N级别处理器内核从User Mode切换到Machine Mode只能通过异常、响应中断或者NMI的方式发生：
响应异常进入异常处理模式。请参见第3.4节了解其详情。
注意：软件可以通过调用ecall指令强行进入ecall异常处理函数。
响应中断进入中断处理模式。请参见第5.6节了解其详情。
响应NMI进入NMI处理模式。请参见第4.3节了解其详情。
根据说明，进入用户模式后，Led()执行ecall命令就会进入异常中断ECALL_Excep
tion_Handler，这时候是在M模式了吗？代码怎么能从异常中断回到主函数继续执行while循环里的语句呢？
/** @addtogroup CM32M4xxR_StdPeriph_Examples * @{ */#define MACHINE_MODE_STACK_SIZE 0x400static uint8_t sMachineModeStack[MACHINE_MODE_STACK_SIZE] = {0};//Configure test_array Acess Permission on PMPvoid PMP_Config() { PMP_Region_InitTypeDef pmp_init; pmp_init.Number = PMP_REGION_NUMBER0; pmp_init.Enable = PMP_REGION_ENABLE; // Enable Configuration pmp_init.Lock = PMP_REGION_UNLOCK; // pmp_init.BaseAddress = 0; // pmp_init.Size = PMP_REGION_SIZE_4GB; //Setting array size pmp_init.AddressMatching = PMP_ADDRESS_MATCHING_NAPOT; //Setting PMP Size to NAPOT mode -> 2^n pmp_init.AccessPermission = PMP_ACCESS_RWX; //Setting array permission is Read Only PMP_ConfigRegion(&pmp_init); sPMP_Region_InitTypeDef spmp_init; spmp_init.Number = SPMP_REGION_NUMBER0; spmp_init.Enable = SPMP_REGION_ENABLE; spmp_init.Lock = SPMP_REGION_UNLOCK; spmp_init.BaseAddress = 0; spmp_init.Size = SPMP_REGION_SIZE_4GB; spmp_init.AddressMatching = SPMP_ADDRESS_MATCHING_NAPOT; spmp_init.UserMode = SPMP_USERMODE_RESET; spmp_init.AccessPermission = SPMP_ACCESS_RWX; sPMP_ConfigRegion(&spmp_init);}/** * @brief Jump User Mode */void JumpUserMode(uint32_t func){ __RV_CSR_WRITE(CSR_MSCRATCH, sMachineModeStack + MACHINE_MODE_STACK_SIZE); __RV_CSR_CLEAR(CSR_MSTATUS, MSTATUS_MPP); __RV_CSR_WRITE(CSR_MEPC, func); __ASM volatile ( "mret ");}void ECALL_Exception_Handler(unsigned long mcause,unsigned long sp){ uint32_t saved_regs = sp; uint32_t mepc = ((uint32_t *)saved_regs)[12]; printf("ECALL Exception Triggerrn"); ((uint32_t *)saved_regs)[12] = mepc + 4;}/** * @brief User Mode Application */void Led(){ LedInit(LED2_PORT, LED2_PIN); __ECALL();}/** * @brief Main function */int main(void) { /* System Clocks Configuration */ PMP_Config(); Exception_Register_EXC(UmodeEcall_EXCn, (unsigned long)ECALL_Exception_Handler); JumpUserMode((uint32_t)Led); while (1) { LedBlink(LED2_PORT, LED2_PIN); delay_ms(1000); }}
