting文件中配置IP、MASK、GATEWAY信息
fi
#echo wlan0 setting #wlan0设置
if [ -x /etc/wlan0-setting ]
then
echo "Found /etc/wlan0-setting..."
wpa_supplicant -B -D wext -c /etc/wpa.conf -i wlan0
wpa_cli add_network
wpa_cli enable_network 0
/etc/rc.d/ifconfig-wlan0 /etc/wlan0-setting
fi
fi
# run rc.local if present and executable
if [ -x /etc/rc.d/rc.local ] #如果/etc/rc.d/rc.local可执行
then
echo "Found /etc/rc.d/rc.local..."
/etc/rc.d/rc.local $mode #那么，直接执行它。整个启动脚本的重点在这里，后面分析它
fi
# run startx if present and executable
if [ -x /usr/bin/startx ]
then
if [ $mode = "start" ]
then
rm /tmp/.X0-lock
startx
fi
fi
/etc/rc.d/rcS脚本可以总结为如下的功能：
a、配置脚本运行的环境变量，文件为/etc/rc.d/rc.conf
b、运行系统服务进程：crond mount-proc-sys hostname depmod modules udev filesystems inetd
c、设置网络IP/etc/rc.d/ifconfig-eth0 /etc/eth0-setting、/etc/rc.d/ifconfig-eth1 /etc/eth1-setting
d、运行/etc/rc.d/rc.local脚本程序
10、分析/etc/rc.d/rc.local start脚本程序
#!/bin/sh
mode=${1:-start} #mode等于传入的参数1为start
if [ $mode = "start" ] #如果mode的值等于start，那么运行以下程序
then
#set sgtl5000 volume 0~127
amixer cset numid=5 65 #猜测是设置蜂鸣器音量，还不确定
echo "start format board mmc..."
echo 1 > /sys/class/leds/user_buzzer/brightness #打开蜂鸣器，说明开始运行此脚本更新系统
/etc/rc.d/spi.sh #运行/etc/rc.d/spi.sh脚本，把/boot/u-boot.spi.512拷贝到Flash中，以后程序从此处取出u-boot运行
以下为/etc/rc.d/spi.sh脚本内容：
#!/bin/sh
echo "****************************************************"
echo "****************************************************"
echo ""
echo " Hua Ming spi Flashing Script - 01/01/2015"
echo ""
echo "erase spi flash..."
flash_erase /dev/mtd0 0 8
echo "write boot/u-boot.spi.512 ..."
flashcp /boot/u-boot.spi.512 /dev/mtd0
echo "done"
/etc/rc.d/mmc.sh /dev/mmcblk1 #运行/etc/rc.d/mmc.sh脚本对mmc以ext3文件系统格式进行分区，分为4个区分别为sys、opt、user、Reserved。
mount /dev/mmcblk1p1 /mnt #挂接sys分区，将rfs_335x.tar.gz的文件进行解压缩，里面存放了最小根文件系统
cd /mnt
tar -xzvf ../rfs_335x.tar.gz
mount /dev/mmcblk1p2 /mnt/opt #挂接opt分区，将HmReader.tar.gz解压缩，里面存放了应用程序
tar -xzvf ../../HmReader.tar.gz
cd boot
ln -s boot.scr.mmc1 boot.scr
sync
echo 0 > /sys/class/leds/user_buzzer/brightness #蜂鸣器关闭
echo "format finished ..." #格式化结束，更新完成
fi
/etc/rc.d/rc.local start脚本程序功能总结如下：
1、将/boot/u-boot.spi.512拷贝到Flash中，在从spi启动时启动u-boot。
2、对MMC1进行分区，分为四个区分别为sys、opt、user、Reserved
3、将最小根文件系统解压缩到sys分区
4、将应用程序解压缩到opt分区
11、对/etc/rc.d/mmc.sh脚本的分析：
#!/bin/sh
echo "****************************************************"
echo "****************************************************"
echo ""
echo " Hua Ming Example Flashing Script - 01/01/2015"
echo ""
if [[ -z $1 ]] #如果传入脚本的参数1不存在的话
then
echo "mmc.sh Usage:"
echo " mmc.sh <Dev>"
echo " Example: mmc.sh /dev/mmcblk0"
exit #退出脚本
fi
STARTTIME=$(date +%s) #获得当前时间
##---------Start of variables---------------------##
## Declare eMMC device name here
#DRIVE="/dev/mmcblk0"
DRIVE=$1 #DRIVE=/dev/mmcblk1
##----------End of variables-----------------------##
## Kill any partition info that might be there
dd if=/dev/zero of=$DRIVE bs=4k count=1 #猜测是清除设备原有的分区
sync
sync #马上写入设备
## Figure out how big the eMMC is in bytes
SIZE=`fdisk -l $DRIVE | grep Disk | awk '{print $5}'` #算出块设备的大小（以字节为单位）
## Translate size into segments, which traditional tools call Cylinders. eMMC is not a spinning disk.
## We are basically ignoring what FDISK and SFDISK are reporting because it is not really accurate.
## we are translating this info to something that makes more sense for eMMC.
CYLINDERS=`echo $SIZE/255/63/512 | bc` #作数学运算算出一个柱面的大小
## Check to see if the eMMC partitions have automatically loaded from the old MBR.
## This might have occured during the boot process if the kernel detected a filesystem
## before we killed the MBR. We will need to unmount and kill them by writing 4k zeros to the
## partitions that were found.
#检查是否已经被挂载，并且已经分区。若有，则清除
check_mounted(){
is_mounted=$(grep ${DRIVE}p /proc/mounts | awk '{print $2}')
if grep -q ${DRIVE}p /proc/mounts; then
echo "Found mounted partition(s) on " ${DRIVE}": " $is_mounted
umount $is_mounted
counter=1
for i in $is_mounted; do \
echo "4k erase on ${DRIVE}p${counter}";
dd if=/dev/zero of=${DRIVE}p${counter} bs=4k count=1;
counter=$((counter+1));
done
else
echo "No partition found. Continuing."
fi
}
check_mounted;
## Partitioning the eMMC using information gathered.
## Here is where you can add/remove partitions.
## We are building 2 partitions:
## 1. FAT, size = 9 cylinders * 255 heads * 63 sectors * 512 bytes/sec = ~70MB
## 2. EXT3, size = 223 ($CYLINDERS-[9 for fat]) cylinders * 255 heads * 63 sectors * 512 bytes/sec = ~1l.7GB
##
## You will need to change the lines ",9,0c0C,*", "10,,,-" to suit your needs. Adding is similar,
## but you will need to be aware of partition sizes and boundaries. Use the man page for sfdisk.
echo "Partitioning the eMMC..."
sfdisk -D -H 255 -S 63 -C $CYLINDERS $DRIVE << EOF
,65,0x0c,*
65,65,,-
130,130,,-
260,,,-
EOF
## This sleep is necessary as there is a service which attempts
## to automount any filesystems that it finds as soon as sfdisk
## finishes partitioning. We sleep to let it run. May need to
## be lengthened if you have more partitions.
sleep 2
## Check here if there has been a partition that automounted.
## This will eliminate the old partition that gets
## automatically found after the sfdisk command. It ONLY
## gets found if there was a previous file system on the same
## partition boundary. Happens when running this script more than once.
## To fix, we just unmount and write some zeros to it.
check_mounted;
## Clean up the dos (FAT) partition as recommended by SFDISK
#dd if=/dev/zero of=${DRIVE}p1 bs=512 count=1
## Make sure posted writes are cleaned up
sync
sync
## Format the eMMC into 2 partitions
echo "Formatting the eMMC into 4 partitions..."
mkfs.ext3 -L "sys" ${DRIVE}p1
mkfs.ext3 -L "opt" ${DRIVE}p2
mkfs.ext3 -L "user" ${DRIVE}p3
mkfs.ext3 -L "Reserved" ${DRIVE}p4
## Make sure posted writes are cleaned up
sync
sync
echo "Formatting done." #格式化完成
## The block below is only necessary if using 'dd'.
## Force check the filesystem for consistency and fix errors if any.
## Resize partition to the length specified by the MBR.
## /sbin/e2fsck -fy ${DRIVE}p2
## /sbin/resize2fs ${DRIVE}p2
ENDTIME=$(date +%s)
echo "It took $(($ENDTIME - $STARTTIME)) seconds to complete this task..." #计算格式化EMMC花了多长时间
## Reboot
echo ""
echo "********************************************"
echo " Hua Ming Example Flash Script is complete."
echo ""
总结为以ext3文件系统格式对MMC进行分区，分为4个区分别为sys、opt、user、Reserved。其中sys存放最小根文件系统，opt存放应用程序，后续可以对它单独进行更新。
12、到这里所有系统更新完成，拔掉SD卡，重新上电后根据Boot Sequence启动配置（LCD0-LCD15引脚，具体可参考TI官方的335X编程手册），从相应的存储设备启动，这里配置的是SPI接口的Flash启动。说启动可能不准确，其实是内部的ROM从SPI的flash中拷贝镜像到RAM中运行。然后启动u-boot，u-boot启动后会从MMC的/sys分区的/boot目录下取出uImage镜像再到RAM中运行，接着Linux系统挂接到/sys系统。然后整个系统正常运行。