我一直在使用STM32 hal驱动程序，我注意到当它们要为寄存器赋值时，它们会使用如下代码：

#define     __IO    volatile

typedef struct
{
  __IO uint32_t CR1;    /*!< USART Control register 1,                 Address offset: 0x00 */
  __IO uint32_t CR2;    /*!< USART Control register 2,                 Address offset: 0x04 */
  __IO uint32_t CR3;    /*!< USART Control register 3,                 Address offset: 0x08 */
  __IO uint32_t BRR;    /*!< USART Baud rate register,                 Address offset: 0x0C */
  __IO uint32_t GTPR;   /*!< USART Guard time and prescaler register,  Address offset: 0x10 */
  __IO uint32_t RTOR;   /*!< USART Receiver Time Out register,         Address offset: 0x14 */
  __IO uint32_t RQR;    /*!< USART Request register,                   Address offset: 0x18 */
  __IO uint32_t ISR;    /*!< USART Interrupt and status register,      Address offset: 0x1C */
  __IO uint32_t ICR;    /*!< USART Interrupt flag Clear register,      Address offset: 0x20 */
  __IO uint32_t RDR;    /*!< USART Receive Data register,              Address offset: 0x24 */
  __IO uint32_t TDR;    /*!< USART Transmit Data register,             Address offset: 0x28 */
  __IO uint32_t PRESC;  /*!< USART clock Prescaler register,           Address offset: 0x2C */
} USART_TypeDef;

#define D2_APB1PERIPH_BASE    (0x40000000UL)
#define UART4_BASE            (D2_APB1PERIPH_BASE + 0x4C00UL)
#define UART4                 ((USART_TypeDef *) UART4_BASE)

UART4->CR = whatever!

所以我试着用C在电脑上测试，但我有点困惑，下面的程序是我写的：

#include <stdio.h>
    #include <stdlib.h>
    #include <stdint.h>
    
    #define startaddress ((mytype*) (0x6d9ff93b))
    typedef struct
    {
        volatile uint32_t myTypeValue;
    }mytype;
    
    int main()
    {
        mytype* inst;
        inst = startaddress;
        inst->myTypeValue = 0x10;
        printf("address is : %x \n",inst);
        return 0;
    }
output : 
Process finished with exit code -1073741819 (0xC0000005)

这程序将崩溃在inst->myTypeValue = 0x10; .否则它将显示这地址已经被分配给它.例如这是这输出：

#include <stdio.h>
    #include <stdlib.h>
    #include <stdint.h>
    
    #define startaddress ((mytype*) (0x6d9ff93b))
    typedef struct
    {
        volatile uint32_t myTypeValue;
    }mytype;
    
    int main()
    {
        mytype* inst;
        inst = startaddress;
  // inst->myTypeValue = 0x10;
        printf("address is : %x \n",inst);
        return 0;
    }

output : 
address is : 6d9ff93b

Process finished with exit code 0

现在我有一些问题，stm32库工作是因为地址是寄存器而不是SRAM吗？因为我记得当我在FMC上使用SDRAM时，我可以在地址0xC0000000处做同样的事情，它工作得很好。
如果赋值是错误的，为什么编译器在第二个例子上工作，并且只在我试图使用结构体内部的变量时崩溃？为什么不在赋值inst = startaddress;时崩溃？