Some Defines from the VFAT file system

#define MSDOS_ROOT_INO 1 /* == MINIX_ROOT_INO */
#define SECTOR_SIZE     512 /* sector size (bytes) */
#define SECTOR_BITS     9 /* log2(SECTOR_SIZE) */
#define MSDOS_DPB       (MSDOS_DPS) /* dir entries per block */
#define MSDOS_DPB_BITS 4 /* log2(MSDOS_DPB) */
#define MSDOS_DPS       (SECTOR_SIZE/sizeof(struct msdos_dir_entry))
#define MSDOS_DPS_BITS 4 /* log2(MSDOS_DPS) */
#define MSDOS_DIR_BITS 5 /* log2(sizeof(struct msdos_dir_entry)) */

#define MSDOS_SUPER_MAGIC 0x4d44 /* MD */
#define MSDOS_MAX_EXTRA 3 /* tolerate up to that number of clusters which are
inaccessible because the FAT is too short */

The Boot sector

struct fat_boot_sector {
        __s8    ignored[3];     /* Boot strap short or near jump */
        __s8    system_id[8];   /* Name - can be used to special case
                                   partition manager volumes */
        __u8    sector_size[2]; /* bytes per logical sector */
        __u8    cluster_size;   /* sectors/cluster */
        __u16   reserved;       /* reserved sectors */
        __u8    fats;           /* number of FATs */
        __u8    dir_entries[2]; /* root directory entries */
        __u8    sectors[2];     /* number of sectors */
        __u8    media;          /* media code (unused) */
        __u16   fat_length;     /* sectors/FAT */
        __u16   secs_track;     /* sectors per track */
        __u16   heads;          /* number of heads */
        __u32   hidden;         /* hidden sectors (unused) */
        __u32   total_sect;     /* number of sectors (if sectors == 0) */

        /* The following fields are only used by FAT32 */
        __u32   fat32_length;   /* sectors/FAT */
        __u16   flags;          /* bit 8: fat mirroring, low 4: active fat */
        __u8    version[2];     /* major, minor filesystem version */
        __u32   root_cluster;   /* first cluster in root directory */
        __u16   info_sector;    /* filesystem info sector */
        __u16   backup_boot;    /* backup boot sector */
        __u16   reserved2[6];   /* Unused */
};

struct fat_boot_fsinfo {
        __u32   reserved1;      /* Nothing as far as I can tell */
        __u32   signature;      /* 0x61417272L */
        __u32   free_clusters; /* Free cluster count. -1 if unknown */
        __u32   next_cluster;   /* Most recently allocated cluster.
                                 * Unused under Linux. */
        __u32   reserved2[4];
};

Two Directory Structures

struct msdos_dir_entry {
        __s8    name[8],ext[3]; /* name and extension */
        __u8    attr;           /* attribute bits */
        __u8    lcase;          /* Case for base and extension */
        __u8    ctime_ms;       /* Creation time, milliseconds */
        __u16   ctime;          /* Creation time */
        __u16   cdate;          /* Creation date */
        __u16   adate;          /* Last access date */
        __u16   starthi;        /* High 16 bits of cluster in FAT32 */
        __u16   time,date,start;/* time, date and first cluster */
        __u32   size;           /* file size (in bytes) */
};

/* Up to 13 characters of the name */
struct msdos_dir_slot {
        __u8    id;             /* sequence number for slot */
        __u8    name0_4[10];    /* first 5 characters in name */
        __u8    attr;           /* attribute byte */
        __u8    reserved;       /* always 0 */
        __u8    alias_checksum; /* checksum for 8.3 alias */
        __u8    name5_10[12];   /* 6 more characters in name */
        __u16   start;          /* starting cluster number, 0 in long slots */
        __u8    name11_12[4];   /* last 2 characters in name */
};