先来引用一下这篇介绍“ARM Linux内核启动要求”的文章ARM Linux Kernel Boot Requirements，是ARM Linux内核的维护者Russell King写的(见 )。
引用：
* CPU register settings
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.
下面对do_bootm_linux函数做一下注释，便于大家理解boot的过程。
void do_bootm_linux (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[],
ulong addr, ulong *len_ptr, int verify)
{
ulong len = 0, checksum;
ulong initrd_start, initrd_end;
ulong data;
void (*theKernel)(int zero, int arch, uint params);
image_header_t *hdr = &header;
bd_t *bd = gd->bd;
#ifdef CONFIG_CMDLINE_TAG
char *commandline = getenv ("bootargs");
#endif
//可以看到theKernel被赋值为hdr->ih_ep，这个hdr是指使用tools/mkimage工具程
//序制作uImage时加在linux.bin.gz前面的一个头部，而ih_ep结构体成员保存的就是使用mkimage时指定
//的-e参数的值，即内核的入口点（Entry Point）。知道了hdr->ih_ep的意义之后，给theKernel赋这个
//值也就是理所当然的了。
theKernel = (void (*)(int, int, uint))ntohl(hdr->ih_ep);
/*
* Check if there is an initrd image
*/
if (argc >= 3) {
SHOW_BOOT_PROGRESS (9);
addr = simple_strtoul (argv[2], NULL, 16);
printf ("## Loading Ramdisk Image at %08lx ...\n", addr);
/* Copy header so we can blank CRC field for re-calculation */
#ifdef CONFIG_HAS_DATAFLASH
if (addr_dataflash (addr)) {
read_dataflash (addr, sizeof (image_header_t),
(char *) &header);
} else
#endif
memcpy (&header, (char *) addr,
sizeof (image_header_t));
if (ntohl (hdr->ih_magic) != IH_MAGIC) {
printf ("Bad Magic Number\n");
SHOW_BOOT_PROGRESS (-10);
do_reset (cmdtp, flag, argc, argv);
}
data = (ulong) & header;
len = sizeof (image_header_t);
checksum = ntohl (hdr->ih_hcrc);
hdr->ih_hcrc = 0;
if (crc32 (0, (unsigned char *) data, len) != checksum) {
printf ("Bad Header Checksum\n");
SHOW_BOOT_PROGRESS (-11);
do_reset (cmdtp, flag, argc, argv);
}
SHOW_BOOT_PROGRESS (10);
print_image_hdr (hdr);
data = addr + sizeof (image_header_t);
len = ntohl (hdr->ih_size);
#ifdef CONFIG_HAS_DATAFLASH
if (addr_dataflash (addr)) {
read_dataflash (data, len, (char *) CFG_LOAD_ADDR);
data = CFG_LOAD_ADDR;
}
#endif
if (verify) {
ulong csum = 0;
printf (" Verifying Checksum ... ");
csum = crc32 (0, (unsigned char *) data, len);
if (csum != ntohl (hdr->ih_dcrc)) {
printf ("Bad Data CRC\n");
SHOW_BOOT_PROGRESS (-12);
do_reset (cmdtp, flag, argc, argv);
}
printf ("OK\n");
}
SHOW_BOOT_PROGRESS (11);
if ((hdr->ih_os != IH_OS_LINUX) ||
(hdr->ih_arch != IH_CPU_ARM) ||
(hdr->ih_type != IH_TYPE_RAMDISK)) {
printf ("No Linux ARM Ramdisk Image\n");
SHOW_BOOT_PROGRESS (-13);
do_reset (cmdtp, flag, argc, argv);
}
#if defined(CONFIG_B2) || defined(CONFIG_EVB4510) || defined(CONFIG_ARMADILLO)
/*
*we need to copy the ramdisk to SRAM to let Linux boot
*/
memmove ((void *) ntohl(hdr->ih_load), (uchar *)data, len);
data = ntohl(hdr->ih_load);
#endif /* CONFIG_B2 || CONFIG_EVB4510 */
/*
* Now check if we have a multifile image
*/
} else if ((hdr->ih_type == IH_TYPE_MULTI) && (len_ptr[1])) {
ulong tail = ntohl (len_ptr[0]) % 4;
int i;
SHOW_BOOT_PROGRESS (13);
/* skip kernel length and terminator */
data = (ulong) (&len_ptr[2]);
/* skip any additional image length fields */
for (i = 1; len_ptr[i]; ++i)
data += 4;
/* add kernel length, and align */
data += ntohl (len_ptr[0]);
if (tail) {
data += 4 - tail;
}
len = ntohl (len_ptr[1]);
} else {
/*
* no initrd image
*/
SHOW_BOOT_PROGRESS (14);
len = data = 0;
}
#ifdef DEBUG
if (!data) {
printf ("No initrd\n");
}
#endif
if (data) {
initrd_start = data;
initrd_end = initrd_start + len;
} else {
initrd_start = 0;
initrd_end = 0;
}
SHOW_BOOT_PROGRESS (15);
debug ("## Transferring control to Linux (at address %08lx) ...\n",
(ulong) theKernel);
#if defined (CONFIG_SETUP_MEMORY_TAGS) || \
defined (CONFIG_CMDLINE_TAG) || \
defined (CONFIG_INITRD_TAG) || \
defined (CONFIG_SERIAL_TAG) || \
defined (CONFIG_REVISION_TAG) || \
defined (CONFIG_LCD) || \
defined (CONFIG_VFD)
setup_start_tag (bd);
#ifdef CONFIG_SERIAL_TAG
setup_serial_tag (¶ms);
#endif
#ifdef CONFIG_REVISION_TAG
setup_revision_tag (¶ms);
#endif
#ifdef CONFIG_SETUP_MEMORY_TAGS
setup_memory_tags (bd);
#endif
#ifdef CONFIG_CMDLINE_TAG
setup_commandline_tag (bd, commandline);
#endif
#ifdef CONFIG_INITRD_TAG
if (initrd_start && initrd_end)
setup_initrd_tag (bd, initrd_start, initrd_end);
#endif
#if defined (CONFIG_VFD) || defined (CONFIG_LCD)
setup_videolfb_tag ((gd_t *) gd);
#endif
setup_end_tag (bd);
#endif
/* we assume that the kernel is in place */
printf ("\nStarting kernel ...\n\n");
#ifdef CONFIG_USB_DEVICE
{
extern void udc_disconnect (void);
//udc_disconnect (); // cancled by
}
#endif
cleanup_before_linux ();
//看看它的名字和参数的命名我们也可以猜到这个其实就是内核的入口函
//数的指针了。几个参数的命名也说明了上文提到的ARM Linux内核启动要求的第一条，
//因为根据ACPS（ARM/Thumb Procedure Call Standard）的规定，这三个参数就是依次
//使用r0，r1和r2来传递的。
调用的时候对参数进行赋值，r0=0，r1=bd->bi_arch_number，r2=bd->bi_boot_params，
一个都不少。至此U-Boot的使命完成，linux开始统治整个世界。
theKernel (0, bd->bi_arch_number, bd->bi_boot_params);
}