register volatile gd_t *gd asm ("r8")

如下的两个结构，我们主要的是关心下面一行声明：

#define DECLARE_GLOBAL_DATA_PTR register volatile gd_t *gd asm ("r8")

这个声明告诉编译器使用寄存器r8来存储gd_t类型的指针gd，即这个定义声明了一个指针，并且指明了它的存储位置。

register表示变量放在机器的寄存器

volatile用于指定变量的值可以由外部过程异步修改

并且这个指针在start_armboot()（board.c)中被初始化：

/* Pointer is writable since we allocated a register for it */

gd = (gd_t*)(_armboot_start - CFG_MALLOC_LEN - sizeof(gd_t));

这样，gd就指向的一个可用的内存地址了。

==============================================================

typedef struct bd_info {

unsigned long bi_baudrate;

unsigned long bi_ip_addr;

unsigned char bi_enetaddr[6];

unsigned char bi_phy_id[4];

struct environment_s *bi_env;

unsigned long bi_board_number;

void *bi_boot_params;

struct {

unsigned long start;

unsigned long size;

} bi_dram[CONFIG_NR_DRAM_BANKS];

unsigned long bi_flashstart;

unsigned long bi_flashsize;

unsigned long bi_flashoffset;

} bd_t;

#ifndef __ASM_GBL_DATA_H

#define __ASM_GBL_DATA_H

/*

* 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)

*/

typedef struct global_data {

bd_t *bd;

unsigned long flags;

unsigned long baudrate;

unsigned long have_console; /* serial_init() was called */

unsigned long reloc_off; /* Relocation Offset */

unsigned long env_addr; /* Address of Environment struct */

unsigned long env_valid; /* Checksum of Environment valid? */

unsigned long fb_base; /* base address of frame buffer */

#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")

#endif /* __ASM_GBL_DATA_H */