uboot_2013.07 start.S 流程分析

基于 smdk 2410.

start.S

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.globl _start //.global 声明一个全局变量(4个字节), 供外部函数调用
_start: b start_code //处理器上电后执行的第一天指令，该条指令占四个字节
ldr pc, _undefined_instruction // 未定义指令异常处理向量地址, 跳转向量表, 和芯片的体系结构有关.
ldr pc, _software_interrupt //软中断. ldr 把第二个操作数指向的地址数据传给 PC
ldr pc, _prefetch_abort //指令预取终止
ldr pc, _data_abort //数据访问终止
ldr pc, _not_used //保留未用
ldr pc, _irq //中断处理
ldr pc, _fiq //快速中断
//下面七条指令是分配一个字(四个字节)的空间，存放异常处理函数的入口地址
_undefined_instruction: .word undefined_instruction //.word 为定义一个 4 字节的空间. undefined_instruction 为地址, 即标号所对应的偏移地址.
_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
//上面的一共15条指令，每条指令占四个字节，一共15*4=60字节
.balignl 16,0xdeadbeef //用四个字节(死牛肉)填充，刚好64字节，以16字节对齐

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.globl _TEXT_BASE
_TEXT_BASE:
#if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_TEXT_BASE)
.word CONFIG_SPL_TEXT_BASE
#else
.word CONFIG_SYS_TEXT_BASE // include/configs/$BOARD.h 例如 CONFIG_SYS_TEXT_BASE = 0x33F80000, 定义为在代码的存放地址.
#endif
/*
* __bss_start，__bss_end__，_end都是定义在链接脚本文件\arch\arm\cpu\arm920t\uboot.lds
*/
.globl _bss_start_ofs
_bss_start_ofs: //存放bss段开始地址的偏移量
.word __bss_start - _start
.globl _bss_end_ofs
_bss_end_ofs: //存放bss段结束地址的偏移量
.word __bss_end - _start
.globl _end_ofs
_end_ofs: //_end标号结束地址偏移量
.word _end - _start
//到此刚好分配了16字节
#ifdef CONFIG_USE_IRQ //分配irq栈空间
/* IRQ stack memory (calculated at run-time) */
.globl IRQ_STACK_START
IRQ_STACK_START: //存放irq栈空间的起始地址, calculated at run-time 运行时分配
.word 0x0badc0de
/* IRQ stack memory (calculated at run-time) */
.globl FIQ_STACK_START
FIQ_STACK_START: //存放fiq栈空间的起始地址, calculated at run-time 运行时分配
.word 0x0badc0de
#endif
/* IRQ stack memory (calculated at run-time) + 8 bytes */
.globl IRQ_STACK_START_IN
IRQ_STACK_START_IN: //存放irq栈空间的起始地址
.word 0x0badc0de

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start_code:
/*
* set the cpu to SVC32 mode
*/
mrs r0, cpsr
bic r0, r0, #0x1f //设置cpu为管理模式
orr r0, r0, #0xd3 //关闭irq，fiq中断
msr cpsr, r0
# define pWTCON 0x53000000
# define INTMSK 0x4A000008 /* Interrupt-Controller base addresses */
# define INTSUBMSK 0x4A00001C
# define CLKDIVN 0x4C000014 /* clock divisor register */
//关闭看门狗
ldr r0, =pWTCON
mov r1, #0x0
str r1, [r0]
/*
* 设置所有的中断掩码，屏蔽所有一级中断
*/
mov r1, #0xffffffff
ldr r0, =INTMSK
str r1, [r0]
//屏蔽二级中断
ldr r1, =0x3ff
ldr r0, =INTSUBMSK
str r1, [r0]
/* FCLK:HCLK:PCLK = 1:4:8 */
ldrr0, =CLKDIVN
movr1, #5
strr1, [r0]
#ifndef CONFIG_SKIP_LOWLEVEL_INIT //该宏用于直接下载到内存中调试用的
blcpu_init_crit //该函数用于I/D caech,tlb的刷新，以及mmu的设置，sdram的设置
#endif
bl_main //_main 定义在 arch/arm/lib/crt0.S 中, 即原来Start.S中部分的汇编代码进行了整合, 一般crt0.S的代码不需要改动.

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/*------------------------------------------------------------------------------*/
.globl c_runtime_cpu_setup
c_runtime_cpu_setup:
mov pc, lr
/*
*************************************************************************
*
* CPU_init_critical registers
*
* setup important registers
* setup memory timing
*
*************************************************************************
*/
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
cpu_init_crit:
/*
* flush v4 I/D caches; 刷新指令和数据缓存，TLB缓存表
*/
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 关闭 mmu
*/
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 /* ./board/samsung/smdk2410/lowlevel_init.S 对内存进行初始化, 串口等初级输出, LED 状态灯点亮等等 */
mov lr, ip
mov pc, lr
#endif /* CONFIG_SKIP_LOWLEVEL_INIT */