在start_armboot()函數中，gd_t和bd_t是u-boot中兩個重要的數據結構，在初始化操作很多都要靠這兩個數據結構來保存或傳遞。分別定義在./include/asm-arm/global_data.h和./include/asm-arm/u-boot.h
相關閱讀：U-Boot源代碼下載地址 http://www.linuxidc.com/Linux/2011-07/38897.htm
1、gd_t: global data數據結構定義，位於文件include/asm-arm/global_data.h。其成員主要是一些全局的系統初始化參數。當使用gd_t時需用宏定義進行聲明：DECLARE_GLOBAL_DATA_PTR，指定占用寄存器R8。
/*
*The following data structure is placed in some memory wich is
*available very early after boot (like DPRAM on MPC8xx/MPC82xx, or
*some locked parts of the data cache) to allow for a minimum set of
*global variables during system initialization (until we have set
*up the memory controller so that we can use RAM).
*
* Keep it*SMALL* and remember to set CFG_GBL_DATA_SIZE > sizeof(gd_t)
*CFG_GBL_DATA_SIZE在config文件中定義，start.S中會根據這個值分配棧空間給global_data
*/
ypedef struct global_data {
bd_t *bd; //struct board_info指針，保存板子信息
unsigned long flags; //指示標志，如設備已經初始化標志等
unsigned long baudrate; //串口波特率
unsigned long have_console; //串口初始化標志
unsigned long env_addr; //Address of Environment struct ，環境參數地址
unsigned long env_valid; //環境參數CRC檢驗有效標志
unsigned long fb_base; //frame buffer的基址
#ifdefCONFIG_VFD
unsigned char vfd_type; /*display type */
#endif
#if 0
unsigned long cpu_clk; /* CPU clock in Hz! */
unsignedlong bus_clk;
unsigned long ram_size; /*RAM size */
unsigned long reset_status; /*reset status register at boot */
#endif
void **jt; /*jump table */
} gd_t;
位於下圖所示CFG_GBL_DATA_SIZE位置,即全局配置數據區
/*
*Global Data Flags
*/
#defineGD_FLG_RELOC0x00001 /*Code was relocated to RAM */
#define GD_FLG_DEVINIT 0x00002 /*Devices have been initialized */
#define GD_FLG_SILENT0x00004 /*Silent mode */
#define GD_FLG_POSTFAIL 0x00008 /*Critical POST test failed */
#define GD_FLG_POSTSTOP 0x00010 /*POST seqeunce aborted */
#define GD_FLG_LOGINIT 0x00020 /*Log Buffer has been initialized */
#defineGD_FLG_DISABLE_CONSOLE 0x00040 /*Disable console (in & out) */
#defineDECLARE_GLOBAL_DATA_PTR register volatile gd_t *gd asm ("r8")
