在start_armboot()函数中，gd_t和bd_t是u-boot中两个重要的数据结构，在初始化操作很多都要靠这两个数据结构来保存或传递。分别定义在./include/asm-arm/global_data.h和./include/asm-arm/u-boot.h
1、gd_t: global data数据结构定义，位于文件include/asm-arm/global_data.h。其成员主要是一些全局的系统初始化参数。当使用gd_t时需用宏定义进行声明：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文件中定义，start.S中会根据这个值分配栈空间给global_data
*/
ypedef struct global_data {
bd_t *bd; //struct board_info指针，保存板子信息
unsigned long flags; //指示标志，如设备已经初始化标志等
unsigned long baudrate; //串口波特率
unsigned long have_console; //串口初始化标志
unsigned long env_addr; //Address of Environment struct ，环境参数地址
unsigned long env_valid; //环境参数CRC检验有效标志
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;
/*
*Global Data Flags
*/
#defineGD_FLG_RELOC0x00001 /*Code was relocated to RAM */
#defineGD_FLG_DEVINIT0x00002 /*Devices have been initialized */
#defineGD_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")
2.、bd_t:board info数据结构定义，位于文件include/asm-arm/u-boot.h。保存板子参数。
typedefstruct bd_info {
intbi_baudrate; //串口波特率
unsigned longbi_ip_addr; //IP地址
struct environment_s*bi_env;
ulongbi_arch_number; //板子ID号
ulongbi_boot_params; //启动参数
struct //DRAM BANKS配置，起始地址与长度
{
ulongstart;
ulongsize;
}bi_dram[CONFIG_NR_DRAM_BANKS];
} bd_t;