text: // On-disk file system format.
  // Both the kernel and user programs use this header file.
  
  // Block 0 is unused.
  // Block 1 is super block.
  // Blocks 2 through sb.ninodes/IPB hold inodes.
  // Then free bitmap blocks holding sb.size bits.
  // Then sb.nblocks data blocks.
  // Then sb.nlog log blocks.
  
  #define ROOTINO 1  // root i-number
  #define BSIZE 512  // block size
  
  // File system super block
  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
  };
  
  #define NDIRECT 10
  #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
    short owner;
    short group;
    uint permissions;
    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)     ((i) / IPB + 2)
  
  // Bitmap bits per block
  #define BPB           (BSIZE*8)
  
  // Block containing bit for block b
  #define BBLOCK(b, ninodes) (b/BPB + (ninodes)/IPB + 3)
  
  // Directory is a file containing a sequence of dirent structures.
  #define DIRSIZ 14
  
  struct dirent {
    ushort inum;
    char name[DIRSIZ];
  };
  