#define IPB (BSIZE / sizeof(struct dinode))

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fs.h
https://github.com/mit-pdos/xv6-public/blob/master/fs.h#L39

#define IPB           (BSIZE / sizeof(struct dinode))

IPBマクロは、ブロック1個当たりに含まれるiノード数を算出します。

fs.h

#define BSIZE 512  // block size

fs.h

// 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
};