#define GPIOA_BASE (AHB1PERIPH_BASE + 0x0000)
typedef struct{ __IO uint32_t MODER; /*!< GPIO port mode register, Address offset: 0x00 */ __IO uint32_t OTYPER; /*!< GPIO port output type register, Address offset: 0x04 */ __IO uint32_t OSPEEDR; /*!< GPIO port output speed register, Address offset: 0x08 */ __IO uint32_t PUPDR; /*!< GPIO port pull-up/pull-down register, Address offset: 0x0C */ __IO uint32_t IDR; /*!< GPIO port input data register, Address offset: 0x10 */ __IO uint32_t ODR; /*!< GPIO port output data register, Address offset: 0x14 */ __IO uint16_t BSRRL; /*!< GPIO port bit set/reset low register, Address offset: 0x18 */ __IO uint16_t BSRRH; /*!< GPIO port bit set/reset high register, Address offset: 0x1A */ __IO uint32_t LCKR; /*!< GPIO port configuration lock register, Address offset: 0x1C */ __IO uint32_t AFR[2]; /*!< GPIO alternate function registers, Address offset: 0x20-0x24 */
} GPIO_TypeDef;
#define GPIOA ((GPIO_TypeDef *) GPIOA_BASE)
void GPIO_PinAFConfig(GPIO_TypeDef* GPIOx, uint16_t GPIO_PinSource, uint8_t GPIO_AF) { uint32_t temp = 0x00; uint32_t temp_2 = 0x00; /* Check the parameters */ assert_param(IS_GPIO_ALL_PERIPH(GPIOx)); assert_param(IS_GPIO_PIN_SOURCE(GPIO_PinSource)); assert_param(IS_GPIO_AF(GPIO_AF)); temp = ((uint32_t)(GPIO_AF) << ((uint32_t)((uint32_t)GPIO_PinSource & (uint32_t)0x07) * 4)) ; GPIOx->AFR[GPIO_PinSource >> 0x03] &= ~((uint32_t)0xF << ((uint32_t)((uint32_t)GPIO_PinSource & (uint32_t)0x07) * 4)) ; temp_2 = GPIOx->AFR[GPIO_PinSource >> 0x03] | temp; GPIOx->AFR[GPIO_PinSource >> 0x03] = temp_2; }这是STM32固件库中的GPIOx操作，今天有一个地方把我绕晕了，还好回过神来了。
大家都知道GPIOA_BASE 是寄存器中的地址。
我搞错的是，误以为将GPIOA替代声明变量的过程，这时候我就纳闷了，怎么声明的变量GPIOA_BASE是一个常数（地址）？
并且还在下面的GPIO_PinAFConfig（）函数中反复调用声明？！
后来我才回过神来，((GPIO_TypeDef *) GPIOA_BASE)不是声明变量，是强制类型转换……
总之，这就是C语言与单片机软硬件对应上的一大关键点吧！
