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备注：下面的描述中n-xxx代表原生u-boot，z-xxx则代表致远的移值u-boot，例如
n-start.S代表原生态u-boot中的start.S文件，而z-start.S则代表致远的u-boot中的start.S.
程序先执行start.S（u-boot\cpu\arm926ejs）
这一点，z-start.S和n-start.S是一模一样的，没有什么变化，具体代码分析如下：
#include <config.h>
#include <common.h>
#include <version.h>
#if defined(CONFIG_OMAP1610)
#include <./configs/omap1510.h>
#elif defined(CONFIG_OMAP730)
#include <./configs/omap730.h>
#endif
/*
*************************************************************************
*
* Jump vector table as in table 3.1 in [1]
*
*************************************************************************
*/
//程序第一个入口_start，紧跟着中断向量表
.globl _start
_start:
b reset //上电reset
ldr pc, _undefined_instruction
ldr pc, _software_interrupt
ldr pc, _prefetch_abort
ldr pc, _data_abort
ldr pc, _not_used
ldr pc, _irq
ldr pc, _fiq
_undefined_instruction:
.word 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
.balignl 16,0xdeadbeef
/*
*************************************************************************
*
* Startup Code (reset vector)
*
* do important init only if we don't start from memory!
* setup Memory and board specific bits prior to relocation.
* relocate armboot to ram
* setup stack
*
*************************************************************************
*/
/*
*************************************************************************
*
* 起始代码（reset后的向量）
*
* 如果我们不是从RAM中启动，会做一些重要的初始化动作，主要包含
* SDRAM和一些关键位的初始化，以及代码relocate工作，也就是将
* 代码从flash中搬迁到SDRAM中，flash的读写速度过慢，最终还是要到
* SDRAM中运行。
* 初始化数据堆栈（C语言的所有函数都需要堆栈存放中间参数，变量）
*
*************************************************************************
*/
_TEXT_BASE:
.word TEXT_BASE //在_TEXT_BASE链接地址处存放一个数值，值为TEXT_BASE
.globl _armboot_start
_armboot_start:
.word _start
/*
* These are defined in the board-specific linker script.
*/
//_bss_start和bss_end的定义在link脚本中，也就是那个lds中
.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
*/
.globl reset
reset:
/*
* set the cpu to SVC32 mode //设定CPU为SVC32模式
*/
mrs r0,cpsr
bic r0,r0,#0x1f
orr r0,r0,#0xd3
msr cpsr,r0
/*
* we do sys-critical inits only at reboot,
* not when booting from ram!
*/
//boot只有不是在RAM中才会进行cpu_init_crit
//正常来说，cpu_init_crit就是实现MMU、SDRAM的初始化，但是就ru
//前面所述，飞思卡尔已经有了boot_prep，所以这里就不用包含SDRAM
//初始化，所以只需要禁止MMU等即可。
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
bl cpu_init_crit
#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
/*
*
* 上面的代码实现的就是将u-boot从flash中搬迁到RAM中，所以从这里也可以确定，在
* 代码搬迁前一定要初始化SDRAM,这里有个关键点，通过伪指令adr判断当前执行代码是
* 在SDRAM中还是flash中，具体的分析在前面有专门文章介绍，
*
*
*/
#endif /* CONFIG_SKIP_RELOCATE_UBOOT */
/* Set up the stack */ //代码搬迁后，自然要先进行堆栈初始化，为C运行打基础
stack_setup:
ldr r0, _TEXT_BASE /* upper 128 KiB: relocated uboot */
sub r0, r0, #CONFIG_SYS_MALLOC_LEN /* malloc area */
sub r0, r0, #CONFIG_SYS_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: //清除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
bl coloured_LED_init //led初始化，这里可有可无，无所谓
bl red_LED_on //点亮LED灯，可有可无吧。
ldr pc, _start_armboot //跳转到_start_armboot函数中
_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
/*
* Go setup Memory and board specific bits prior to relocation.
*/
mov ip, lr /* perserve link reg across call */
bl lowlevel_init /* go setup pll,mux,memory */
mov lr, ip /* restore link */
mov pc, lr /* back to my caller */
#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
@ carve out a frame on current user stack
sub sp, sp, #S_FRAME_SIZE
stmia sp, {r0 - r12} @ Save user registers (now in svc mode) r0-r12
ldr r2, _armboot_start
sub r2, r2, #(CONFIG_STACKSIZE+CONFIG_SYS_MALLOC_LEN)
sub r2, r2, #(CONFIG_SYS_GBL_DATA_SIZE+8) @ set base 2 words into abort stack
@ get values for "aborted" pc and cpsr (into parm regs)
ldmia r2, {r2 - r3}
add r0, sp, #S_FRAME_SIZE @ grab pointer to old stack
add r5, sp, #S_SP
mov r1, lr
stmia r5, {r0 - r3} @ save sp_SVC, lr_SVC, pc, cpsr
mov r0, sp @ save current stack into r0 (param register)
.endm
.macro irq_save_user_regs
sub sp, sp, #S_FRAME_SIZE
stmia sp, {r0 - r12} @ Calling r0-r12
@ !!!! R8 NEEDS to be saved !!!! a reserved stack spot would be good.
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+CONFIG_SYS_MALLOC_LEN)
sub r13, r13, #(CONFIG_SYS_GBL_DATA_SIZE+8) @ reserved a couple spots in abort stack
str lr, [r13] @ save caller lr in position 0 of saved stack
mrs lr, spsr @ get the spsr
str lr, [r13, #4] @ save spsr in position 1 of saved stack
mov r13, #MODE_SVC @ prepare SVC-Mode
@ msr spsr_c, r13
msr spsr, r13 @ switch modes, make sure moves will execute
mov lr, pc @ capture return pc
movs pc, lr @ jump to next instruction & switch modes.
.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