The disk is 512 byte blocks, with a superblock:
struct sup { UINT isize; /* number of blocks devoted to ilist */ UINT fsize; /* largest file block + 1 */ UINT nfree; /* number of free blocks in free[] */ UINT free[100]; /* free block numbers */ UINT ninode; /* number of free inumbers in inode[] */ UINT inode[100];/* free inode numbers */ char flock; /* irrelevant on disk */ char ilock; /* irrelevant on disk */ ULONG time; /* time of last umount */};this is followed by inode blocks, fully packed with 16 disk inodes
struct dsknod { UINT mode; /* see below */ char nlinks; /* number of directory links */ UCHAR uid; /* user id of owner */ UCHAR gid; /* group id of owner */ UCHAR size0; /* high byte of 24-bit size */ UINT size1; /* low word of 24-bit size */ UINT addr[8]; /* block numbers or device number */ ULONG rtime; /* time of last read */ ULONG wtime; /* time of last write */};the freelist of blocks is a chain.
code to deal with all this is in src/fstools/fslib.c