text: struct file {
    enum { FD_NONE, FD_PIPE, FD_INODE, FD_DEVICE } type;
    int ref; // reference count
    char readable;
    char writable;
    struct pipe *pipe; // FD_PIPE
    struct inode *ip;  // FD_INODE and FD_DEVICE
    uint off;          // FD_INODE
    short major;       // FD_DEVICE
  };
  
  #define major(dev)  ((dev) >> 16 & 0xFFFF)
  #define minor(dev)  ((dev) & 0xFFFF)
  #define	mkdev(m,n)  ((uint)((m)<<16| (n)))
  
  // in-memory copy of an inode
  struct inode {
    uint dev;           // Device number
    uint inum;          // Inode number
    int ref;            // Reference count
    struct sleeplock lock; // protects everything below here
    int valid;          // inode has been read from disk?
  
    short type;         // copy of disk inode
    short major;
    short minor;
    short nlink;
    uint size;
    uint addrs[NDIRECT+1];
  };
  
  // map major device number to device functions.
  struct devsw {
    int (*read)(int, uint64, int);
    int (*write)(int, uint64, int);
  };
  
  extern struct devsw devsw[];
  
  #define CONSOLE 1
  #define CONSOLE1 1
  #define CONSOLE2 2
  #define CONSOLE3 3
  