1. cpu/arm920t/start.S:_start
   1. reset
   2. stack_setup
   3. clear_bss
   4. call start_armboot
2. lib_arm/board.c:start_armboot
   1. for(;;){(*init_func_ptr)()}
   2. mem_malloc_init()
   3. env_relocate()
   4. device_init()
   5. jumptable_init()
   6. for(;;)main_loop();
      1. parse_input
      2. run_cmd
3. How to Add a new Command
   1. common/cmd_xxx.c
   2. include/command.h:#define U_BOOT_CMD