bootm内核的启动

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在U-Boot源码阅读（六）大佬的命令这篇文章中，知道了内核的启动是通过命令nand read.i c0008000 80000 500000;bootm c0008000，先把flash的数据读到内存中，然后执行这段内存的命令。看上去bootm和go还是很相近的嘛。但是go仅仅是开始执行而已，并没做任何的操作。而内核的启动需要满足下列条件：

（1）CPU 寄存器的设置。
? R0=0。
? R1=机器类型ID；对于ARM 结构的CPU，其机器类型ID 可以参见linux/arch/arm/tools/mach-types。
? R2=启动参数标记列表在RAM 中起始基地址。
（2）CPU 工作模式。
? 必须禁止中断（IRQs 和FIQs）。
? CPU 必须为SVC 模式。
（3）Cache 和MMU 的设置。
? MMU 必须关闭。
? 指令Cache 可以打开也可以关闭。
? 数据Cache 必须关闭。

bootm命令：

int do_bootm (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
{
ulong iflag;
ulong addr;
ulong data, len, checksum;
ulong *len_ptr = NULL; /* not to make warning. by scsuh */
uint unc_len = CFG_BOOTM_LEN;
int i, verify;
char *name, *s;
int (*appl)(int, char *[]);
image_header_t *hdr = &header;
s = getenv ("verify");
verify = (s && (*s == 'n')) ? 0 : 1;
if (argc < 2) {
addr = load_addr;
} else {
addr = simple_strtoul(argv[1], NULL, 16);
}
#ifdef CONFIG_ZIMAGE_BOOT
#define LINUX_ZIMAGE_MAGIC 0x016f2818
if (*(ulong *)(addr + 9*4) == LINUX_ZIMAGE_MAGIC) {
printf("Boot with zImage\n");
addr = virt_to_phys(addr);
hdr->ih_os = IH_OS_LINUX;
hdr->ih_ep = ntohl(addr);
goto after_header_check;
}
#endif
SHOW_BOOT_PROGRESS (1);
printf ("## Booting 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
memmove (&header, (char *)addr, sizeof(image_header_t));
if (ntohl(hdr->ih_magic) != IH_MAGIC) {
#ifdef __I386__ /* correct image format not implemented yet - fake it */
if (fake_header(hdr, (void*)addr, -1) != NULL) {
/* to compensate for the addition below */
addr -= sizeof(image_header_t);
/* turnof verify,
* fake_header() does not fake the data crc
*/
verify = 0;
} else
#endif /* __I386__ */
{
#ifdef CONFIG_IMAGE_BOOT
printf("Boot with Image\n");
addr = virt_to_phys(addr);
hdr->ih_os = IH_OS_LINUX;
hdr->ih_ep = ntohl(addr);
hdr->ih_comp = IH_COMP_NONE;
goto after_header_check;
#endif
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) {
puts ("Bad Header Checksum\n");
SHOW_BOOT_PROGRESS (-2);
return 1;
}
SHOW_BOOT_PROGRESS (3);
#ifdef CONFIG_HAS_DATAFLASH
if (addr_dataflash(addr)){
len = ntohl(hdr->ih_size) + sizeof(image_header_t);
read_dataflash(addr, len, (char *)CFG_LOAD_ADDR);
addr = CFG_LOAD_ADDR;
}
#endif
/* for multi-file images we need the data part, too */
print_image_hdr ((image_header_t *)addr);
data = addr + sizeof(image_header_t);
len = ntohl(hdr->ih_size);
if (verify) {
puts (" Verifying Checksum ... ");
if (crc32 (0, (uchar *)data, len) != ntohl(hdr->ih_dcrc)) {
printf ("Bad Data CRC\n");
SHOW_BOOT_PROGRESS (-3);
return 1;
}
puts ("OK\n");
}
SHOW_BOOT_PROGRESS (4);
len_ptr = (ulong *)data;
SHOW_BOOT_PROGRESS (5);
switch (hdr->ih_type) {
case IH_TYPE_STANDALONE:
break;
case IH_TYPE_KERNEL:
name = "Kernel Image";
break;
case IH_TYPE_MULTI:
break;
}
SHOW_BOOT_PROGRESS (6);
/*
* We have reached the point of no return: we are going to
* overwrite all exception vector code, so we cannot easily
* recover from any failures any more...
*/
iflag = disable_interrupts();
#ifdef CONFIG_AMIGAONEG3SE
/*
* We've possible left the caches enabled during
* bios emulation, so turn them off again
*/
icache_disable();
invalidate_l1_instruction_cache();
flush_data_cache();
dcache_disable();
#endif
switch (hdr->ih_comp) {
case IH_COMP_NONE:
break;
case IH_COMP_GZIP: //这里使用gzip对内核进行压缩
printf (" Uncompressing %s ... ", name);
if (gunzip ((void *)ntohl(hdr->ih_load), unc_len,
(uchar *)data, &len) != 0) {
puts ("GUNZIP ERROR - must RESET board to recover\n");
SHOW_BOOT_PROGRESS (-6);
do_reset (cmdtp, flag, argc, argv);
}
break;
}
puts ("OK\n");
SHOW_BOOT_PROGRESS (7);
switch (hdr->ih_type) {
case IH_TYPE_STANDALONE:
return 0;
case IH_TYPE_KERNEL:
case IH_TYPE_MULTI:
/* handled below */
break;
default:
return 1;
}
SHOW_BOOT_PROGRESS (8);
#if defined(CONFIG_ZIMAGE_BOOT) || defined(CONFIG_IMAGE_BOOT)
after_header_check:
#endif
switch (hdr->ih_os) {
default: /* handled by (original) Linux case */
case IH_OS_LINUX:
#ifdef CONFIG_SILENT_CONSOLE
fixup_silent_linux();
#endif
do_bootm_linux (cmdtp, flag, argc, argv,
addr, len_ptr, verify);
break;
}
SHOW_BOOT_PROGRESS (-9);
return 1;
}