__IO uint16_t

STM32里的类型定义，见如下说明：

typedef volatile unsigned short vu16;

typedef __IO uint16_t  vu16 

/* IO definitions (access restrictions to peripheral registers) */

#ifdef __cplusplus

  #define   __I     volatile             /*!< defines 'read only' permissions                 */

#else

  #define   __I     volatile const       /*!< defines 'read only' permissions                 */

#endif

#define     __O     volatile             /*!< defines 'write only' permissions                */

#define     __IO    volatile             /*!< defines 'read / write' permissions              */ 

/* Exported types ------------------------------------------------------------*/
typedef signed long  s32;
typedef signed short s16;
typedef signed char  s8;
typedef signed long  const sc32;  /* Read Only */
typedef signed short const sc16;  /* Read Only */
typedef signed char  const sc8;   /* Read Only */
typedef volatile signed long  vs32;
typedef volatile signed short vs16;
typedef volatile signed char  vs8;
typedef volatile signed long  const vsc32;  /* Read Only */
typedef volatile signed short const vsc16;  /* Read Only */
typedef volatile signed char  const vsc8;   /* Read Only */
typedef unsigned long  u32;
typedef unsigned short u16;
typedef unsigned char  u8;
typedef unsigned long  const uc32;  /* Read Only */
typedef unsigned short const uc16;  /* Read Only */
typedef unsigned char  const uc8;   /* Read Only */
typedef volatile unsigned long  vu32;
typedef volatile unsigned short vu16;
typedef volatile unsigned char  vu8;
typedef volatile unsigned long  const vuc32;  /* Read Only */
typedef volatile unsigned short const vuc16;  /* Read Only */
typedef volatile unsigned char  const vuc8;   /* Read Only */

Exported_types
<Stm32f10x>
类型定义 
typedef int32_t  s32 
typedef int16_t  s16 
typedef int8_t  s8 
typedef const int32_t  sc32 
typedef const int16_t  sc16 
typedef const int8_t  sc8 
typedef __IO int32_t  vs32 
typedef __IO int16_t  vs16 
typedef __IO int8_t  vs8 
typedef __I int32_t  vsc32 
typedef __I int16_t  vsc16 
typedef __I int8_t  vsc8 
typedef uint32_t  u32 
typedef uint16_t  u16 
typedef uint8_t  u8 
typedef const uint32_t  uc32 
typedef const uint16_t  uc16 
typedef const uint8_t  uc8 
typedef __IO uint32_t  vu32 
typedef __IO uint16_t  vu16 
typedef __IO uint8_t  vu8 
typedef __I uint32_t  vuc32 
typedef __I uint16_t  vuc16 
typedef __I uint8_t  vuc8