typedef struct image_header {
uint32_t    ih_magic;    /* Image Header Magic Number(镜像头部幻数,为#define IH_MAGIC    0x27051956    )    */ //幻数:用来标记文件的格式 
uint32_t    ih_hcrc;    /* Image Header CRC Checksum(镜像头部CRC校验码)    */
uint32_t    ih_time;    /* Image Creation Timestamp(镜像创建时间戳)*/
uint32_t    ih_size;    /* Image Data Size(镜像数据大小(不算头部) )    */
uint32_t    ih_load;    /* Data    Load Address(镜像数据将要载入的内存地址)    */ 
uint32_t    ih_ep;      /* Entry Point Address(镜像入口地址)    */
uint32_t    ih_dcrc;    /* Image Data CRC Checksum(镜像数据CRC校验码)    */
uint8_t    ih_os;       /* Operating System(操作系统类型)    */
uint8_t    ih_arch;     /* CPU architecture(CPU架构)    */
uint8_t    ih_type;     /* Image Type(镜像类型)    */
uint8_t    ih_comp;     /* Compression Type(压缩类型)    */
uint8_t    ih_name[IH_NMLEN];    /* Image Name(镜像名字ih_name,共32字节 #define IH_NMLEN    32)    */
} image_header_t;

int do_bootm (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{
ulong    iflag;
ulong    addr;
ulong    data, len, checksum;
ulong *len_ptr;
uint    unc_len = CFG_BOOTM_LEN; 
int    i, verify;
char    *name, *s;
int    (*appl)(int, char *[]);
image_header_t *hdr = &header; //定义头部结构体指针hdr等于header的地址.

s = getenv ("verify"); //读取uboot环境变量verify
verify = (s && (*s == ‘n‘)) ? 0 : 1; //如果verify==n,局部变量verify=0,否则verify=1.

if (argc < 2) { //如果argc==1(只输入了bootm),则使用缺省加载地址load_addr 
addr = load_addr;
} else { //否则使用argv[1](0x30007FC0)为加载地址
addr = simple_strtoul(argv[1], NULL, 16);
}
SHOW_BOOT_PROGRESS (1);
printf ("## Booting image at %08lx ...\n", addr); //打印"## Booting image at 0x30007FC0 ...\n" 

#ifdef CONFIG_HAS_DATAFLASH
if (addr_dataflash(addr)){
read_dataflash(addr, sizeof(image_header_t), (char *)&header);
} else
#endif
memmove (&header, (char *)addr, sizeof(image_header_t)); 
//在加载地址中前64B大小的头部结构体提取到image_header_t结构变量header中,为下面的分析校验做准备

if (ntohl(hdr->ih_magic) != IH_MAGIC) //判断幻数Magic number 是否匹配,不匹配说明下载过程中错误.
{
...
} else
#endif    /* __I386__ */
{
puts ("Bad Magic Number\n");
SHOW_BOOT_PROGRESS (-1);
return 1;
}
}
SHOW_BOOT_PROGRESS (2);

data = (ulong)&header; 
len = sizeof(image_header_t);

checksum = ntohl(hdr->ih_hcrc);
hdr->ih_hcrc = 0;

if (crc32 (0, (uchar *)data    , len) != checksum) { //判断校验和
puts ("Bad Header Checksum\n");
SHOW_BOOT_PROGRESS (-2);
return 1;
}
SHOW_BOOT_PROGRESS (3);

....

#if defined(__PPC__) //判断体系结构,校验CPU类型是否正确
if (hdr->ih_arch != IH_CPU_PPC)
#elif defined(__ARM__)
if (hdr->ih_arch != IH_CPU_ARM)
#elif defined(__I386__)
if (hdr->ih_arch != IH_CPU_I386)
#elif defined(__mips__)
if (hdr->ih_arch != IH_CPU_MIPS)
#elif defined(__nios__)
if (hdr->ih_arch != IH_CPU_NIOS)
#elif defined(__M68K__)
if (hdr->ih_arch != IH_CPU_M68K)
#elif defined(__microblaze__)
if (hdr->ih_arch != IH_CPU_MICROBLAZE)
#elif defined(__nios2__)
if (hdr->ih_arch != IH_CPU_NIOS2)
#elif defined(__blackfin__)
if (hdr->ih_arch != IH_CPU_BLACKFIN)
#elif defined(__avr32__)
if (hdr->ih_arch != IH_CPU_AVR32)
#else
# error Unknown CPU type //没有找到CPU类型
#endif
...
switch (hdr->ih_type) //判断镜像image类型
{ ...}

switch (hdr->ih_comp) //根据镜像压缩(compression)类型把内核镜像解压到指定的地址
{
case IH_COMP_NONE: //使用的是没有压缩,执行该段case
if(ntohl(hdr->ih_load) == data) //该data内核地址刚好位于ih_load加载地址,不需要移动,直接运行
{ 
printf (" XIP %s ... ", name); //打印
} 
else //else执行内核移动,将内核data地址移到 hdr->ih_load (加载地址)中
{ ...
memmove ((void *) ntohl(hdr->ih_load), (uchar *)data, len); 
...}    
break;
case IH_COMP_GZIP:
....
}
...
switch (hdr->ih_os) //根据不同的操作系统类型来启动内核
{ 
case IH_OS_LINUX: //LINUX系统,执行该段case
#ifdef CONFIG_SILENT_CONSOLE
fixup_silent_linux();
#endif
do_bootm_linux(cmdtp, flag, argc, argv,addr, len_ptr, verify); //执行do_bootm_linux()函数启动内核
break;
case IH_OS_NETBSD: //NETBSD系统 
....
....
}

void do_bootm_linux (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[],ulong addr, ulong *len_ptr, int verify) 
{
void (*theKernel)(int zero, int arch, uint params); //定义一个函数指针theKernel
... ...
theKernel = (void (*)(int, int, uint))ntohl(hdr->ih_ep); //1.设置theKernel地址=ih_ep镜像入口地址,用于后面启动内核
... ...
char *commandline = getenv ("bootargs"); 
//commandline指向"bootargs"命令环境参数. 用于后面setup_commandline_tag的形参
//在本uboot界面中输入print指令就能得到"bootargs=noinitrd root=/dev/mtdblock3 init=/linuxrc console=ttySAC0" 
//root=/dev/mtdblock3:表示根文件系统root位于第4个flsh分区(mtdblock3), mtdblock0=bootloader,mtd1=参数,mtd2=内核 
//init=/linuxrc:指定内核启动后运行的第一个脚本是当前目录下linuxrc脚本
//console=ttySAC0:指定选择串口0(ttySAC0)来打印信息、
... ...

/*2.设置tag 参数*/ 
setup_start_tag (bd);           //在0X30000100地址保存start_tag数据，tag:用于u-boot给Linux kernel传递参数数据,因为内核启动后不能使用uboot了.
setup_memory_tags (bd);                   //保存memory_tag数据,让LINUX知道内存多大
setup_commandline_tag (bd, commandline); //保存commandline_tag数据
setup_end_tag (bd);                      //初始化tag结构体结束
....
cleanup_before_linux ();                   //3.启动内核之前需要做一些清理工作,禁止中断,关闭cache

theKernel (0, bd->bi_arch_number, bd->bi_boot_params); 
//4.通过ih_ep镜像入口地址启动内核,然后从0X30000010处读取tag参数，
//其中"bd->bi_arch_number"参数是向内核传递的机器ID，用于内核确定机器ID是否正确,    bd->bi_arch_number是在start_armboot函数中board_init里赋了值

}

static void setup_start_tag (bd_t *bd)
{
params = (struct tag *) bd->bi_boot_params; 

//初始化(struct tag *)型全局变量params= bd->bi_boot_params=0x30000100,
// 之后的memory_tag和commandline_tag等tag数据都保存在params后面的偏移地址. 
params->hdr.tag = ATAG_CORE; //存放srat常量:params->hdr.tag = ATAG_CORE=0x54410001, tag表示tag类型的常量。 
params->hdr.size = tag_size (tag_core); //存放srat长度:params->hdr.size=5, size表示start_tag的结构大小。
//因为tag_size (tag_core)=((sizeof(struct tag_header) + sizeof(struct tag_core)) >> 2)
//其中tag_header结构体里有2个4字节成员(size,tag),
//tag_core结构体里有3个4字节成员(flags,pagesize,rootdev)
//所以tag_size (tag_core)=(2*4+3*4)>>2=5; 单位是4字节
params->u.core.flags = 0;    //存放params的(tag_core型)结构体成员u.core.flags=0
params->u.core.pagesize = 0;//存放params的(tag_core型)结构体成员u.core.pagesize=0
params->u.core.rootdev = 0;//存放params的(tag_core型)结构体成员u.core.rootdev=0

params = tag_next (params); //params指向下一个tag(setup_memory_tags),params=(0x30000100+size*4)=0x30000114 
} 

static void setup_memory_tags (bd_t *bd) 
{
int i;

for (i = 0; i < CONFIG_NR_DRAM_BANKS; i++) {
params->hdr.tag = ATAG_MEM; //存放内存tag常量: params->hdr.tag =ATAG_MEM= 0x54410002 
params->hdr.size = tag_size (tag_mem32); //存放内存长度:params->hdr.size =4 (len+ATAG_MEM+u.mem.size+u.mem.start)

params->u.mem.start = bd->bi_dram[i].start; 
//存放内存(sdram)的的首地址,
// bd->bi_dram[i].start在start_armboot()函数中init_sequence->dram_init结构函数成员里被复制：
// gd->bd->bi_dram[0].start = PHYS_SDRAM_1;其中"PHYS_SDRAM_1"在./include/configs/100ask24x0.h中定义为0X30000000(bank6首地址)
//所以,这里存放内存(sdram)首地址:params->u.mem.start =0X30000000; 
params->u.mem.size = bd->bi_dram[i].size;
//同上,gd->bd->bi_dram[0].size = PHYS_SDRAM_1_SIZE;"PHYS_SDRAM_1_SIZE"被定义为0X04000000(64Mb)
//所以,这里存放内存(sdram)长度: params->u.mem.size=0X04000000; 
params = tag_next (params); //params指向下一个tag(setup_commandline_tag),params=(0x30000114+size*4)=0x30000124 
}
}

static void setup_commandline_tag (bd_t *bd, char *commandline) //commandline:指向"bootargs"命令环境参数
{
char *p;

if (!commandline) // 判断bootargs是否为空,
return;


for (p = commandline; *p == ‘ ‘; p++); //去掉空格

if (*p == ‘\0‘) //判断*p是否为空
return;

params->hdr.tag = ATAG_CMDLINE; //存放命令行产量: params->hdr.tag =ATAG_MEM= 0x54410009 
params->hdr.size =
(sizeof (struct tag_header) + strlen (p) + 1 + 4) >> 2; //存放命令行长度 params->hdr.size 
/* 其中 strlen (p) + 1 + 4: +1表示添加结束符‘/0‘ */
/* +4 表示向上取整,比如当len=(4,5,6,7)时，size=(len+4）>>2=2; 实现4字节对齐 */

strcpy (params->u.cmdline.cmdline, p); 
//存放命令行参数:params->u.cmdline.cmdline=boottargs=noinitrd root=/dev/mtdblock3 init=/linuxrc console=ttySAC0

params = tag_next (params); //params指向下一个tag(setup_end_tag)
}

static void setup_end_tag (bd_t *bd)  
{
params->hdr.tag = ATAG_NONE; //params->hdr.tag =ATAG_NONE=0
params->hdr.size = 0; //size=0
}

int cleanup_before_linux (void)
{
unsigned long i;

disable_interrupts (); //禁止中断
/* turn off I/D-cache */ //关闭 指令Icache和数据Dcache
asm ("mrc p15, 0, %0, c1, c0, 0":"=r" (i));
i &= ~(C1_DC | C1_IC);
asm ("mcr p15, 0, %0, c1, c0, 0": :"r" (i));

/* flush I/D-cache */
i = 0;
asm ("mcr p15, 0, %0, c7, c7, 0": :"r" (i));

return (0);
}