do_bootm()函数个人注释

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;

/*为是否对镜像头做校验做准备，读取uboot的环境变量verify，
如果环境变量verify等于’n’，则局部变量verify赋值成为0；如果环境变量verify为空（即没有
定义环境变量verify）或者环境变量verify不等于’n’，则局部变量verify赋值成为1
*/
	s = getenv ("verify");
	verify = (s && (*s == 'n')) ? 0 : 1;

	if (argc < 2) //获取镜像存放的内存首地址，如果参数个数小于2（即只是输入了bootm）,使用缺省加载地址CFG_LOAD_ADDR。
	{
		addr = load_addr;
	}
	 else  //否则使用第二个参数作为加载地址
	{
		addr = simple_strtoul(argv[1], NULL, 16);
	}

	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

//从镜像内存首地址读取镜像头部，为下面的分析校验做准备
	memmove (&header, (char *)addr, sizeof(image_header_t));


//判断内核头部编号属性是否为IH_MAGIC，如果不是u-boot镜像识别的内核镜像格式，会输出提示信息”Bad Magic Number”
	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__ */
	  {
			puts ("Bad Magic Number\n");
			SHOW_BOOT_PROGRESS (-1);
			return 1;
	  }
	}
	SHOW_BOOT_PROGRESS (2);

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

//对内核的镜像头部做CRC检验
	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) //对镜像头部的数据做CRC校验
	{
		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;


//检验CPU的类型
#if defined(__PPC__)
	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
#endif
	{
		printf ("Unsupported Architecture 0x%x\n", hdr->ih_arch);
		SHOW_BOOT_PROGRESS (-4);
		return 1;
	}
	SHOW_BOOT_PROGRESS (5);

	switch (hdr->ih_type) {
	case IH_TYPE_STANDALONE:
		name = "Standalone Application";
		/* A second argument overwrites the load address */
		if (argc > 2) {
			hdr->ih_load = htonl(simple_strtoul(argv[2], NULL, 16));
		}
		break;
	case IH_TYPE_KERNEL:
		name = "Kernel Image";
		break;
	case IH_TYPE_MULTI:
		name = "Multi-File Image";
		len  = ntohl(len_ptr[0]);
		/* OS kernel is always the first image */
		data += 8; /* kernel_len + terminator */
		for (i=1; len_ptr[i]; ++i)
			data += 4;
		break;
	default: printf ("Wrong Image Type for %s command\n", cmdtp->name);
		SHOW_BOOT_PROGRESS (-5);
		return 1;
	}
	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) //开始判断是否对内核进行了压缩
{
	//首先清楚这几个地址的定义addr = CFG_LOAD_ADDR;  data = addr + sizeof(image_header_t);
	case IH_COMP_NONE:  //如果没有被压缩，只是在内核的前面加了一个头部信息
//在利用mkimage时指定-a选项后面就是指定的内核加载的地址，这个地址会被存放到镜像头结构的ih_load成员变量中，如在smdk2410中ih_load为0x30008000
//如果加载地址等于addr(即：bootm后面接的地址)就不移动内核，可以在这个addr上开始加载，此时的内核入口地址就是addr后的64字节，即data
		if(ntohl(hdr->ih_load) == addr)
		{ 
			printf ("   XIP %s ... ", name);
		}
/*
ih_load不等于addr，此时：data=addr+64还是指向真正的内核部分，这时候就要将内核拷贝到ih_load指定的地方才能执行，这种情况ih_load==ih_ep加载地址等于内核的入口地址
*/
		else 
		{
#if defined(CONFIG_HW_WATCHDOG) || defined(CONFIG_WATCHDOG)
			size_t l = len;
			void *to = (void *)ntohl(hdr->ih_load);
			void *from = (void *)data;

			printf ("   Loading %s ... ", name);

			while (l > 0) {
				size_t tail = (l > CHUNKSZ) ? CHUNKSZ : l;
				WATCHDOG_RESET();
				memmove (to, from, tail);
				to += tail;
				from += tail;
				l -= tail;
			}
#else	/* !(CONFIG_HW_WATCHDOG || CONFIG_WATCHDOG) */
			memmove ((void *) ntohl(hdr->ih_load), (uchar *)data, len);
			//内核移动函数：将data地址处的真正内核复制到指定的ih_load处
			
#endif	/* CONFIG_HW_WATCHDOG || CONFIG_WATCHDOG */
		}
		break;
		
	case IH_COMP_GZIP:  //内核文件有压缩成gzip格式情况：需要解压缩
		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;
		
#ifdef CONFIG_BZIP2//内核文件有压缩成bzip2格式情况：需要解压缩
	case IH_COMP_BZIP2:
		printf ("   Uncompressing %s ... ", name);
		/*
		 * If we've got less than 4 MB of malloc() space,
		 * use slower decompression algorithm which requires
		 * at most 2300 KB of memory.
		 */
		i = BZ2_bzBuffToBuffDecompress ((char*)ntohl(hdr->ih_load),
						&unc_len, (char *)data, len,
						CFG_MALLOC_LEN < (4096 * 1024), 0);
		if (i != BZ_OK) {
			printf ("BUNZIP2 ERROR %d - must RESET board to recover\n", i);
			SHOW_BOOT_PROGRESS (-6);
			udelay(100000);
			do_reset (cmdtp, flag, argc, argv);
		}
		break;
		
#endif /* CONFIG_BZIP2 */
	default:
		if (iflag)
			enable_interrupts();
		printf ("Unimplemented compression type %d\n", hdr->ih_comp);
		SHOW_BOOT_PROGRESS (-7);
		return 1;
}
	puts ("OK\n");
	SHOW_BOOT_PROGRESS (7);

	switch (hdr->ih_type) {
	case IH_TYPE_STANDALONE:
		if (iflag)
			enable_interrupts();

		/* load (and uncompress), but don't start if "autostart"
		 * is set to "no"
		 */
		if (((s = getenv("autostart")) != NULL) && (strcmp(s,"no") == 0)) {
			char buf[32];
			sprintf(buf, "%lX", len);
			setenv("filesize", buf);
			return 0;
		}
		appl = (int (*)(int, char *[]))ntohl(hdr->ih_ep);
		(*appl)(argc-1, &argv[1]);
		return 0;
	case IH_TYPE_KERNEL:
	case IH_TYPE_MULTI:
		/* handled below */
		break;
	default:
		if (iflag)
			enable_interrupts();
		printf ("Can't boot image type %d\n", hdr->ih_type);
		SHOW_BOOT_PROGRESS (-8);
		return 1;
	}
	SHOW_BOOT_PROGRESS (8);

switch (hdr->ih_os) //确定内核的操作系统
{
	default:			/* handled by (original) Linux case */
	case IH_OS_LINUX: //选定为Linux操作系统
#ifdef CONFIG_SILENT_CONSOLE
	    fixup_silent_linux();
#endif

		//调用do_bootm_linux函数启动内核，接着分析armlinux.c文件下的这个函数，从这里传入的参数有：
	    do_bootm_linux  (cmdtp, flag, argc, argv,
			     addr, len_ptr, verify);
	    break;
	case IH_OS_NETBSD:
	    do_bootm_netbsd (cmdtp, flag, argc, argv,
			     addr, len_ptr, verify);
	    break;

#ifdef CONFIG_LYNXKDI
	case IH_OS_LYNXOS:
	    do_bootm_lynxkdi (cmdtp, flag, argc, argv,
			     addr, len_ptr, verify);
	    break;
#endif

	case IH_OS_RTEMS:
	    do_bootm_rtems (cmdtp, flag, argc, argv,
			     addr, len_ptr, verify);
	    break;

#if (CONFIG_COMMANDS & CFG_CMD_ELF)
	case IH_OS_VXWORKS:
	    do_bootm_vxworks (cmdtp, flag, argc, argv,
			      addr, len_ptr, verify);
	    break;
	case IH_OS_QNX:
	    do_bootm_qnxelf (cmdtp, flag, argc, argv,
			      addr, len_ptr, verify);
	    break;
#endif /* CFG_CMD_ELF */
#ifdef CONFIG_ARTOS
	case IH_OS_ARTOS:
	    do_bootm_artos  (cmdtp, flag, argc, argv,
			     addr, len_ptr, verify);
	    break;
#endif
}

	SHOW_BOOT_PROGRESS (-9);
#ifdef DEBUG
	puts ("\n## Control returned to monitor - resetting...\n");
	do_reset (cmdtp, flag, argc, argv);
#endif
	return 1;
}