*** am335x开发板的疑问以及解答

参考文件：

1，AM335x ARM Cortex-A8 Microprocessors (MPUs) Technical Reference Manual.pdf；

2，am3359.pdf；

1，am335x的cpu上电后，会跳到哪个地址去执行？

答：

芯片到uboot启动流程 ：ROM → MLO(SPL)→ uboot.img

AM335x 中bootloader被分成了 3 个部分：

第一级 bootloader：引导加载程序，板子上电后会自动执行这些代码，如选择哪种方式启动（NAND，SDcard，UART。。。），然后跳转转到第二级 bootloader。这些代码应该是存放在 176KB 的 ROM 中。

第二级 bootloader：MLO（SPL），用以硬件初始化：关闭看门狗，关闭中断，设置 CPU 时钟频率、速度等操作。然后会跳转到第三级bootloader。MLO文件应该会被映射到 64 KB的 Internal SRAM 中。

第三级 bootloader：uboot.img，C代码的入口。

其中第一级 bootloader 是板子固化的，第二级和第三级是通过编译 uboot 所得的。

2，第二级 bootloader：MLO（SPL）做了哪些事情？

MLO(SPL)内存分布如下：

SPL内存重映射：

< PATH : /arch/arm/cpu/armv7/omap-common/u-boot-spl.lds >
MEMORY { .sram : ORIGIN = CONFIG_SPL_TEXT_BASE,\
		LENGTH = CONFIG_SPL_MAX_SIZE }
MEMORY { .sdram : ORIGIN = CONFIG_SPL_BSS_START_ADDR, \
		LENGTH = CONFIG_SPL_BSS_MAX_SIZE }

OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
OUTPUT_ARCH(arm)
ENTRY(_start)
SECTIONS
{
	.text      :
	{
	__start = .;
	  arch/arm/cpu/armv7/start.o	(.text)
	  *(.text*)
	} >.sram

	. = ALIGN(4);
	.rodata : { *(SORT_BY_ALIGNMENT(.rodata*)) } >.sram

	. = ALIGN(4);
	.data : { *(SORT_BY_ALIGNMENT(.data*)) } >.sram
	. = ALIGN(4);
	__image_copy_end = .;
	_end = .;

	.bss :
	{
		. = ALIGN(4);
		__bss_start = .;
		*(.bss*)
		. = ALIGN(4);
		__bss_end__ = .;
	} >.sdram
}

#define CONFIG_SPL_TEXT_BASE		0x402F0400
#define CONFIG_SPL_MAX_SIZE		(46 * 1024)
#define CONFIG_SPL_STACK		LOW_LEVEL_SRAM_STACK

#define CONFIG_SPL_BSS_START_ADDR	0x80000000
#define CONFIG_SPL_BSS_MAX_SIZE		0x80000		/* 512 KB */ 

    @1@ 保存启动参数 bl    save_boot_params

/*
 * the actual reset code
 */

reset:
	bl	save_boot_params

.global save_boot_params
save_boot_params:
	/*
	 * See if the rom code passed pointer is valid:
	 * It is not valid if it is not in non-secure SRAM
	 * This may happen if you are booting with the help of
	 * debugger
	 */
	ldr     r2, =NON_SECURE_SRAM_START
	cmp	r2, r0
	bgt	1f 
	ldr	r2, =NON_SECURE_SRAM_END
	cmp	r2, r0
	blt	1f

	/*
	 * store the boot params passed from rom code or saved
	 * and passed by SPL
	 */
	cmp	r0, #0
	beq	1f
	ldr	r1, =boot_params
	str	r0, [r1]

/*《PATH: /arch/arm/include/asm/arch-ti81xx/omap.h》
 * Non-secure SRAM Addresses
 * Non-secure RAM starts at 0x40300000 for GP devices. But we keep SRAM_BASE
 * at 0x40304000(EMU base) so that our code works for both EMU and GP
 */
#define NON_SECURE_SRAM_START	0x40304000
#define NON_SECURE_SRAM_END	0x4030E000
#define LOW_LEVEL_SRAM_STACK	0x4030B7FC

问题：这些参数是保存在哪里的？大概有哪些参数？

答：

这些参数保存的内存地址为 64 KB 的 OCM RAM 中：

注：Dowloaded Image 区域：是用来保存 MLO（SPL） 文件的，其最大可达到 109 KB

    @a2@ 设置 CPU 为 SVC32 模式

	        
        /*
	 * set the cpu to SVC32 mode
	 */
	mrs	r0, cpsr
	bic	r0, r0, #0x1f
	orr	r0, r0, #0xd3
	msr	cpsr,r0

    CPSR:程序状态寄存器(current program status register) (当前程序状态寄存器)，在任何处理器模式下被访问。它包含了条件标志位、中断禁止位、当前处理器模式标志以及其他的一些控制和状态位。
CPSR在用户级编程时用于存储条件码。

   SPSR:程序状态保存寄存器（saved program status register）,每一种处理器模式下都有一个状态寄存器SPSR,SPSR用于保存CPSR的状态，以便异常返回后恢复异常发生时的工作状态。当特定 的异常中断发生时，这个寄存器用于存放当前程序状态寄存器的内容。在异常中断退出时，可以用SPSR来恢复CPSR。由于用户模式和系统模式不是异常中断 模式，所以他没有SPSR。当用户在用户模式或系统模式访问SPSR，将产生不可预知的后果。

CPSR格式如下所示。SPSR和CPSR格式相同。
31 30 29 28 27 26 7 6 5 4 3 2 1 0
N Z C V Q DNM(RAZ) I F T M4 M3 M2 M1 M0

详解：http://blog.chinaunix.net/uid-28458801-id-3487199.html

    @a3@ CPU的初始化

《PATH : /arch/arm/cpu/armv7/start.S》
	/* the mask ROM code should have PLL and others stable */
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
	bl	cpu_init_crit
#endif

.globl lowlevel_init
lowlevel_init:
	/*
	 * Setup a temporary stack
	 */
	ldr	sp, =LOW_LEVEL_SRAM_STACK

	/*
	 * Save the old lr(passed in ip) and the current lr to stack
	 */
	push	{ip, lr}

	/*
	 * go setup pll, mux, memory
	 */
	bl	s_init
	pop	{ip, pc}

问题：CPU的初始化有哪些内容？

答：

            @b1@ 首先要设置堆栈区，因为将会调用 C函数来实现CPU的初始化

问题：这个堆栈在什么位置，其内存大小是多少？

答

《PATH ：/arch/arm/include/asm/arch-ti81xx/omap.h》
#define LOW_LEVEL_SRAM_STACK	0x4030B7FC 

            @b2@ 执行 s_init() 函数，实现 CPU 的初始化

/*
 * early system init of muxing and clocks.
 */
void s_init(void)
{
	/* Can be removed as A8 comes up with L2 enabled */
	l2_cache_enable();

	/* WDT1 is already running when the bootloader gets control
	 * Disable it to avoid "random" resets
	 */
	__raw_writel(0xAAAA, WDT_WSPR);
	while(__raw_readl(WDT_WWPS) != 0x0);
	__raw_writel(0x5555, WDT_WSPR);
	while(__raw_readl(WDT_WWPS) != 0x0);

#ifdef CONFIG_SPL_BUILD
	/* Setup the PLLs and the clocks for the peripherals */
	pll_init();

	/* Enable RTC32K clock */
	rtc32k_enable();

	/* UART softreset */
	u32 regVal;
	u32 uart_base = DEFAULT_UART_BASE;

	enable_uart0_pin_mux();
	/* IA Motor Control Board has default console on UART3*/
	/* XXX: This is before we've probed / set board_id */
	if (board_id == IA_BOARD) {
		uart_base = UART3_BASE;
	}

	regVal = __raw_readl(uart_base + UART_SYSCFG_OFFSET);
	regVal |= UART_RESET;
	__raw_writel(regVal, (uart_base + UART_SYSCFG_OFFSET) );
	while ((__raw_readl(uart_base + UART_SYSSTS_OFFSET) &
			UART_CLK_RUNNING_MASK) != UART_CLK_RUNNING_MASK);

	/* Disable smart idle */
	regVal = __raw_readl((uart_base + UART_SYSCFG_OFFSET));
	regVal |= UART_SMART_IDLE_EN;
	__raw_writel(regVal, (uart_base + UART_SYSCFG_OFFSET));

	/* Initialize the Timer */
	init_timer();

	preloader_console_init();

	printf("\nlocation /board/ti/am335x\n");		//@@
/*@@*/
//	led();
/*@@*/
	
	config_am335x_ddr();

#endif
}

                    @c1@ 使能第二级缓冲区

	/* Can be removed as A8 comes up with L2 enabled */
	l2_cache_enable();


l2_cache_enable:
	push	{r0, r1, r2, lr}
	mrc	15, 0, r3, cr1, cr0, 1
	orr	r3, r3, #2
	mcr	15, 0, r3, cr1, cr0, 1
	pop	{r1, r2, r3, pc}

                    @c2@ 关闭看门狗（WDT）

	/* WDT1 is already running when the bootloader gets control
	 * Disable it to avoid "random" resets
	 */
	__raw_writel(0xAAAA, WDT_WSPR);
	while(__raw_readl(WDT_WWPS) != 0x0);
	__raw_writel(0x5555, WDT_WSPR);
	while(__raw_readl(WDT_WWPS) != 0x0);

#define WDT_WSPR	(WDT_BASE + 0x048)



/* Watchdog Timer */
#ifdef CONFIG_AM335X
#define WDT_BASE			0x44E35000
#else
#define WDT_BASE			0x480C2000
#endif

                    @c3@ 给外设设置好 PLL 和 时钟频率等

	/* Setup the PLLs and the clocks for the peripherals */
	pll_init();



/*
 * Configure the PLL/PRCM for necessary peripherals
 */
void pll_init()
{
	mpu_pll_config(MPUPLL_M_500);
	core_pll_config();
	per_pll_config();
	ddr_pll_config();
	/* Enable the required interconnect clocks */
	interface_clocks_enable();
	/* Enable power domain transition */
	power_domain_transition_enable();
	/* Enable the required peripherals */
	per_clocks_enable();
}

                    @c4@ 使能 32-KHz 频率的实时时钟

	/* Enable RTC32K clock */
	rtc32k_enable();


《PATH : /board/ti/am335x/evm.c》
static void rtc32k_enable(void)
{
	/* Unlock the rtc's registers */
	__raw_writel(0x83e70b13, (AM335X_RTC_BASE + RTC_KICK0_REG));
	__raw_writel(0x95a4f1e0, (AM335X_RTC_BASE + RTC_KICK1_REG));

	/* Enable the RTC 32K OSC */
	__raw_writel(0x48, (AM335X_RTC_BASE + RTC_OSC_REG));
}


/* RTC base address */
#define AM335X_RTC_BASE            0x44E3E000


#define RTC_KICK0_REG        0x6c
#define RTC_KICK1_REG        0x70
#define RTC_OSC_REG        0x54

                    @c5@ 使能UART0

	/* UART softreset */
	u32 regVal;
	u32 uart_base = DEFAULT_UART_BASE;

	enable_uart0_pin_mux();
	/* IA Motor Control Board has default console on UART3*/
	/* XXX: This is before we've probed / set board_id */
	if (board_id == IA_BOARD) {
		uart_base = UART3_BASE;
	}

	regVal = __raw_readl(uart_base + UART_SYSCFG_OFFSET);
	regVal |= UART_RESET;
	__raw_writel(regVal, (uart_base + UART_SYSCFG_OFFSET) );
	while ((__raw_readl(uart_base + UART_SYSSTS_OFFSET) &
			UART_CLK_RUNNING_MASK) != UART_CLK_RUNNING_MASK);

	/* Disable smart idle */
	regVal = __raw_readl((uart_base + UART_SYSCFG_OFFSET));
	regVal |= UART_SMART_IDLE_EN;
	__raw_writel(regVal, (uart_base + UART_SYSCFG_OFFSET));



#ifdef CONFIG_AM335X
#define DEFAULT_UART_BASE		UART0_BASE
#endif


#ifdef CONFIG_AM335X
#define UART0_BASE			0x44E09000
#else
#define UART0_BASE			0x48020000
#endif

                    @c6@ 初始化 定时器

	/* Initialize the Timer */
	init_timer();



static void init_timer(void)
{
	/* Reset the Timer */
	__raw_writel(0x2, (DM_TIMER2_BASE + TSICR_REG));

	/* Wait until the reset is done */
	while (__raw_readl(DM_TIMER2_BASE + TIOCP_CFG_REG) & 1);

	/* Start the Timer */
	__raw_writel(0x1, (DM_TIMER2_BASE + TCLR_REG));
}


/* DM Timer base addresses */
#define DM_TIMER0_BASE			0x4802C000
#define DM_TIMER1_BASE			0x4802E000
#define DM_TIMER2_BASE			0x48040000
#define DM_TIMER3_BASE			0x48042000
#define DM_TIMER4_BASE			0x48044000
#define DM_TIMER5_BASE			0x48046000
#define DM_TIMER6_BASE			0x48048000
#define DM_TIMER7_BASE			0x4804A000

                    @c7@ 初始化控制台，通过UART可以查看相关信息

	preloader_console_init();

《PATH : /arch/arm/cpu/armv7/omap-common/spl.c》
/* This requires UART clocks to be enabled */
void preloader_console_init(void)
{
	const char *u_boot_rev = U_BOOT_VERSION;
	char rev_string_buffer[50];

	gd = &gdata;
	gd->bd = &bdata;
	gd->flags |= GD_FLG_RELOC;
	gd->baudrate = CONFIG_BAUDRATE;

	serial_init();		/* serial communications setup */

	/* Avoid a second "U-Boot" coming from this string */
	u_boot_rev = &u_boot_rev[7];

	printf("\nU-Boot SPL %s (%s - %s)\n", u_boot_rev, U_BOOT_DATE,
		U_BOOT_TIME);
	omap_rev_string(rev_string_buffer);
	printf("Texas Instruments %s\n", rev_string_buffer);
}  

                    @c8@ 配置 DDR

	config_am335x_ddr();

《PATH ：》
/*  void DDR2_EMIF_Config(void); */
static void config_am335x_ddr(void)
{
	int data_macro_0 = 0;
	int data_macro_1 = 1;

	enable_ddr_clocks();

	config_vtp();

	Cmd_Macro_Config();

	Data_Macro_Config(data_macro_0);
	Data_Macro_Config(data_macro_1);

	__raw_writel(PHY_RANK0_DELAY, DATA0_RANK0_DELAYS_0);
	__raw_writel(PHY_RANK0_DELAY, DATA1_RANK0_DELAYS_0);

	__raw_writel(DDR_IOCTRL_VALUE, DDR_CMD0_IOCTRL);
	__raw_writel(DDR_IOCTRL_VALUE, DDR_CMD1_IOCTRL);
	__raw_writel(DDR_IOCTRL_VALUE, DDR_CMD2_IOCTRL);
	__raw_writel(DDR_IOCTRL_VALUE, DDR_DATA0_IOCTRL);
	__raw_writel(DDR_IOCTRL_VALUE, DDR_DATA1_IOCTRL);

	__raw_writel(__raw_readl(DDR_IO_CTRL) & 0xefffffff, DDR_IO_CTRL);
	__raw_writel(__raw_readl(DDR_CKE_CTRL) | 0x00000001, DDR_CKE_CTRL);

	config_emif_ddr2();
}


《PATH : /arm/include/asm/arch-ti81xx/cpu.h》
#define DATA0_RANK0_DELAYS_0		(DDR_PHY_BASE_ADDR + 0x134)
#define	DATA1_RANK0_DELAYS_0		(DDR_PHY_BASE_ADDR + 0x1D8)

/* DDR offsets */
#define	DDR_PHY_BASE_ADDR		0x44E12000
#define	DDR_IO_CTRL			0x44E10E04
#define	DDR_CKE_CTRL			0x44E1131C
#define	CONTROL_BASE_ADDR		0x44E10000

                    @c DONE@

            @b DONE@

    @a4@ 设置 internal RAM 内存空间的栈指针，调用 board_init_f()函数

/* Set stackpointer in internal RAM to call board_init_f */
call_board_init_f:
	ldr	sp, =(CONFIG_SYS_INIT_SP_ADDR)
	bic	sp, sp, #7 /* 8-byte alignment for ABI compliance */
	ldr	r0,=0x00000000
	bl	board_init_f

#define CONFIG_SYS_INIT_SP_ADDR		(CONFIG_SYS_INIT_RAM_ADDR + \
					 CONFIG_SYS_INIT_RAM_SIZE - \
					 GENERATED_GBL_DATA_SIZE)

#define CONFIG_SYS_INIT_RAM_ADDR	SRAM0_START
#define CONFIG_SYS_INIT_RAM_SIZE	SRAM0_SIZE


#ifdef CONFIG_AM335X
#define SRAM0_START			0x402F0400
#else
#define SRAM0_START			0x40300000
#endif



#if defined(CONFIG_AM335X) || defined(CONFIG_TI814X)
#define SRAM0_SIZE			(0x1B400) /* 109 KB */
#define SRAM_GPMC_STACK_SIZE		(0x40)
#endif



#define GENERATED_GBL_DATA_SIZE (128) /* (sizeof(struct global_data) + 15) & ~15 */

void board_init_f(ulong dummy)
{
	/*
	 * We call relocate_code() with relocation target same as the
	 * CONFIG_SYS_SPL_TEXT_BASE. This will result in relocation getting
	 * skipped. Instead, only .bss initialization will happen. That's
	 * all we need
	 */
	debug(">>board_init_f()\n");
	relocate_code(CONFIG_SPL_STACK, &gdata, CONFIG_SPL_TEXT_BASE);
}


#define CONFIG_SPL_TEXT_BASE		0x402F0400
#define CONFIG_SPL_MAX_SIZE		(46 * 1024)
#define CONFIG_SPL_STACK		LOW_LEVEL_SRAM_STACK



#define LOW_LEVEL_SRAM_STACK	0x4030B7FC

/*
 * void relocate_code (addr_sp, gd, addr_moni)
 *
 * This "function" does not return, instead it continues in RAM
 * after relocating the monitor code.
 *
 */
	.globl	relocate_code
relocate_code:
	mov	r4, r0	/* save addr_sp */
	mov	r5, r1	/* save addr of gd */
	mov	r6, r2	/* save addr of destination 0x402F0400*/

    @a5@ 代码重定位

代码重定向，它首先检测自己(MLO)是否已经在内存中：

如果是直接跳到下面的堆栈初始化代码 clear_bss。

如果不是就将自己从Nor Flash中拷贝到内存中。

Nor Flash 和Nand Flash 本质区别就在于是否进行代码拷贝，也就是下面代码所表述：无论
是Nor Flash 还是Nand Flash，核心思想就是将 uboot 代码搬运到内存中去运行，但是没有拷
贝bss 后面这段代码，只拷贝bss 前面的代码，bss 代码是放置全局变量的。Bss 段代码是为
了清零，拷贝过去再清零重复操作。

	/* Set up the stack						    */
stack_setup:
	mov	sp, r4

	adr	r0, _start
	cmp	r0, r6
	moveq	r9, #0		/* no relocation. relocation offset(r9) = 0 */
	beq	clear_bss		/* skip relocation */
	mov	r1, r6			/* r1 <- scratch for copy_loop */
	ldr	r3, _image_copy_end_ofs
	add	r2, r0, r3		/* r2 <- source end address	    */

copy_loop:                              /* 自拷贝 */
	ldmia	r0!, {r9-r10}		/* copy from source address [r0]    */
	stmia	r1!, {r9-r10}		/* copy to   target address [r1]    */
	cmp	r0, r2			/* until source end address [r2]    */
	blo	copy_loop

    @a6@ 清空 bss 段

clear_bss:

	ldr	r0, _bss_start_ofs
	ldr	r1, _bss_end_ofs
	mov	r4, r6			/* reloc addr */
	add	r0, r0, r4
	add	r1, r1, r4

	mov	r2, #0x00000000		/* clear			    */

clbss_l:str	r2, [r0]		/* clear loop...		    */
	add	r0, r0, #4
	cmp	r0, r1
	bne	clbss_l

/*
 * These are defined in the board-specific linker script.
 */
.globl _bss_start_ofs
_bss_start_ofs:
    .word __bss_start - _start          /* __bss_start = 0x80000000 */

    @a7@ 调用函数 board_init_r，用以完成 MLO（SPI）阶段的所有初始化，并跳转到 uboot.img 阶段

/*
 * We are done. Do not return, instead branch to second part of board
 * initialization, now running from RAM.
 */
jump_2_ram:
/*
 * If I-cache is enabled invalidate it
 */
#ifndef CONFIG_SYS_ICACHE_OFF
	mcr	p15, 0, r0, c7, c5, 0	@ invalidate icache
	mcr     p15, 0, r0, c7, c10, 4	@ DSB
	mcr     p15, 0, r0, c7, c5, 4	@ ISB
#endif
	ldr	r0, _board_init_r_ofs
	adr	r1, _start
	add	lr, r0, r1
	add	lr, lr, r9
	/* setup parameters for board_init_r */
	mov	r0, r5		/* gd_t */
	mov	r1, r6		/* dest_addr */
	/* jump to it ... */
	mov	pc, lr

_board_init_r_ofs:
	.word board_init_r - _start

《PATH : /arch/arm/cpu/armv7/omap-common/spl.c 》
void board_init_r(gd_t *id, ulong dummy)
{
	u32 boot_device;
	debug(">>spl:board_init_r()\n");

	timer_init();
	i2c_init(CONFIG_SYS_I2C_SPEED, CONFIG_SYS_I2C_SLAVE);

#ifdef CONFIG_SPL_BOARD_INIT
	spl_board_init();
#endif

	boot_device = omap_boot_device();
	debug("boot device - %d\n", boot_device);
	switch (boot_device) {
#ifdef CONFIG_SPL_MMC_SUPPORT
	case BOOT_DEVICE_MMC1:
	case BOOT_DEVICE_MMC2:
		spl_mmc_load_image();
		break;
#endif
#ifdef CONFIG_SPL_NAND_SUPPORT
	case BOOT_DEVICE_NAND:
		spl_nand_load_image();
		break;
#endif
#ifdef CONFIG_SPL_YMODEM_SUPPORT
	case BOOT_DEVICE_UART:
		spl_ymodem_load_image();
		break;
#endif
	default:
		printf("SPL: Un-supported Boot Device - %d!!!\n", boot_device);
		hang();
		break;
	}

	switch (spl_image.os) {
	case IH_OS_U_BOOT:
		debug("Jumping to U-Boot\n");
		jump_to_image_no_args();
		break;
	default:
		puts("Unsupported OS image.. Jumping nevertheless..\n");
		jump_to_image_no_args();
	}
}

    @a DONE@

3，第三级 bootloader：uboot.img 做了哪些事情？

uboot.img 内存分布如下：

访问 /arch/arm/lib/board.c 中 的 board_init_f() 函数：

在 uboot.img 运行过程中，有两个非常重要的结构体：gd_t 和 bd_t 。

其中 gd_t ：global_data 数据结构的定义，位于：/arch/arm/include/asm/global_data.h 中。

                 其成员主要是一些全局的系统初始化参数。

其中 bd_t ：bd_info 数据结构的定义，位于：/arch/arm/include/asm/u-boot.h 中。

                 其成员是开发板的相关参数。

/*
 * The following data structure is placed in some memory which 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 GENERATED_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	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_FSL_ESDHC
	unsigned long	sdhc_clk;
#endif
#ifdef CONFIG_AT91FAMILY
	/* "static data" needed by at91's clock.c */
	unsigned long	cpu_clk_rate_hz;
	unsigned long	main_clk_rate_hz;
	unsigned long	mck_rate_hz;
	unsigned long	plla_rate_hz;
	unsigned long	pllb_rate_hz;
	unsigned long	at91_pllb_usb_init;
#endif
#ifdef CONFIG_ARM
	/* "static data" needed by most of timer.c on ARM platforms */
	unsigned long	timer_rate_hz;
	unsigned long	tbl;
	unsigned long	tbu;
	unsigned long long	timer_reset_value;
	unsigned long	lastinc;
#endif
#ifdef CONFIG_IXP425
	unsigned long	timestamp;
#endif
	unsigned long	relocaddr;	/* Start address of U-Boot in RAM */
	phys_size_t	ram_size;	/* RAM size */
	unsigned long	mon_len;	/* monitor len */
	unsigned long	irq_sp;		/* irq stack pointer */
	unsigned long	start_addr_sp;	/* start_addr_stackpointer */
	unsigned long	reloc_off;
#if !(defined(CONFIG_SYS_ICACHE_OFF) && defined(CONFIG_SYS_DCACHE_OFF))
	unsigned long	tlb_addr;
#endif
	void		**jt;		/* jump table */
	char		env_buf[32];	/* buffer for getenv() before reloc. */
} gd_t;

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



typedef struct bd_info {
    int			bi_baudrate;	/* serial console baudrate */
    unsigned long	bi_ip_addr;	/* IP Address */
    ulong	        bi_arch_number;	/* unique id for this board */
    ulong	        bi_boot_params;	/* where this board expects params */
    struct				/* RAM configuration */
    {
	ulong start;
	ulong size;
    }			bi_dram[CONFIG_NR_DRAM_BANKS];
} bd_t;

其中 DECLARE_GLOBAL_DATA_PTR 宏定义在系统初始化过程中会被频繁调用，

其的作用是，声明gd这么一个全局的指针，这个指针指向gd_t结构体类型，并且这个gd指针是保存在ARM的r8这个寄存器里面的。

uboot.img 第一个运行的文件还是 start.o，其在运行访问的 board_init_f() 函数定义在 /arch/arm/lib/board.c 中：

void board_init_f(ulong bootflag)
{
	bd_t *bd;
	init_fnc_t **init_fnc_ptr;
	gd_t *id;
	ulong addr, addr_sp;

	/* Pointer is writable since we allocated a register for it */
	gd = (gd_t *) ((CONFIG_SYS_INIT_SP_ADDR) & ~0x07);
	/* compiler optimization barrier needed for GCC >= 3.4 */
	__asm__ __volatile__("": : :"memory");

	memset((void *)gd, 0, sizeof(gd_t));

        ...
}

#define CONFIG_SYS_INIT_RAM_ADDR	SRAM0_START
#define CONFIG_SYS_INIT_RAM_SIZE	SRAM0_SIZE
#define CONFIG_SYS_INIT_SP_ADDR		(CONFIG_SYS_INIT_RAM_ADDR + \
					 CONFIG_SYS_INIT_RAM_SIZE - \
					 GENERATED_GBL_DATA_SIZE)


#define SRAM0_START			0x402F0400


#define SRAM0_SIZE			(0x1B400) /* 109 KB */


#define GENERATED_GBL_DATA_SIZE (128) /* (sizeof(struct global_data) + 15) & ~15 */

因此，系统初始化参数将会被保存在 （保存 MLO（SPL）文件的内存空间的）末尾 2 KB 处。

通过计算的 gb 指针指向的内存空间地址为 gb = 0x4030B000

gb_t 结构体中某些元素的值是来自于 uboot.img's header，这个header的数据保存在内存的0x807FFFCO，大小为 64字节

/*
 * Legacy format image header,
 * all data in network byte order (aka natural aka bigendian).
 */
typedef struct image_header {
	uint32_t	ih_magic;	/* Image Header Magic Number	*/
	uint32_t	ih_hcrc;	/* Image Header CRC Checksum	*/
	uint32_t	ih_time;	/* Image Creation Timestamp	*/
	uint32_t	ih_size;	/* Image Data Size		*/
	uint32_t	ih_load;	/* Data	 Load  Address		*/
	uint32_t	ih_ep;		/* Entry Point Address		*/
	uint32_t	ih_dcrc;	/* Image Data CRC Checksum	*/
	uint8_t		ih_os;		/* Operating System		*/
	uint8_t		ih_arch;	/* CPU architecture		*/
	uint8_t		ih_type;	/* Image Type			*/
	uint8_t		ih_comp;	/* Compression Type		*/
	uint8_t		ih_name[IH_NMLEN];	/* Image Name		*/
} image_header_t;

/*
 * 8MB into the SDRAM to allow for SPL's bss at the beginning of SDRAM.
 * 64 bytes before this address should be set aside for u-boot.img's
 * header. That is 0x807FFFC0--0x80800000 should not be used for any
 * other needs.
 */
#define CONFIG_SYS_TEXT_BASE		0x80800000