分类： 嵌入式
2010-01-11 00:06:39
因为其他的事情，关于u-boot的学习搁浅了有几周了，想要了解u-boot.很好的移植u-boot到一块新的板子上，有必要了解启动代码，从最开始的start.s学习起。
/*
* armboot - Startup Code for ARM920 CPU-core
*
* Copyright (c) 2001 Marius Gr鰃er <mag@sysgo.de>
* Copyright (c) 2002 Alex Z黳ke <azu@sysgo.de>
* Copyright (c) 2002
*
* See file CREDITS for list of people who contributed to this
* project.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation; either version 2 of
* the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc.,
* MA 02111-1307
*/
#include <config.h>
//包含头文件，这个文件在目录../u-boot-
//其实这个头文件里只包含有一句：”#include
//这个头文件的创建要回到mkconfig里后面才知道，在这里有
// 85 echo “#include
//这个头文件就是对硬地址的定义
#include <version.h>
//版本信息
/*
*************************************************************************
*
* Jump vector table as in table
*跳转中断向量表
*
*************************************************************************
*/
.globl _start @.global 全局声明标志，这样声明的标号将可以被外部使用。(与armasm中的EXPORT相同)。
_start: b reset @跳转到复位函数reset，这个函数在下面定义，这里使用b是不带返回的跳转,即不将PC值保存到LR寄存器里
@这里_start的链接地并不是0x00000000而是TEXT_BASE（这个在../board/smdk2410/config.mk）,但它却是程序
@的入口位置
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!
* relocate armboot to ram
* setup stack
* jump to second stage
*
*************************************************************************
*/
_TEXT_BASE:
.word TEXT_BASE
.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 @设置程序状态寄存器，设置为ARM指令，超级用户状态
bic r0,r0,#0x
orr r0,r0,#0xd3 @再置位相应的位
msr cpsr,r0 @将值写回CPSR内
/* turn off the watchdog */ @关闭看门狗
#if defined(CONFIG_S
# define pWTCON 0x15300000
# define INTMSK 0x14400008 /* Interupt-Controller base addresses */
# define CLKDIVN 0x14800014 /* clock divisor register */
#elif defined(CONFIG_S
# define pWTCON 0x53000000
# define INTMSK 0x
# define INTSUBMSK 0x
# define CLKDIVN 0x
#endif
#if defined(CONFIG_S
ldr r0, =pWTCON @将寄存器pWTCON的地装载至r0寄存器，以便赋值
mov r1, #0x0 @将立即数0x0装载到r1中
str r1, [r0] @将r1内的数值存入到pWTCON中，即修改了pWTCON内的值
//对于特殊寄存器的操作，据根手册得知需要用指令LDR/STR进行操作，或者使用整型指针
/*
* mask all IRQs by setting all bits in the INTMR – default @屏蔽所有的中断，通过设置INTMSR寄存器里的所有位
*/
mov r1, #0xffffffff
ldr r0, =INTMSK @在这里我一直有个疑问，为什么反汇编后，这里会变成ldr r0, [pc, #864]
str r1, [r0]
# if defined(CONFIG_S
ldr r1, =0x3ff @如果是S
ldr r0, =INTSUBMSK @在这里我也疑问，为什么反汇编后，这里会变成ldr r0, [pc, #860]
str r1, [r0]
# endif
/* FCLK:HCLK:PCLK = 1:2:4 */
/* default FCLK is 120 MHz ! */
ldr r0, =CLKDIVN @设置时钟，设置分频比，想修改时钟频率就可以在这里修改 反汇编后，这里会变成ldr r0, [pc, #856]
mov r1, #3
str r1, [r0]
#endif /* CONFIG_S
/*
* we do sys-critical inits only at reboot,
* not when booting from ram!
*/
#ifndef CONFIG_SKIP_LOWLEVEL_INIT
bl cpu_init_crit @如果配置了底层初始化就跳转至 子函数处初始化cpu_init_crit,这里使用的是带返回的跳转指令BL
@跳转同时还将当前PC值保存到LR中,以便程序返回
#endif
#ifndef CONFIG_SKIP_RELOCATE_UBOOT @如果配置了重定位项就执行下面程序
relocate: /* relocate U-Boot to RAM */ @将U-boot代码搬运至RAM中
adr r0, _start /* r0 <- current position of code */ @读取当前程序起始位置地址,adr是将基于当前pc（注意还是在flash中）地址加载到r0
ldr r1, _TEXT_BASE /* test if we run from flash or RAM */ @要搬运的基地，在这里是内存的基地址
cmp r0, r1 /* don't reloc during debug */ @比较确定是否已在内存中运行
beq stack_setup @如果是调试则跳到stack_setup中执行,建立堆栈
ldr r2, _armboot_start @将u-boot要搬运的代码的起始地址装载到r2中
ldr r3, _bss_start @bss段起始地址
sub r2, r3, r2 /* r2 <- size of armboot */ @计算出u-boot要搬运代码的大小
add r2, r0, r2 /* r2 <- source end address */ @代码的结束地址
copy_loop:
ldmia r0!, {r3-r10} /* copy from source address [r0] */ @先从flash中拷贝出来，这里r0保存的是要拷贝的代码地址
stmia r1!, {r3-r10} /* copy to target address [r1] */ @再放到内存中，这里r1是要放置代码的地址
cmp r0, r2 /* until source end addreee [r2] */ @比较一下拷贝是否完成
ble copy_loop
#endif /* CONFIG_SKIP_RELOCATE_UBOOT */
/* Set up the stack */
stack_setup:
ldr r0, _TEXT_BASE /* upper 128 KiB: relocated uboot */ @建立堆栈
sub r0, r0, #CFG_MALLOC_LEN /* malloc area */ @向下为动态内存分配空间，即为malloc分配空间
sub r0, r0, #CFG_GBL_DATA_SIZE /* bdinfo */ @为bd结构分配空间
#ifdef CONFIG_USE_IRQ
sub r0, r0, #(CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ)
#endif
sub sp, r0, #12 /* leave 3 words for abort-stack */ @为abort异常预留12字节的空间，并将当前的地址赋给sp，这样就为内存栈
@设置好了，之后如果在u-boot中运行程序时需要使用栈的时候就从这里开始。
clear_bss: @清除数据段
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 @第一次跳入bank6的内存执行
_start_armboot: .word start_armboot @跳转到_start_armboot，也就是函数 start_armboot，
@此函数存放在u-boot-2009.03/lib_arm/board.c，这样就到了u-boot的第二阶段了
/*
*************************************************************************
*
* CPU_init_critical registers
*
* setup important registers @建立一些重要的寄存器
* setup memory timing @建立相关的存储器时序
*
*************************************************************************
*/
#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 */ @禁止指令缓存与数据缓存，这里协处理器的命令要参考手册
mcr p15, 0, r0, c8, c7, 0 /* flush v4 TLB */ @禁止后备缓存
/*
* disable MMU stuff and caches
*/
mrc p15, 0, r0, c1, c0, 0 @禁止内存管理
bic r0, r0, #0x00002300 @ clear bits 13, 9:8 (--V- --RS)
bic r0, r0, #0x00000087 @ clear bits 7, 2:0 (B--- -
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.
*/
@在重定位之前，我们必须对RAM时序进行初始化，因存储时序是与开发板相关的，所以你可以在你的
@开发板目录下可以找到lowlevel_init.S这个文件(这是移植要修改的文件之一)
mov ip, lr @将保存在lr内的值写入ip，即将上次跳转函数时的程序指针保存，以便在子函数再跳转
bl lowlevel_init @跳转至函数lowlevel_init当前的程序指针保存入lr，以便返回
mov lr, ip @将ip的值赋给lr，就是恢复到跳转入此子函数时保存的程序指针
mov pc, lr @子函数返回，即返回到bl cpu_init_crit下一个地址
#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
