中容量STM32处理器启动代码的理解

  上个星期还看到ARM被日本软银给收购了，由此大家都很熟悉的ARM变成了一家日资企业，祝愿它能更好的发展~

  今天要介绍一下STM32的启动代码，我这里使用的是中容量的STM32f103c8t6,对应的启动文件就是startup_stm32f10x_md.s，我这里启动文件版本是V3.6.2

  不多说废话，先上我注释过的源码：

/**
  ******************************************************************************
  * @file      startup_stm32f10x_md.s
  * @author    MCD Application Team
  * @version   V3.6.2
  * @date      28-February-2013
  * @brief     STM32F10x Medium Density Devices vector table for RIDE7 toolchain.
  *            This module performs:
  *                - Set the initial SP                                              -设置最初的SP寄存器值
  *                - Set the initial PC == Reset_Handler,							 -设置最初的PC寄存器值 == Reset_Handler
  *                - Set the vector table entries with the exceptions ISR address    -在有特殊的ISR(中断服务程序)地址的情况下设置向量表入口
  *                - Configure the clock system 									 -设置时钟系统
  *                - Branches to main in the C library (which eventually			 -C库文件中由分支到主函数（最后调用了主函数）
  *                  calls main()).
  *            After Reset the Cortex-M3 processor is in Thread mode,				 在线程模式下复位了CM3处理器后，此时优先级特权化，栈顶设置为主函数
  *            priority is Privileged, and the Stack is set to Main.
  ******************************************************************************
  * @attention
  *
  * 
© COPYRIGHT 2013 STMicroelectronics
  *
  * 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.
  *
  ******************************************************************************
  */
    
  .syntax unified                                                                   ;表示使用了统一汇编语言语法
	.cpu cortex-m3                                                                  ;MCU为CM3
	.fpu softvfp
	.thumb                                                                          ;指令集

.global	g_pfnVectors                                                                ;global使得g_pfnVectors可以被其他目标文件使用
.global	Default_Handler

/* start address for the initialization values of the .data section.                初始化.data 块的起始地址，这个地址在链接脚本中被定义
defined in linker script */
.word	_sidata
/* start address for the .data section. defined in linker script                    .data块的起始地址，这个地址在链接脚本中被定义*/  
.word	_sdata
/* end address for the .data section. defined in linker script                      .data块的结束地址，这个地址在链接脚本中被定义*/
.word	_edata
/* start address for the .bss section. defined in linker script                     .bss块的起始地址，这个地址在链接脚本中被定义*/
.word	_sbss
/* end address for the .bss section. defined in linker script                       .bss块的结束地址，这个地址在链接脚本中被定义*/
.word	_ebss

.equ  BootRAM, 0xF108F85F                                                           /*这里跟c中的宏定义类似，即BootRAM = 0xF108F85F,不参与编译*/
/**
 * @brief  This is the code that gets called when the processor first
 *          starts execution following a reset event. Only the absolutely
 *          necessary set is performed, after which the application
 *          supplied main() routine is called. 
 * @param  None
 * @retval : None
*/

    .section	.text.Reset_Handler
	.weak	Reset_Handler
	.type	Reset_Handler, %function
Reset_Handler:	

/* Copy the data segment initializers from flash to SRAM                             将初始化了的数据段从flash复制到SRAM*/  
  movs	r1, #0                                                                       ;将立即数0赋值给r1寄存器
  b	LoopCopyDataInit                                                                 ;程序转移到LoopCopyDataInit处

CopyDataInit:                                                                        ;从FLASH中拷贝地址在sdata和edata之间的代码到SRAM中
	ldr	r3, =_sidata                                                                 ;从存储器中将_sidata加载到寄存器r3中
	ldr	r3, [r3, r1]                                                                 ;从地址r3+r1处读取一个字（32bit）到r3中
	str	r3, [r0, r1]                                                                 ;把寄存器r3的值存储到存储器中地址为r0+r1地址处
	adds	r1, r1, #4                                                               ;r1 = r1 + 4
    
LoopCopyDataInit:
	ldr	r0, =_sdata                                                                  ;从存储器中将_sidata加载到寄存器r0中
	ldr	r3, =_edata                                                                  ;从存储器中将_edata加载到寄存器r3中
	adds	r2, r0, r1                                                               ;r2=r0+r1
	cmp	r2, r3                                                                       ;计算r2 - r3,若小于0，标志位为0，反之为1
	bcc	CopyDataInit                                                                 ;如果标志位为0（无借位）即r2