关于stm32上电之后的启动流程，梳理一下思路，如有错误，回头来改。
加载程序：当用户选择想单片机烧录程序的时候，BOOT0、BOOT1来决定代码存放的位置。并将这片存储空间重新映射到0x00000000开始的位置。
启动程序：然后程序从开始处执行，首先执行的不是main()，而是启动文件
异常信号处理：当用户按下复位键(或者产生其他异常信号)的时候，CPU查询中断向量表，跳转到相对应的处理。
stm32的启动模式
程序映像通常是从地址 0x00000000 开始的、系统复位后，处理器首先读取向量表中的前两个字（8 个字节），第一个字存入 MSP，第二个字为复位向量，它表示程序执行的起始地址（复位处理）
启动模式的设置
BOOT0和 BOOT1用于设置 STM32的启动方式，启动模式只决定程序烧录的位置，加载完 程序之后会有一个重映射(映射到0x00000000地址位置)；真正产生复位信号的时候，CPU还是从开始位置执行。
C语言内存分布
在运行C用户程序的时候，内存将数据和代码分成几个部分
启动文件分析
以STM32F051启动文件为例，与其他系列的stm32单片机的启动过程都是差不多的。
首先声明堆栈空间，定义了异常向量表(其中有很多异常处理的入口地址)，然后进行系统时钟初始化，堆栈空间的初始化，最后才跳转到C程序中的main()函数，执行用户程序。总言之，启动文件设置了用户程序的运行环境。
;******************** (C) COPYRIGHT 2016 STMicroelectronics ********************
;* File Name : startup_stm32f051x8.s
;* Author : MCD Application Team
;* Description : STM32F051x4/STM32F051x6/STM32F051x8 devices vector table for MDK-ARM toolchain.
;* This module performs:
;* - Set the initial SP
;* - Set the initial PC == Reset_Handler
;* - Set the vector table entries with the exceptions ISR address
;* - Branches to __main in the C library (which eventually
;* calls main()).
;* After Reset the CortexM0 processor is in Thread mode,
;* priority is Privileged, and the Stack is set to Main.
;* <<< Use Configuration Wizard in Context Menu >>>
;*******************************************************************************
;*
;* Redistribution and use in source and binary forms, with or without modification,
;* are permitted provided that the following conditions are met:
;* 1. Redistributions of source code must retain the above copyright notice,
;* this list of conditions and the following disclaimer.
;* 2. Redistributions in binary form must reproduce the above copyright notice,
;* this list of conditions and the following disclaimer in the documentation
;* and/or other materials provided with the distribution.
;* 3. Neither the name of STMicroelectronics nor the names of its contributors
;* may be used to endorse or promote products derived from this software
;* without specific prior written permission.
;*
;* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
;* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
;* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
;* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
;* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
;* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
;* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
;* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
;* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
;* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
;
;*******************************************************************************
; Amount of memory (in bytes) allocated for Stack
; Tailor this value to your application needs
; <h> Stack Configuration
; <o> Stack Size (in Bytes) <0x0-0xFFFFFFFF:8>
; </h>
Stack_Size EQU 0x400 ;定义栈的大小1K
AREA STACK, NOINIT, READWRITE, ALIGN=3 ;AREA 是定义一个段，此处定义的是栈段，没有初始化的段，可读可写，align是说段的起始地址应该是8的倍数
Stack_Mem SPACE Stack_Size ;SPACE 是为栈分配空间，开辟栈空间大小为0x400
__initial_sp ;__initial_sp是个标号，这个标号指向栈的栈顶
; <h> Heap Configuration
; <o> Heap Size (in Bytes) <0x0-0xFFFFFFFF:8>
; </h>
Heap_Size EQU 0x200 ;定义堆的大小 512B
AREA HEAP, NOINIT, READWRITE, ALIGN=3
__heap_base ;堆的起始地址
Heap_Mem SPACE Heap_Size ;堆的大小
__heap_limit ;堆的结束地址
PRESERVE8
THUMB
; Vector Table Mapped to Address 0 at Reset
AREA RESET, DATA, READONLY ;此处定义的数据段，数据段放的是中断向量表，只读
EXPORT __Vectors ;EXPORT在程序中声明一个全局的标号__Vectors，该标号可在其它的文件中引用
EXPORT __Vectors_End
EXPORT __Vectors_Size
;DCD指令以字为单位分配内存，要求4字节对齐，并要求初始化这些内存。
;在向量表中，DCD 分配了一堆内存，并且以 ESR 的入口地址初始化它们。
__Vectors DCD __initial_sp ; Top of Stack 存放__initial_sp，也就是堆栈栈顶的地址
DCD Reset_Handler ; Reset Handler
DCD NMI_Handler ; NMI Handler
DCD HardFault_Handler ; Hard Fault Handler
DCD 0 ; Reserved
DCD 0 ; Reserved
DCD 0 ; Reserved
DCD 0 ; Reserved
DCD 0 ; Reserved
DCD 0 ; Reserved
DCD 0 ; Reserved
DCD SVC_Handler ; SVCall Handler
DCD 0 ; Reserved
DCD 0 ; Reserved
DCD PendSV_Handler ; PendSV Handler
DCD SysTick_Handler ; SysTick Handler
; External Interrupts
DCD WWDG_IRQHandler ; Window Watchdog
DCD PVD_IRQHandler ; PVD through EXTI Line detect
DCD RTC_IRQHandler ; RTC through EXTI Line
DCD FLASH_IRQHandler ; FLASH
DCD RCC_IRQHandler ; RCC
DCD EXTI0_1_IRQHandler ; EXTI Line 0 and 1
DCD EXTI2_3_IRQHandler ; EXTI Line 2 and 3
DCD EXTI4_15_IRQHandler ; EXTI Line 4 to 15
DCD TSC_IRQHandler ; TS
DCD DMA1_Channel1_IRQHandler ; DMA1 Channel 1
DCD DMA1_Channel2_3_IRQHandler ; DMA1 Channel 2 and Channel 3
DCD DMA1_Channel4_5_IRQHandler ; DMA1 Channel 4 and Channel 5
DCD ADC1_COMP_IRQHandler ; ADC1, COMP1 and COMP2
DCD TIM1_BRK_UP_TRG_COM_IRQHandler ; TIM1 Break, Update, Trigger and Commutation
DCD TIM1_CC_IRQHandler ; TIM1 Capture Compare
DCD TIM2_IRQHandler ; TIM2
DCD TIM3_IRQHandler ; TIM3
DCD TIM6_DAC_IRQHandler ; TIM6 and DAC
DCD 0 ; Reserved
DCD TIM14_IRQHandler ; TIM14
DCD TIM15_IRQHandler ; TIM15
DCD TIM16_IRQHandler ; TIM16
DCD TIM17_IRQHandler ; TIM17
DCD I2C1_IRQHandler ; I2C1
DCD I2C2_IRQHandler ; I2C2
DCD SPI1_IRQHandler ; SPI1
DCD SPI2_IRQHandler ; SPI2
DCD USART1_IRQHandler ; USART1
DCD USART2_IRQHandler ; USART2
DCD 0 ; Reserved
DCD CEC_IRQHandler ; CEC
__Vectors_End
__Vectors_Size EQU __Vectors_End - __Vectors
AREA |.text|, CODE, READONLY ;text 一般代表代码段
; Reset handler routine
Reset_Handler PROC
EXPORT Reset_Handler [WEAK] ;“[WEAK]”是一个弱符号声明，就是告诉编译器，我这里声明的标号的优先权低于其它同名的标号，也就是说如果在工程中还有别的函数和我同名，那么你就调用和我同名的其它函数，如果没有同名的，那你就调用我。
IMPORT __main ;IMPORT则表明后面标号所代表的函数是调用其它文件的
IMPORT SystemInit
LDR R0, =SystemInit ;复位子程序是系统上电后第一个执行的程序，调用 SystemInit 函数初始化系统时钟
BLX R0
LDR R0, =__main ;__main 是一个标准的 C 库函数，主要作用是初始化用户堆栈，并在函数的最后调用main 函数去到 C 的世界
BX R0
ENDP
; Dummy Exception Handlers (infinite loops which can be modified)
NMI_Handler PROC
EXPORT NMI_Handler [WEAK]
B .
ENDP
HardFault_Handler\
PROC
EXPORT HardFault_Handler [WEAK]
B .
ENDP
SVC_Handler PROC
EXPORT SVC_Handler [WEAK]
B .
ENDP
PendSV_Handler PROC
EXPORT PendSV_Handler [WEAK]
B .
ENDP
SysTick_Handler PROC
EXPORT SysTick_Handler [WEAK]
B .
ENDP
Default_Handler PROC
EXPORT WWDG_IRQHandler [WEAK]
EXPORT PVD_IRQHandler [WEAK]
EXPORT RTC_IRQHandler [WEAK]
EXPORT FLASH_IRQHandler [WEAK]
EXPORT RCC_IRQHandler [WEAK]
EXPORT EXTI0_1_IRQHandler [WEAK]
EXPORT EXTI2_3_IRQHandler [WEAK]
EXPORT EXTI4_15_IRQHandler [WEAK]
EXPORT TSC_IRQHandler [WEAK]
EXPORT DMA1_Channel1_IRQHandler [WEAK]
EXPORT DMA1_Channel2_3_IRQHandler [WEAK]
EXPORT DMA1_Channel4_5_IRQHandler [WEAK]
EXPORT ADC1_COMP_IRQHandler [WEAK]
EXPORT TIM1_BRK_UP_TRG_COM_IRQHandler [WEAK]
EXPORT TIM1_CC_IRQHandler [WEAK]
EXPORT TIM2_IRQHandler [WEAK]
EXPORT TIM3_IRQHandler [WEAK]
EXPORT TIM6_DAC_IRQHandler [WEAK]
EXPORT TIM14_IRQHandler [WEAK]
EXPORT TIM15_IRQHandler [WEAK]
EXPORT TIM16_IRQHandler [WEAK]
EXPORT TIM17_IRQHandler [WEAK]
EXPORT I2C1_IRQHandler [WEAK]
EXPORT I2C2_IRQHandler [WEAK]
EXPORT SPI1_IRQHandler [WEAK]
EXPORT SPI2_IRQHandler [WEAK]
EXPORT USART1_IRQHandler [WEAK]
EXPORT USART2_IRQHandler [WEAK]
EXPORT CEC_IRQHandler [WEAK]
WWDG_IRQHandler
PVD_IRQHandler
RTC_IRQHandler
FLASH_IRQHandler
RCC_IRQHandler
EXTI0_1_IRQHandler
EXTI2_3_IRQHandler
EXTI4_15_IRQHandler
TSC_IRQHandler
DMA1_Channel1_IRQHandler
DMA1_Channel2_3_IRQHandler
DMA1_Channel4_5_IRQHandler
ADC1_COMP_IRQHandler
TIM1_BRK_UP_TRG_COM_IRQHandler
TIM1_CC_IRQHandler
TIM2_IRQHandler
TIM3_IRQHandler
TIM6_DAC_IRQHandler
TIM14_IRQHandler
TIM15_IRQHandler
TIM16_IRQHandler
TIM17_IRQHandler
I2C1_IRQHandler
I2C2_IRQHandler
SPI1_IRQHandler
SPI2_IRQHandler
USART1_IRQHandler
USART2_IRQHandler
CEC_IRQHandler
B .
ENDP
ALIGN
;*******************************************************************************
; User Stack and Heap initialization
;*******************************************************************************
IF :DEF:__MICROLIB ;检查是否定义了__MICROLIB,如果定义了则条件成立。用户可以在配置软件中勾选。
;用户栈和堆初始化,由C库函数_main来完成
EXPORT __initial_sp ;栈顶地址
EXPORT __heap_base ;堆起始地址
EXPORT __heap_limit ;堆结束地址
ELSE
IMPORT __use_two_region_memory
EXPORT __user_initial_stackheap ;用户自己来初始化堆栈
__user_initial_stackheap
LDR R0, = Heap_Mem
LDR R1, = (Stack_Mem + Stack_Size)
LDR R2, = (Heap_Mem + Heap_Size)
LDR R3, = Stack_Mem
BX LR
ALIGN
ENDIF
END
;************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE*****
Q&A
Q:为什么驱动文件是汇编语言写而不是C语言?
A:汇编语言处理一些更加底层的工作，如设置堆栈空间等，这一点C这个"高级语言"无法做到。
Q:启动程序阶段，CPU对启动文件是从上向下足以读取吗？
A:编译之前的是源程序，编译之后的指令的机器码以地址进行排列。实际上按地址逐一进行读取，要看源码的组织结构。
Q:用户通过Keil软件配置堆栈空间和自己定义堆栈有什么区别？
A:
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