void main_loop (void)
{
#ifndef CONFIG_SYS_HUSH_PARSER
	static char lastcommand[CONFIG_SYS_CBSIZE] = { 0, };
	int len;
	int rc = 1;
	int flag;
#endif

#if defined(CONFIG_BOOTDELAY) && (CONFIG_BOOTDELAY >= 0)
	char *s;
	int bootdelay;
#endif
#ifdef CONFIG_PREBOOT
	char *p;
#endif
#ifdef CONFIG_BOOTCOUNT_LIMIT
	unsigned long bootcount = 0;
	unsigned long bootlimit = 0;
	char *bcs;
	char bcs_set[16];
#endif /* CONFIG_BOOTCOUNT_LIMIT */

#ifdef CONFIG_BOOTCOUNT_LIMIT
	bootcount = bootcount_load();
	bootcount++;
	bootcount_store (bootcount);
	sprintf (bcs_set, "%lu", bootcount);
	setenv ("bootcount", bcs_set);
	bcs = getenv ("bootlimit");
	bootlimit = bcs ? simple_strtoul (bcs, NULL, 10) : 0;
#endif /* CONFIG_BOOTCOUNT_LIMIT */

#ifdef CONFIG_MODEM_SUPPORT
	debug ("DEBUG: main_loop:   do_mdm_init=%d\n", do_mdm_init);
	if (do_mdm_init) {
		char *str = strdup(getenv("mdm_cmd"));
		setenv ("preboot", str);  /* set or delete definition */
		if (str != NULL)
			free (str);
		mdm_init(); /* wait for modem connection */
	}
#endif  /* CONFIG_MODEM_SUPPORT */

#ifdef CONFIG_VERSION_VARIABLE
	{
		setenv ("ver", version_string);  /* set version variable */
	}
#endif /* CONFIG_VERSION_VARIABLE */

#ifdef CONFIG_SYS_HUSH_PARSER
	u_boot_hush_start ();
#endif

#if defined(CONFIG_HUSH_INIT_VAR)
	hush_init_var ();
#endif

#ifdef CONFIG_PREBOOT
	if ((p = getenv ("preboot")) != NULL) {
# ifdef CONFIG_AUTOBOOT_KEYED
		int prev = disable_ctrlc(1);	/* disable Control C checking */
# endif

# ifndef CONFIG_SYS_HUSH_PARSER
		run_command (p, 0);
# else
		parse_string_outer(p, FLAG_PARSE_SEMICOLON |
				    FLAG_EXIT_FROM_LOOP);
# endif

# ifdef CONFIG_AUTOBOOT_KEYED
		disable_ctrlc(prev);	/* restore Control C checking */
# endif
	}
#endif /* CONFIG_PREBOOT */

#if defined(CONFIG_UPDATE_TFTP)
	update_tftp (0UL);
#endif /* CONFIG_UPDATE_TFTP */


#if defined(CONFIG_BOOTDELAY) && (CONFIG_BOOTDELAY >= 0)
	/*获得bootdelay的环境变量*/
	s = getenv ("bootdelay");
	/*将bootdelay的字符串变量变成int,传给bootdelay*/
	bootdelay = s ? (int)simple_strtol(s, NULL, 10) : CONFIG_BOOTDELAY;


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

# ifdef CONFIG_BOOT_RETRY_TIME
	init_cmd_timeout ();
# endif	/* CONFIG_BOOT_RETRY_TIME */

#ifdef CONFIG_POST
	if (gd->flags & GD_FLG_POSTFAIL) {
		s = getenv("failbootcmd");
	}
	else
#endif /* CONFIG_POST */
#ifdef CONFIG_BOOTCOUNT_LIMIT
	if (bootlimit && (bootcount > bootlimit)) {
		printf ("Warning: Bootlimit (%u) exceeded. Using altbootcmd.\n",
		        (unsigned)bootlimit);
		s = getenv ("altbootcmd");
	}
	else
#endif /* CONFIG_BOOTCOUNT_LIMIT */
	/*获得bootcmd的环境变量,以便启动*/
		s = getenv ("bootcmd");

	debug ("### main_loop: bootcmd=\"%s\"\n", s ? s : "<UNDEFINED>");
	/*如果bootdelay不为0,abortboot将被执行*/
	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
	/*执行从"bootcmd"获得的环境变量,启动内核*/
		run_command (s, 0);
# else
		parse_string_outer(s, FLAG_PARSE_SEMICOLON |
				    FLAG_EXIT_FROM_LOOP);
# endif

# ifdef CONFIG_AUTOBOOT_KEYED
		disable_ctrlc(prev);	/* restore Control C checking */
# endif

	/*执行menu菜单命令*/
#ifdef CONFIG_CMD_MENU
	run_command("menu", 0);
#endif

  /*进入uboot自带的命令行*/
# ifdef CONFIG_MENUKEY
	if (menukey == CONFIG_MENUKEY) {
		s = getenv("menucmd");
		if (s) {
# ifndef CONFIG_SYS_HUSH_PARSER
			run_command(s, 0);
# else
			parse_string_outer(s, FLAG_PARSE_SEMICOLON |
						FLAG_EXIT_FROM_LOOP);
# endif
		}
	}
#endif /* CONFIG_MENUKEY */
#endif /* CONFIG_BOOTDELAY */

	/*
	 * 
	 */
#ifdef CONFIG_SYS_HUSH_PARSER
	parse_file_outer();
	/* This point is never reached */
	for (;;);
#else
	for (;;) {
#ifdef CONFIG_BOOT_RETRY_TIME
		if (rc >= 0) {
			/* Saw enough of a valid command to
			 * restart the timeout.
			 */
			reset_cmd_timeout();
		}
#endif
		len = readline (CONFIG_SYS_PROMPT);

		flag = 0;	/* assume no special flags for now */
		if (len > 0)
			strcpy (lastcommand, console_buffer);
		else if (len == 0)
			flag |= CMD_FLAG_REPEAT;
#ifdef CONFIG_BOOT_RETRY_TIME
		else if (len == -2) {
			/* -2 means timed out, retry autoboot
			 */
			puts ("\nTimed out waiting for command\n");
# ifdef CONFIG_RESET_TO_RETRY
			/* Reinit board to run initialization code again */
			do_reset (NULL, 0, 0, NULL);
# else
			return;		/* retry autoboot */
# endif
		}
#endif

		if (len == -1)
			puts ("<INTERRUPT>\n");
		else
			rc = run_command (lastcommand, flag);

		if (rc <= 0) {
			/* invalid command or not repeatable, forget it */
			lastcommand[0] = 0;
		}
	}
#endif /*CONFIG_SYS_HUSH_PARSER*/
}