startup.s 功能

* 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 Cortex-M4 processor is in Thread mode,
* priority is Privileged, and the Stack is set to Main.

函式Reset_Handler 內呼叫函式 entry()，entry() 函式新增一些我們要在 main() 函式之前執行的程式碼，entry() 函式的最後呼叫 main()。

向量表

在 startup.s 內，定義了一個向量表，從地址0開始（對於STM32，就是 flash 的起始地址 0x08000000）存每個向量，向量指向的位置就是對應的中斷服務函式

所以 0x20000D84 就是主堆疊指標的初始值，0x08059545-1就是復位向量，指向中斷服務函式Reset_Handler

可以看 rtthread.map 得到驗證

startup.s 檔案內容

以下是 GCC RT-Thread 版本的 startup.s

    .section  .text.Reset_Handler
  .weak  Reset_Handler
  .type  Reset_Handler, %function
Reset_Handler:  
  ldr   sp, =_estack     /* set stack pointer */

/* Copy the data segment initializers from flash to SRAM */  
  movs  r1, #
 
0
  b  LoopCopyDataInit

CopyDataInit:
  ldr  r3, =_sidata
  ldr  r3, [r3, r1]
  str  r3, [r0, r1]
  adds  r1, r1, #4
    
LoopCopyDataInit:
  ldr  r0, =_sdata
  ldr  r3, =_edata
  adds  r2, r0, r1
  cmp  r2, r3
  bcc  CopyDataInit
  ldr  r2, =_sbss
  b  LoopFillZerobss
/* Zero fill the bss segment. */  
FillZerobss:
  movs  r3, #0
  str  r3, [r2], #4
    
LoopFillZerobss:
  ldr  r3, = _ebss
  cmp  r2, r3
  bcc  FillZerobss

/* Call the clock system intitialization function.*/
  bl  SystemInit   
/* Call static constructors */
    /* bl __libc_init_array */
/* Call the application's entry point.*/
  bl  entry
  bx  lr    
.size  Reset_Handler, .-Reset_Handler

/**
 * @brief  This is the code that gets called when the processor receives an 
 *         unexpected interrupt.  This simply enters an infinite loop, preserving
 *         the system state for examination by a debugger.
 * @param  None     
 * @retval None       
*/
    .section  .text.Default_Handler,"ax",%progbits
Default_Handler:
Infinite_Loop:
  b  Infinite_Loop
  .size  Default_Handler, .-Default_Handler
/******************************************************************************
*
* The minimal vector table for a Cortex M3. Note that the proper constructs
* must be placed on this to ensure that it ends up at physical address
* 0x0000.0000.
* 
*******************************************************************************/
   .section  .isr_vector,"a",%progbits
  .type  g_pfnVectors, %object
  .size  g_pfnVectors, .-g_pfnVectors
    
    
g_pfnVectors:
  .word  _estack
  .word  Reset_Handler
  .word  NMI_Handler
  .word  HardFault_Handler
  .word  MemManage_Handler
  .word  BusFault_Handler
  .word  UsageFault_Handler
  .word  0
  .word  0
  .word  0
  .word  0
  .word  SVC_Handler
  .word  DebugMon_Handler
  .word  0
  .word  PendSV_Handler
  .word  SysTick_Handler
  
  /* External Interrupts */
  .word     WWDG_IRQHandler                   /* Window WatchDog              */                                        
  .word     PVD_IRQHandler                    /* PVD through EXTI Line detection */                        
  .word     TAMP_STAMP_IRQHandler             /* Tamper and TimeStamps through the EXTI line */            
  .word     RTC_WKUP_IRQHandler               /* RTC Wakeup through the EXTI line */                      
  .word     FLASH_IRQHandler                  /* FLASH                        */                                          
  .word     RCC_IRQHandler                    /* RCC                          */                                            
  .word     EXTI0_IRQHandler                  /* EXTI Line0                   */                        
  .word     EXTI1_IRQHandler                  /* EXTI Line1                   */                          
  .word     EXTI2_IRQHandler                  /* EXTI Line2                   */                          
  .word     EXTI3_IRQHandler                  /* EXTI Line3                   */                          
......
                         
                         
/*******************************************************************************
*
* Provide weak aliases for each Exception handler to the Default_Handler. 
* As they are weak aliases, any function with the same name will override 
* this definition.
* 
*******************************************************************************/
   .weak      NMI_Handler
   .thumb_set NMI_Handler,Default_Handler
  
   .weak      HardFault_Handler
   .thumb_set HardFault_Handler,Default_Handler
  
   .weak      MemManage_Handler
   .thumb_set MemManage_Handler,Default_Handler
  
   .weak      BusFault_Handler
   .thumb_set BusFault_Handler,Default_Handler

   .weak      UsageFault_Handler
   .thumb_set UsageFault_Handler,Default_Handler

......

程式從Reset_Handler 開始執行，第一條語句就是對 SP 進行初始化。

不同於 IAR 版本，這個版本除了中斷服務函式Reset_Handler 外，其他中斷服務函式都是呼叫函式 Default_Handler，Default_Handler是一個死迴圈。功能上和 IAR 版本一樣都是死迴圈。