启动文件：startup_M451Series.s
1 ; Reset Handler
2
3 Reset_Handler PROC
4 EXPORT Reset_Handler [WEAK]
5 IMPORT SystemInit
6 IMPORT __main
7
8 LDR R0, =0x40000100
9 ; Unlock Register
10 LDR R1, =0x59
11 STR R1, [R0]
12 LDR R1, =0x16
13 STR R1, [R0]
14 LDR R1, =0x88
15 STR R1, [R0]
16
17 ; Init POR
18 LDR R2, =0x40000024
19 LDR R1, =0x00005AA5
20 STR R1, [R2]
21
22 ; Select INV Type
23 LDR R2, =0x40000200
24 LDR R1, [R2]
25 BIC R1, R1, #0x1000
26 STR R1, [R2]
27
28 ; Lock register
29 MOVS R1, #0
30 STR R1, [R0]
31
32
33 LDR R0, = SystemInit
34 BLX R0
35 LDR R0, = __main
36 BX R0
37 ENDP
从复位中断函数可以知道，系统复位后先 执行 SystemInit 函数，后执行 __main 函数，由 __main 转到用户 main() 函数。
1 /** 2 * Initialize the system 3 * 4 * @param None 5 * @return None 6 * 7 * @brief Setup the microcontroller system. 8 * Initialize the System. 9 */ 10 void SystemInit(void) 11 { 12 /* ToDo: add code to initialize the system 13 do not use global variables because this function is called before 14 reaching pre-main. RW section maybe overwritten afterwards. */ 15 16 SYS_UnlockReg(); 17 /* One-time POR18 */ 18 if((SYS->PDID >> 12) == 0x945) 19 { 20 M32(GCR_BASE+0x14) |= BIT7; 21 } 22 /* Force to use INV type with HXT */ 23 CLK->PWRCTL &= ~CLK_PWRCTL_HXTSELTYP_Msk; 24 SYS_LockReg(); 25 26 27 #ifdef EBI_INIT 28 extern void SYS_Init(); 29 extern void EBI_Init(); 30 31 SYS_UnlockReg(); 32 SYS_Init(); 33 EBI_Init(); 34 SYS_LockReg(); 35 #endif 36 37 /* FPU settings ------------------------------------------------------------*/ 38 #if (__FPU_PRESENT == 1) && (__FPU_USED == 1) 39 SCB->CPACR |= ((3UL << 10 * 2) | /* set CP10 Full Access */ 40 (3UL << 11 * 2)); /* set CP11 Full Access */ 41 #endif 42 43 }
__main 函数 完成C语言加载所需的资源，后执行跳转用户 main 函数。