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/global_data.h和./include/asm/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。需要用到时用宏定义: 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
*/
typedef 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 reloc_off; // ?? 重定位偏移，即实际定向位置与连接指定之差
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
#ifdef CONFIG_VFD
unsigned char vfd_type; /* display type */
#endif
#if 0
unsigned long cpu_clk; /* CPU clock in Hz! */
unsigned long 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
*/
#define GD_FLG_RELOC 0x00001 /* Code was relocated to RAM */
#define GD_FLG_DEVINIT 0x00002 /* Devices have been initialized */
#define GD_FLG_SILENT 0x00004 /* Silent mode */
#define DECLARE_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。
typedef struct bd_info {
int bi_baudrate; // 串口波特率
unsigned long bi_ip_addr; // IP地址
unsigned char bi_enetaddr[6]; // MAC地址
struct environment_s *bi_env;
ulong bi_arch_number; // 板子ID号
ulong bi_boot_params; // 启动参数
struct // DRAM BANKS配置，起始地址与长度
{
ulong start;
ulong size;
} bi_dram[CONFIG_NR_DRAM_BANKS];
#ifdef CONFIG_HAS_ETH1
unsigned char bi_enet1addr[6]; // 如果有第二块网卡，指定MAC地址
#endif
} bd_t;