* 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.
函数 Reset_Handler 内调用函数 entry()，entry() 函数添加一些我们要在 main() 函数之前执行的代码，entry() 函数的最后调用 main()。
在 startup.s 内，定义了一个向量表，从地址0开始（对于STM32，就是 flash 的起始地址 0x08000000）存每个向量，向量指向的位置就是对应的中断服务函数
所以 0x20000D84 就是主堆栈指针的初始值，0x08059545-1就是复位向量，指向中断服务函数 Reset_Handler
可以看 rtthread.map 得到验证
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 ......
程序从 Reset_Handler 开始运行，第一条语句就是对 SP 进行初始化。
不同于 IAR 版本，这个版本除了中断服务函数 Reset_Handler 外，其他中断服务函数都是调用函数 Default_Handler，Default_Handler是一个死循环。功能上和 IAR 版本一样都是死循环。
STM32 CM3/CM4 ------ startup.s 文件分析 ------ GCC RT-Thread Studio 版本
原文：https://www.cnblogs.com/god-of-death/p/14870876.html