_TEXT_BASE:
 .word TEXT_BASE //這里是獲得代碼段的起始地址，我的是0x33F80000(在board/xxx/config.mk中
                  //可到找到“TEXT_BASE=0x33F80000”
.globl lowlevel_init //這里相當于定義一個全局的lowlevel_init以方便調(diào)用

lowlevel_init:
 /* memory control configuration */
 /* make r0 relative the current location so that it */
 /* reads SMRDATA out of FLASH rather than memory ! */
 ldr r0, =SMRDATA //SMDATA表示這 13個寄存器的值存放的開始地址，值為0x33F8xxxx,處于內(nèi)
                       //存中，這一句的作用是把其值加載到r0中
 
 ldr r1, _TEXT_BASE // 把代碼的起始地址(0x33F80000)加載到r1中

 sub r0, r0, r1 //r0減去r1其結(jié)果存入r0，也即SMDATA中的起始地址0x33F8xxxx減去
                       //0x33F80000，其結(jié)果就是13個寄存器的值在NOR Flash存放的開始地址

 ldr r1, =BWSCON /* Bus Width Status Controller */ //存儲控制器的基地址
 add r2, r0, #13*4 //在計算出來的存放地址加上#13*4，然后其結(jié)果保存在r2中
                           //13 個寄存器，每個寄存器占4個字節(jié)

0:
 ldr r3, [r0], #4 //內(nèi)存中r0的值加載到r3中，然后r0加4，即下一個寄存器的

 str r3, [r1], #4 //讀出寄存器的值保存到r1中，然后r1也偏移4

 cmp r2, r0 //比較r0與r2的值，如果不等繼續(xù)返回0:執(zhí)行，也即13個寄存器的值
                           // 是否讀完
 bne 0b
 /* everything is fine now */
 mov pc, lr //程序跳轉(zhuǎn)，返回到cpu_init_crit中
 .ltorg
/* the literal pools origin */
SMRDATA:
...................

relocate:                /* 將U-Boot復(fù)制到RAM中     */
    adr    r0, _start        /* r0：當前代碼的開始地址 */
    ldr    r1, _TEXT_BASE        /* r1:代碼段的連接地址*/
    cmp r0, r1 /* 測試現(xiàn)在是在FLash中，還在是RAM中，如果要從NandFlash啟動的話，這里要根據(jù)需要修改 */
    beq stack_setup /*如果已經(jīng)在RAM中，則不需要復(fù)制*/

    ldr    r2, _armboot_start /*_armboot_start在前面定義，是第一條指令的運行地址*/
    ldr    r3, _bss_start /*在連接腳本U-Boot.lds中定義，是代碼段的結(jié)束地址*/
    sub    r2, r3, r2        /* r2 <- 代碼段長度 */
    add    r2, r0, r2        /* r2 <-代碼段的結(jié)束地址 */

copy_loop:
    ldmia     {r3-r10}        /* 從地址[r0]處獲得數(shù)據(jù) */
    stmia     {r3-r10}        /* 復(fù)制到地址[r1]處 */
    cmp    r0, r2            /* 判斷是否復(fù)制完畢 */
    ble    copy_loop /*沒有復(fù)制完，則繼續(xù)*/
#endif    /* CONFIG_SKIP_RELOCATE_UBOOT */

stack_setup:
    ldr    r0, _TEXT_BASE        /* _TEXT_BASE 為代碼段的開始地址，值為0x33F80000 */
    sub    r0, r0, #CONFIG_SYS_MALLOC_LEN    /* 代碼段下面，留出一段內(nèi)存以實現(xiàn)malloc */
    sub    r0, r0, #CONFIG_SYS_GBL_DATA_SIZE /* 再留出一段內(nèi)存，存一些全局參數(shù) */
#ifdef CONFIG_USE_IRQ
    sub    r0, r0, #(CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ)
#endif
    sub    sp, r0, #12        /* 最后，留出12字節(jié)的內(nèi)存給abort異常 */

clear_bss:
    ldr    r0, _bss_start        /* 下面的都是棧 */
    ldr    r1, _bss_end        /* stop here */
    mov    r2, #0x00000000        

clear_bss:
    ldr    r0, _bss_start        /* BSS段的開始地址，它的值在連接腳本中U-Boot.lds中確定 */
    ldr    r1, _bss_end        /* BSS段的結(jié)束地址，它的值在連接腳本u-Boot.lds中確定 */
    mov    r2, #0x00000000        /* clear */

clbss_l:str    r2, [r0]        /* 向BSS段中寫入0值 */
    add    r0, r0, #4
    cmp    r0, r1
    ble    clbss_l

ldr    pc, _start_armboot

_start_armboot:    .word start_armboot

int dram_init (void)
{
    gd->bd->bi_dram[0].start = PHYS_SDRAM_1;
    gd->bd->bi_dram[0].size = PHYS_SDRAM_1_SIZE;

    return 0;
}

#define CONFIG_SETUP_MEMORY_TAGS 1
#define　CONFIG＿CMDLINE＿TAG　1

#if defined(CONFIG_BOOTDELAY) && (CONFIG_BOOTDELAY >= 0)
    s = getenv ("bootdelay");   //得到環(huán)境變量中bootdelay
    bootdelay = s ? (int)simple_strtol(s, NULL, 10) : CONFIG_BOOTDELAY;

    debug ("### main_loop entered: bootdelay=%d\n\n", bootdelay);


#ifdef CONFIG_BOOTCOUNT_LIMIT //啟動次數(shù)的限制功能，如果到達一定次數(shù)，將不能啟動u-boot.
    if (bootlimit && (bootcount > bootlimit)) {//檢查是否超出啟動次數(shù)限制
        printf ("Warning: Bootlimit (%u) exceeded. Using altbootcmd.\n",
         (unsigned)bootlimit);
        s = getenv ("altbootcmd");//啟動延時
    }
    else
#endif /* CONFIG_BOOTCOUNT_LIMIT */
        s = getenv ("bootcmd");// 獲得啟動參數(shù)

    debug ("### main_loop: bootcmd=\"%s\"\n", s ? s : "<UNDEFINED>");
// 這里如果bootdelay大于0，并且中間沒有被中斷的話，執(zhí)行命令行參數(shù)
    if (bootdelay >= 0 && s && !abortboot (bootdelay)) {
# ifdef CONFIG_AUTOBOOT_KEYED
  int prev = disable_ctrlc(1); /* disable Control C checking */
# endif
# ifndef CONFIG_SYS_HUSH_PARSER
  run_command (s, 0); //運行啟動的命令行，例如 可以使用tftp命令
# else
  parse_string_outer(s, FLAG_PARSE_SEMICOLON |
        FLAG_EXIT_FROM_LOOP);
# endif


#define CONFIG_BOOTM_LINUX 1
#define CONFIG_BOOTM_NETBSD 1
#define CONFIG_BOOTM_RTEMS 1

#ifdef CONFIG_BOOTM_LINUX
extern boot_os_fn do_bootm_linux;
#endif
#ifdef CONFIG_BOOTM_NETBSD
static boot_os_fn do_bootm_netbsd;
#endif

void (*theKernel)(int zero, int arch, uint params);

theKernel = (void (*)(int, int, uint))images->ep;


typedef struct bootm_headers {
     ............................

    int        fit_noffset_fdt;/* FDT blob subimage node offset */
#endif

#ifndef USE_HOSTCC
    image_info_t    os;        /* os image info */
    ulong        ep;        /* entry point of OS */

    ulong        rd_start, rd_end;/* ramdisk start/end */
...............

}

theKernel (0, machid, bd->bi_boot_params);

int do_bootm (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{
    ulong        iflag;
    ulong        load_end = 0;
    int        ret;
    boot_os_fn    *boot_fn;
#ifndef CONFIG_RELOC_FIXUP_WORKS
    static int relocated = 0;
    /* relocate boot function table */
    if (!relocated) {
        int i;
        for (i = 0; i < ARRAY_SIZE(boot_os); i++)
            if (boot_os[i] != NULL)
                boot_os[i] += gd->reloc_off;
        relocated = 1;
    }
#endif
    /* determine if we have a sub command */
    if (argc > 1) {
        char *endp;

        simple_strtoul(argv[1], &endp, 16);
    
        if ((*endp != 0) && (*endp != ':') && (*endp != '#'))
            return do_bootm_subcommand(cmdtp, flag, argc, argv);
    }

    if (bootm_start(cmdtp, flag, argc, argv)) //提取mkimage生成的文件頭部，放到bootm_headers_t結(jié)構(gòu)體中
        return 1;

    iflag = disable_interrupts();

#if defined(CONFIG_CMD_USB)
       usb_stop();
#endif

#ifdef CONFIG_AMIGAONEG3SE
    /*
     * We've possible left the caches enabled during
     * bios emulation, so turn them off again
     */
    icache_disable();
    dcache_disable();
#endif

    ret = bootm_load_os(images.os, &load_end, 1); //加載操作系統(tǒng)的關(guān)鍵部分 確定使用的地址
    if (ret < 0) { //出錯處理
        if (ret == BOOTM_ERR_RESET)
            do_reset (cmdtp, flag, argc, argv);
        if (ret == BOOTM_ERR_OVERLAP) {
            if (images.legacy_hdr_valid) {
                if (image_get_type (&images.legacy_hdr_os_copy) == IH_TYPE_MULTI)
                    puts ("WARNING: legacy format multi component "
                        "image overwritten\n");
            } else {
                puts ("ERROR: new format image overwritten - "
                    "must RESET the board to recover\n");
                show_boot_progress (-113);
                do_reset (cmdtp, flag, argc, argv);
            }
        }
        if (ret == BOOTM_ERR_UNIMPLEMENTED) {
            if (iflag)
                enable_interrupts();
            show_boot_progress (-7);
            return 1;
        }
    }
    lmb_reserve(&images.lmb, images.os.load, (load_end - images.os.load));

    if (images.os.type == IH_TYPE_STANDALONE) {//獨立的應(yīng)用程序
        if (iflag)
            enable_interrupts();
        /* This may return when 'autostart' is 'no' */
        bootm_start_standalone(iflag, argc, argv);
        return 0;
    }
    show_boot_progress (8);

#ifdef CONFIG_SILENT_CONSOLE //這里處理Linux操作系統(tǒng)
    if (images.os.os == IH_OS_LINUX)
        fixup_silent_linux(); //該函數(shù)中處理bootarg參數(shù)
#endif
    boot_fn = boot_os[images.os.os];
    if (boot_fn == NULL) {
        if (iflag)
            enable_interrupts();
        printf ("ERROR: booting os '%s' (%d) is not supported\n",
            genimg_get_os_name(images.os.os), images.os.os);
        show_boot_progress (-8);
        return 1;
    }

    arch_preboot_os();
/*下面的函數(shù)，繼續(xù)引導(dǎo)內(nèi)核的鏡像，復(fù)制image header 到全局變量header；
檢查header的魔數(shù)，檢查數(shù)，header和image中的這兩個。確定image的體系結(jié)構(gòu)和類型（KERNEL  or MULTI）,關(guān)閉中斷，加載image到header中的加載地址*/
    boot_fn(0, argc, argv, &images); //調(diào)用do_bootm_linux()函數(shù)
    show_boot_progress (-9);
#ifdef DEBUG
    puts ("\n## Control returned to monitor - resetting...\n");
#endif
    do_reset (cmdtp, flag, argc, argv);

    return 1;
}

static int bootm_load_os(image_info_t os, ulong *load_end, int boot_progress)
{
    uint8_t comp = os.comp;
    ulong load = os.load;
    ulong blob_start = os.start;
    ulong blob_end = os.end;
    ulong image_start = os.image_start;
    ulong image_len = os.image_len;
    uint unc_len = CONFIG_SYS_BOOTM_LEN;

    const char *type_name = genimg_get_type_name (os.type);

    switch (comp) { //判斷image的壓縮類型
    case IH_COMP_NONE:
        if (load == blob_start) {
            printf (" XIP %s ... ", type_name);
        } else {
            printf (" Loading %s ... ", type_name);
//如果在Image head中加載的地址和bootm命令參數(shù)2指定的地址相同，則不需要復(fù)制，直接執(zhí)行
            if (load != image_start) {
                memmove_wd ((void *)load,
                        (void *)image_start, image_len, CHUNKSZ);
            }
        }
        *load_end = load + image_len;
        puts("OK\n");
        break;
    case IH_COMP_GZIP:
        printf (" Uncompressing %s ... ", type_name);
        if (gunzip ((void *)load, unc_len,
                    (uchar *)image_start, &image_len) != 0) {
            puts ("GUNZIP: uncompress, out-of-mem or overwrite error "
                "- must RESET board to recover\n");
            if (boot_progress)
                show_boot_progress (-6);
            return BOOTM_ERR_RESET;
        }
        *load_end = load + image_len;
        break;
#ifdef CONFIG_BZIP2
    case IH_COMP_BZIP2: //判斷是什么類型的壓縮類型
        printf (" Uncompressing %s ... ", type_name);
         int i = BZ2_bzBuffToBuffDecompress ((char*)load,
                    &unc_len, (char *)image_start, image_len,
                    CONFIG_SYS_MALLOC_LEN < (4096 * 1024), 0);
        if (i != BZ_OK) {
            printf ("BUNZIP2: uncompress or overwrite error %d "
                "- must RESET board to recover\n", i);
            if (boot_progress)
                show_boot_progress (-6);
            return BOOTM_ERR_RESET;
        }
        *load_end = load + unc_len;
        break;
#endif /* CONFIG_BZIP2 */
#ifdef CONFIG_LZMA
    case IH_COMP_LZMA:
        printf (" Uncompressing %s ... ", type_name);

....................
   return 0;
}

* CPU register settings //這里也就是我們的theKernel中的作用
          o r0 = 0.
          o r1 = machine type number.
          o r2 = physical address of tagged list in system RAM.
    * CPU mode
          o All forms of interrupts must be disabled (IRQs and FIQs.)
          o The CPU must be in SVC mode. (A special exception exists for Angel.)
    * Caches, MMUs
          o The MMU must be off.
          o Instruction cache may be on or off.
          o Data cache must be off and must not contain any stale data.
    * Devices
          o DMA to/from devices should be quiesced.
    * The boot loader is expected to call the kernel image by jumping directly to the first instruction of the kernel image.