• FAT filesystem
  • Layout of a FAT volume
  • FAT bootsector and BPB
  • FAT directory entries
  • The FAT
• ext2 filesystem
• ISO-9660 (CD-ROM) filesystem
• Overview of common filesystems
• Filesystem code snippets
• Links
• REPORT BUGS OR ERRORS IN THIS DOCUMENT

Newly-purchased floppy disks advertised as 'formatted' usually contain a FAT12 filesystem.

Layout of a FAT volume

• boot sector (more than one for FAT32)
• root directory (not present for FAT32)
• one or more copies of the FAT (almost always 2 copies)
• the data area, where files and subdirectories are stored. For FAT32, the root directory is stored in the data area as well.

• Code to load and run the DOS kernel
• The poorly-named BIOS Parameter Block (BPB), with disk geometry and filesystem info. The BPB corresponds to the superblock of UNIX filesystems.
• Magic values: 55h at offset 510, 0AAh at offset 511.

typedef unsigned char u8;
typedef unsigned short u16;	/* Note: multi-byte values are little-endian */
typedef unsigned long u32;
struct fat_bootsector		/* Warning: this struct must be packed */
{
	u8 jump[3];		/* 16-bit JMP to boot code, or 8-bit JMP + NOP */
	u8 oem_id[8];		/* e.g. 'MSWIN4.0' */
	u16 bytes_per_sector;	/* usu. =512 */
	u8 sectors_per_cluster;
	u16 num_boot_sectors;	/* usu. =1 */
	u8 num_fats;		/* usu. =2 */
	u16 num_root_dir_ents;
	u16 total_sectors;	/* 16-bit; 0 if num sectors > 65535 */
	u8 media_ID_byte;       /* usu. =0F0h */
	u16 sectors_per_fat;
	u16 sectors_per_track;
	u16 heads;
	u32 hidden_sectors;	/* =LBA partition start */
	u32 total_sectors_large;/* 32-bit; 0 if num sectors < 65536 */
	u8 boot_code[474];
	u8 magic[2];		/* 55h, 0AAh */
};                              /* 512 bytes total */

typedef unsigned char u8;
typedef unsigned short u16;	/* Note: multi-byte values are little-endian */
typedef unsigned long u32;
struct fat_dirent               /* Warning: this struct must be packed */
{
        u8 name[8];             /* ALL-CAPS, pad right with spaces */
        u8 ext[3];              /* ALL-CAPS, pad right with spaces */
        u8 attrib;              /* attribute byte */
        u8 reserved;            /* =0 */
        u8 ctime_ms;            /* file creation time, 10ms units */
        u16 ctime;              /* file creation time, in DOS format */
        u16 cdate;              /* file creation date, in DOS format */
        u16 adate;              /* DOS date of last file access */
        u16 st_clust_msw;       /* high 16 bits of starting cluster (FAT32) */
        u16 mtime;              /* DOS time of last file modification */
        u16 mdate;              /* DOS date of last file modification */
        u16 st_clust;           /* starting cluster */
        u32 file_size;          /* in bytes */
};                              /* 32 bytes total */

struct dos_time                 /* Warning: this struct must be packed */
{
        unsigned two_secs : 5;  /* low 5 bits: 2-second increments */
        unsigned minutes : 6;   /* middle 6 bits: minutes */
        unsigned hours : 5;     /* high 5 bits: hours (0-23) */
};                              /* 2 bytes total */

struct dos_date                 /* Warning: this struct must be packed */
{
        unsigned date : 5;      /* low 5 bits: date (1-31) */
        unsigned month : 4;     /* middle 4 bits: month (1-12) */
        unsigned year : 7;      /* high 7 bits: year - 1980 */
};                              /* 2 bytes total */

struct attrib                   /* Warning: this struct must be packed */
{
        int read_only : 1;      /* b0 */
        int hidden : 1;
        int system : 1;
        int volume_label : 1;
        int directory : 1;
        int archive : 1;
        int reserved : 2;       /* b6, b7 */
};                              /* 1 byte total */

• FAT12: 1...4084 (0FF4h) clusters
• FAT16: 4085...65524 (0FFF4h) clusters
• FAT32: 65525... clusters

- linked list for used and free sectors (FAT family)
- tree for used sectors, bitmap for free (UNIX family)
- subdirectories
- advanced: journalling, striping
- sorted directories let you perform a fast binary search