PIC的程序疑惑

extern volatile unsigned int  PORTB __attribute__((__sfr__));
typedef struct tagPORTBBITS {
  unsigned RB0:1;
  unsigned RB1:1;
  unsigned RB2:1;
  unsigned RB3:1;
  unsigned RB4:1;
  unsigned RB5:1;
  unsigned RB6:1;
  unsigned RB7:1;
  unsigned :4;
  unsigned RB12:1;
  unsigned RB13:1;
  unsigned RB14:1;
  unsigned RB15:1;
} PORTBBITS;
不知该格式的含义，((__sfr__))指代什么，求指导