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

最新推荐文章于 2021-11-08 22:46:47 发布

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本文深入解析GD全局数据结构及其寄存器优化技术，详细讲解了如何通过寄存器r8存储gd_t类型指针，以及GD全局数据结构在系统初始化阶段的配置与作用。

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 */

#Linux