text: // On-disk file system format.
  // Both the kernel and user programs use this header file.
  
  
  #define ROOTINO 1  // root i-number
  #define BSIZE 512  // block size
  
  // Disk layout:
  // [ boot block | super block | log | inode blocks |
  //                                          free bit map | data blocks]
  //
  // mkfs computes the super block and builds an initial file system. The
  // super block describes the disk layout:
  struct superblock {
    uint size;         // Size of file system image (blocks)
    uint nblocks;      // Number of data blocks
    uint ninodes;      // Number of inodes.
    uint nlog;         // Number of log blocks
    uint logstart;     // Block number of first log block
    uint inodestart;   // Block number of first inode block
    uint bmapstart;    // Block number of first free map block
  };
  
  #define NDIRECT 12
  #define NINDIRECT (BSIZE / sizeof(uint))
  #define MAXFILE (NDIRECT + NINDIRECT)
  
  // On-disk inode structure
  struct dinode {
    short type;           // File type
    short major;          // Major device number (T_DEV only)
    short minor;          // Minor device number (T_DEV only)
    short nlink;          // Number of links to inode in file system
    uint size;            // Size of file (bytes)
    uint addrs[NDIRECT+1];   // Data block addresses
  };
  
  // Inodes per block.
  #define IPB           (BSIZE / sizeof(struct dinode))
  
  // Block containing inode i
  #define IBLOCK(i, sb)     ((i) / IPB + sb.inodestart)
  
  // Bitmap bits per block
  #define BPB           (BSIZE*8)
  
  // Block of free map containing bit for block b
  #define BBLOCK(b, sb) (b/BPB + sb.bmapstart)
  
  // Directory is a file containing a sequence of dirent structures.
  #define DIRSIZ 14
  
  struct dirent {
    ushort inum;
    char name[DIRSIZ];
  };
  
  
  uint balloc_page(uint dev);
  void bfree_page(int dev, uint b);
  void write_page_to_disk(uint dev, char *pg, uint blk);
  void read_page_from_disk(uint dev, char *pg, uint blk);
  