STM32F4启动代码分析

;******************** (C) COPYRIGHT 2014 STMicroelectronics ********************
;* File Name          : startup_stm32f40_41xxx.s
;* Author             : MCD Application Team
;* @version           : V1.4.0
;* @date              : 04-August-2014
;* Description        : STM32F40xxx/41xxx 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
;*                      - Configure the system clock and the external SRAM mounted on 
;*                        STM324xG-EVAL board to be used as data memory (optional, 
;*                        to be enabled by user)
;*                      - Branches to __main in the C library (which eventually
;*                        calls main()).
;*                      After Reset the CortexM4 processor is in Thread mode,
;*                      priority is Privileged, and the Stack is set to Main.
;* <<< Use Configuration Wizard in Context Menu >>>   
;*******************************************************************************
; 
; Licensed under MCD-ST Liberty SW License Agreement V2, (the "License");
; You may not use this file except in compliance with the License.
; You may obtain a copy of the License at:
; 
;        http://www.st.com/software_license_agreement_liberty_v2
; 
; Unless required by applicable law or agreed to in writing, software 
; distributed under the License is distributed on an "AS IS" BASIS, 
; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
; See the License for the specific language governing permissions and
; limitations under the License.
; 
;*******************************************************************************

; 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     0x00000400   ;开辟栈的大小为0x00000400（1KB）

                AREA    STACK, NOINIT, READWRITE, ALIGN=3 ;一个新的代码段或者数据段。STACK表示段名，这个可以任意命名；
														  ;NOINIT表示不初始化；READWRITE表示可读可写，ALIGN=3，表示按照2^3对齐，即8字节对齐。
														  
Stack_Mem       SPACE   Stack_Size ;分配一定大小的内存空间，单位为字节。这里指定大小等于Stack_Size
__initial_sp     ;表示栈的结束地址，即栈顶地址，栈是由高向低生长的。


; <h> Heap Configuration
;   <o>  Heap Size (in Bytes) <0x0-0xFFFFFFFF:8>
; </h>

Heap_Size       EQU     0x00000200 ;;开辟堆的大小为0x00000200（512字节）

                AREA    HEAP, NOINIT, READWRITE, ALIGN=3 ;名字为HEAP，NOINIT即不初始化，可读可写，8（2^3）字节对齐。
__heap_base                                ;__heap_base表示堆的起始地址
Heap_Mem        SPACE   Heap_Size         ; 分配一定大小的内存空间
__heap_limit                               ;__heap_limit表示堆的结束地址。堆是由低向高生长的，跟栈的生长方向相反。

                PRESERVE8              ; 指定当前文件的堆栈按照8字节对齐
                THUMB                  ;后面指令兼容THUMB指令,THUBM是ARM以前的指令集，16bit，
									;现在Cortex-M系列的都使用THUMB-2指令集，THUMB-2是32位的，兼容16位和32位的指令，是THUMB的超级。


; Vector Table Mapped to Address 0 at Reset
                AREA    RESET, DATA, READONLY    ;定义一个数据段，名字为RESET，可读。
                EXPORT  __Vectors                ;声明 __Vectors、__Vectors_End和__Vectors_Size这三个标号具有全局属性，可供外部的文件调用
                EXPORT  __Vectors_End
                EXPORT  __Vectors_Size

__Vectors       DCD     __initial_sp               ; Top of Stack   ;向量表起始地址
                DCD     Reset_Handler              ; Reset Handler   ;分配一个或者多个以字为单位的内存，以四字节对齐，并要求初始化这些内存。
                DCD     NMI_Handler                ; NMI Handler
                DCD     HardFault_Handler          ; Hard Fault Handler
                DCD     MemManage_Handler          ; MPU Fault Handler
                DCD     BusFault_Handler           ; Bus Fault Handler
                DCD     UsageFault_Handler         ; Usage Fault Handler
                DCD     0                          ; Reserved             ; 0 表示保留
                DCD     0                          ; Reserved
                DCD     0                          ; Reserved
                DCD     0                          ; Reserved
                DCD     SVC_Handler                ; SVCall Handler
                DCD     DebugMon_Handler           ; Debug Monitor Handler
                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 detection                        
                DCD     TAMP_STAMP_IRQHandler             ; Tamper and TimeStamps through the EXTI line            
                DCD     RTC_WKUP_IRQHandler               ; RTC Wakeup through the EXTI line                       
                DCD     FLASH_IRQHandler                  ; FLASH                                           
                DCD     RCC_IRQHandler                    ; RCC                                             
                DCD     EXTI0_IRQHandler                  ; EXTI Line0                                             
                DCD     EXTI1_IRQHandler                  ; EXTI Line1                                             
                DCD     EXTI2_IRQHandler                  ; EXTI Line2                                             
                DCD     EXTI3_IRQHandler                  ; EXTI Line3                                             
                DCD     EXTI4_IRQHandler                  ; EXTI Line4                                             
                DCD     DMA1_Stream0_IRQHandler           ; DMA1 Stream 0                                   
                DCD     DMA1_Stream1_IRQHandler           ; DMA1 Stream 1                                   
                DCD     DMA1_Stream2_IRQHandler           ; DMA1 Stream 2