Uboot 分析（4）

还记得上节中，u-boot.lds文件连接的内容吗？？

代码段  数据段  命令段   bss段

下面来看第一部分：代码段，对应于uboot的第一阶段 start.S文件

附上代码：

 

<span style= "font-size: 14px;" >#include <config.h>
#include <version.h>


/*
 *************************************************************************
 *
 * Jump vector table as in table 3.1 in [1]
 *
 *************************************************************************
 */


.globl _start 　　 /*声明一个全局变量，可以被其他文件引用*/
_start: b       reset 　　　　  /* 复位 */ b是不带返回的跳转(bl是带返回的跳转)，意思是无条件直接跳转到_start执行程序
    <span style= "color: #ff0000;" >ldr   pc, _undefined_instruction</span>  　　 /* 未定义指令向量 ldr相当于mov操作*/
    ldr pc, _software_interrupt
    ldr pc, _prefetch_abort
    ldr pc, _data_abort
    ldr pc, _not_used
    ldr pc, _irq
    ldr pc, _fiq
/*  中断向量表入口地址 */
_undefined_instruction: .word undefined_instruction　　　　  /*就是在当前地址，即_undefined_instruction 处存放 undefined_instruction*/
_software_interrupt:    .word software_interrupt
_prefetch_abort:    .word prefetch_abort
_data_abort:        .word data_abort
_not_used:      .word not_used
_irq:           .word irq
_fiq:           .word fiq
/*　　word伪操作用于分配一段字内存单元(分配的单元都是字对齐的)，并用伪操作中的expr初始化*/ <br><br>
    .balignl 16,0xdeadbeef

/*</span><br><br>

他

<span style= "font-size: 14px;" >*/
/*
 *************************************************************************
 *
 * Startup Code (reset vector)
 *
 * do important init only if we don't start from memory!
 * relocate armboot to ram
 * setup stack
 * jump to second stage
 *
 *************************************************************************
 */

_TEXT_BASE:
    .word   TEXT_BASE　　　　</span><br><br><span style= "font-size: 14px;" >/*_TEXT_BASE:   .word    TEXT_BASE  //0x33F80000,在board/smdk2410/config.mk中</span>

<span style= "font-size: 14px;" >这段话表示，用户告诉编译器<span style= "color: #ff0000;" >编译地址</span>的起始地址*/

.globl _armboot_start
_armboot_start:
    .word _start

/*
 * These are defined in the board-specific linker script.
 */
.globl _bss_start
_bss_start:
    .word __bss_start

.globl _bss_end
_bss_end:
    .word _end

#ifdef CONFIG_USE_IRQ
/* IRQ stack memory (calculated at run-time) */
.globl IRQ_STACK_START
IRQ_STACK_START:
    .word   0x0badc0de

/* IRQ stack memory (calculated at run-time) */
.globl FIQ_STACK_START
FIQ_STACK_START:
    .word 0x0badc0de
#endif


/*
 * the actual reset code
 */

reset:
    /*
     * set the cpu to SVC32 mode
     */
    mrs r0,cpsr
    bic r0,r0,#0x1f　　 /*工作模式位清零*/
    orr r0,r0,#0xd3　　 /*工作模式位设置为“10011”（管理模式），并将中断禁止位和快中断禁止位置1 */
    msr cpsr,r0
/*<span> 以上代码将CPU的工作模式位设置为管理模式，即设置相应的CPSR程序状态字，并将中断禁止位和快中断禁止位置一，<br>从而屏蔽了IRQ和FIQ中断。</span>*/ <br>
/* turn off the watchdog */
#if defined(CONFIG_S3C2400)
# define pWTCON     0x15300000
# define INTMSK     0x14400008  /* Interupt-Controller base addresses */
# define CLKDIVN    0x14800014  /* clock divisor register */
#elif defined(CONFIG_S3C2410)
# define pWTCON     0x53000000
# define INTMSK     0x4A000008  /* Interupt-Controller base addresses */
# define INTSUBMSK  0x4A00001C
# define CLKDIVN    0x4C000014  /* clock divisor register */
#endif
<br> /*<span> 以上代码完成了WATCHDOG，INTMSK，INTSUBMSK，CLKDIVN四个寄存器的地址的设置</span>*/
#if defined(CONFIG_S3C2400) || defined(CONFIG_S3C2410)
    ldr     r0, =pWTCON
    mov     r1, #0x0
    str     r1, [r0]

    /*
    <span style="color: #ff0000;"> * mask all IRQs by setting all bits in the INTMR - default</span>
     */
    mov r1, #0xffffffff
    ldr r0, =INTMSK
    str r1, [r0]
# if defined(CONFIG_S3C2410)
    ldr r1, =0x3ff
    ldr r0, =INTSUBMSK
    str r1, [r0]
# endif
/*<span style="color: #ff0000;"><strong>设置MPLLCON,UPLLCON, CLKDIVN</strong>*</span>/
    /* FCLK:HCLK:PCLK = 1:2:4 */
    /* default FCLK is 120 MHz ! */
    ldr r0, =CLKDIVN
    mov r1, #3
    str r1, [r0]
#endif  /* CONFIG_S3C2400 || CONFIG_S3C2410 */

    /*
     * we do sys-critical inits only at reboot,
     * not when booting from ram!
     */ <br> /*接着往下看：*/
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
    bl  cpu_init_crit　<span> /* </span><span>;跳转并把转移后面紧接的一条指令地址保存到链接寄存器LR(R14)中，<br>　　　　　　　　　　　　　　　　　　　　　　以此来完成子程序的调用*/ </span>
#endif

#ifndef CONFIG_SKIP_RELOCATE_UBOOT
relocate:               /* relocate U-Boot to RAM       */
    adr r0, _start      /* r0 <- current position of code   */
    ldr r1, _TEXT_BASE      /* test if we run from flash or RAM */
    cmp     r0, r1                  /* don't reloc during debug         */
    beq     stack_setup

    ldr r2, _armboot_start
    ldr r3, _bss_start
    sub r2, r3, r2      /* r2 <- size of armboot            */
    add r2, r0, r2      /* r2 <- source end address         */

copy_loop:
    ldmia   r0!, {r3-r10}       /* copy from source address [r0]    */
    stmia   r1!, {r3-r10}       /* copy to   target address [r1]    */
    cmp r0, r2          /* until source end addreee [r2]    */
    ble copy_loop
#endif  /* CONFIG_SKIP_RELOCATE_UBOOT */

    /* Set up the stack                         */
stack_setup:
    ldr r0, _TEXT_BASE      /* upper 128 KiB: relocated uboot   */
    sub r0, r0, #CFG_MALLOC_LEN /* malloc area                      */
    sub r0, r0, #CFG_GBL_DATA_SIZE /* bdinfo                        */
#ifdef CONFIG_USE_IRQ
    sub r0, r0, #(CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ)
#endif
    sub sp, r0, #12     /* leave 3 words for abort-stack    */

clear_bss:
    ldr r0, _bss_start      /* find start of bss segment        */
    ldr r1, _bss_end        /* stop here                        */
    mov     r2, #0x00000000     /* clear                            */

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

#if 0
    /* try doing this stuff after the relocation */
    ldr     r0, =pWTCON
    mov     r1, #0x0
    str     r1, [r0]

    /*
     * mask all IRQs by setting all bits in the INTMR - default
     */
    mov r1, #0xffffffff
    ldr r0, =INTMR
    str r1, [r0]

    /* FCLK:HCLK:PCLK = 1:2:4 */
    /* default FCLK is 120 MHz ! */
    ldr r0, =CLKDIVN
    mov r1, #3
    str r1, [r0]
    /* END stuff after relocation */
#endif

    ldr pc, _start_armboot

_start_armboot: .word start_armboot


/*
 *************************************************************************
 *
 * CPU_init_critical registers
 *
 * setup important registers
 * setup memory timing
 *
 *************************************************************************
 */


#ifndef CONFIG_SKIP_LOWLEVEL_INIT
cpu_init_crit:
    /*
     * flush v4 I/D caches
     */
    mov r0, #0
    mcr p15, 0, r0, c7, c7, 0   /* flush v3/v4 cache */
    mcr p15, 0, r0, c8, c7, 0   /* flush v4 TLB */

    /*
     * disable MMU stuff and caches
     */
    mrc p15, 0, r0, c1, c0, 0
    bic r0, r0, #0x00002300 @ clear bits 13, 9:8 (--V- --RS)
    bic r0, r0, #0x00000087 @ clear bits 7, 2:0 (B--- -CAM)
    orr r0, r0, #0x00000002 @ set bit 2 (A) Align
    orr r0, r0, #0x00001000 @ set bit 12 (I) I-Cache
    mcr p15, 0, r0, c1, c0, 0

    /*
     * before relocating, we have to setup RAM timing
     * because memory timing is board-dependend, you will
     * find a lowlevel_init.S in your board directory.
     */
    mov ip, lr
    bl  lowlevel_init
    mov lr, ip
    mov pc, lr
#endif /* CONFIG_SKIP_LOWLEVEL_INIT */

/*
 *************************************************************************
 *
 * Interrupt handling
 *
 *************************************************************************
 */

@
@ IRQ stack frame.
@
#define S_FRAME_SIZE    72

#define S_OLD_R0    68
#define S_PSR       64
#define S_PC        60
#define S_LR        56
#define S_SP        52

#define S_IP        48
#define S_FP        44
#define S_R10       40
#define S_R9        36
#define S_R8        32
#define S_R7        28
#define S_R6        24
#define S_R5        20
#define S_R4        16
#define S_R3        12
#define S_R2        8
#define S_R1        4
#define S_R0        0

#define MODE_SVC 0x13
#define I_BIT    0x80

/*
 * use bad_save_user_regs for abort/prefetch/undef/swi ...
 * use irq_save_user_regs / irq_restore_user_regs for IRQ/FIQ handling
 */

    .macro  bad_save_user_regs
    sub sp, sp, #S_FRAME_SIZE
    stmia   sp, {r0 - r12}          @ Calling r0-r12
    ldr r2, _armboot_start
    sub r2, r2, #(CONFIG_STACKSIZE+CFG_MALLOC_LEN)
    sub r2, r2, #(CFG_GBL_DATA_SIZE+8)  @ set base 2 words into abort  stack
    ldmia   r2, {r2 - r3}           @ get pc, cpsr
    add r0, sp, #S_FRAME_SIZE       @ restore sp_SVC

    add r5, sp, #S_SP
    mov r1, lr
    stmia   r5, {r0 - r3}           @ save sp_SVC, lr_SVC, pc, cpsr
    mov r0, sp
    .endm

    .macro  irq_save_user_regs
    sub sp, sp, #S_FRAME_SIZE
    stmia   sp, {r0 - r12}          @ Calling r0-r12
    add     r8, sp, #S_PC
    stmdb   r8, {sp, lr}^                   @ Calling SP, LR
    str     lr, [r8, #0]                    @ Save calling PC
    mrs     r6, spsr
    str     r6, [r8, #4]                    @ Save CPSR
    str     r0, [r8, #8]                    @ Save OLD_R0
    mov r0, sp
    .endm

    .macro  irq_restore_user_regs
    ldmia   sp, {r0 - lr}^          @ Calling r0 - lr
    mov r0, r0
    ldr lr, [sp, #S_PC]         @ Get PC
    add sp, sp, #S_FRAME_SIZE
    subs    pc, lr, #4          @ return  & move spsr_svc into cpsr
    .endm

    .macro get_bad_stack
    ldr r13, _armboot_start     @ setup our mode stack
    sub r13, r13, #(CONFIG_STACKSIZE+CFG_MALLOC_LEN)
    sub r13, r13, #(CFG_GBL_DATA_SIZE+8) @ reserved a couple spots in abort  stack

    str lr, [r13]           @ save caller lr / spsr
    mrs lr, spsr
    str     lr, [r13, #4]

    mov r13, #MODE_SVC          @ prepare SVC-Mode
    @ msr   spsr_c, r13
    msr spsr, r13
    mov lr, pc
    movs    pc, lr
    .endm

    .macro get_irq_stack            @ setup IRQ stack
    ldr sp, IRQ_STACK_START
    .endm

    .macro get_fiq_stack            @ setup FIQ stack
    ldr sp, FIQ_STACK_START
    .endm

/*
 * exception handlers
 */
    .align  5
undefined_instruction:
    get_bad_stack
    bad_save_user_regs
    bl  do_undefined_instruction

    .align  5
software_interrupt:
    get_bad_stack
    bad_save_user_regs
    bl  do_software_interrupt

    .align  5
prefetch_abort:
    get_bad_stack
    bad_save_user_regs
    bl  do_prefetch_abort

    .align  5
data_abort:
    get_bad_stack
    bad_save_user_regs
    bl  do_data_abort

    .align  5
not_used:
    get_bad_stack
    bad_save_user_regs
    bl  do_not_used

#ifdef CONFIG_USE_IRQ

    .align  5
irq:
    get_irq_stack
    irq_save_user_regs
    bl  do_irq
    irq_restore_user_regs

    .align  5
fiq:
    get_fiq_stack
    /* someone ought to write a more effiction fiq_save_user_regs */
    irq_save_user_regs
    bl  do_fiq
    irq_restore_user_regs

#else

    .align  5
irq:
    get_bad_stack
    bad_save_user_regs
    bl  do_irq

    .align  5
fiq:
    get_bad_stack
    bad_save_user_regs
    bl  do_fiq

#endif
</span>

　　上面给出了代码的部分相关注释，个人学习uboot的总结是，要仔细的反复熟读源码框架，然后再进行不停地分析明一条指令

　　下节具体分析第一阶段的源码，以及为什么要那么做？？