• # - 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

FAT filesystem

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

Layout of a FAT volume

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

FAT bootsector and BPB

• 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	uint8_t;	/* or #include <stdint.h> */
typedef unsigned short	uint16_t;	/* Note: multi-byte values are little-endian */
typedef unsigned long	uint32_t;
struct fat_bootsector			/* Warning: this struct must be packed */
{
        uint8_t  jump[3];               /* 16-bit JMP to boot code, or 8-bit JMP + NOP */
        uint8_t  oem_id[8];             /* e.g. 'MSWIN4.0' */
	uint16_t bytes_per_sector;	/* usu. =512 */
        uint8_t  sectors_per_cluster;
	uint16_t num_boot_sectors;	/* usu. =1 */
        uint8_t  num_fats;              /* usu. =2 */
	uint16_t num_root_dir_ents;
        uint16_t total_sectors;         /* 16-bit; 0 if num sectors > 65535 */
        uint8_t  media_ID_byte;         /* usu. =0F0h */
	uint16_t sectors_per_fat;
	uint16_t sectors_per_track;
	uint16_t heads;
	uint32_t hidden_sectors;	/* =LBA partition start */
        uint32_t total_sectors_large;   /* 32-bit; 0 if num sectors < 65536 */
        uint8_t  boot_code[474];
        uint8_t  magic[2];              /* 55h, 0AAh */
};                              /* 512 bytes total */

FAT directory entries

typedef unsigned char	uint8_t;	/* or #include <stdint.h> */
typedef unsigned short	uint16_t;	/* Note: multi-byte values are little-endian */
typedef unsigned long	uint32_t;
struct fat_dirent               	/* Warning: this struct must be packed */
{
        uint8_t  name[8];               /* ALL-CAPS, pad right with spaces */
        uint8_t  ext[3];                /* ALL-CAPS, pad right with spaces */
        uint8_t  attrib;                /* attribute byte */
        uint8_t  reserved;              /* =0 */
        uint8_t  ctime_ms;              /* file creation time, 10ms units */
	uint16_t ctime;              	/* file creation time, in DOS format */
	uint16_t cdate;              	/* file creation date, in DOS format */
	uint16_t adate;              	/* DOS date of last file access */
	uint16_t st_clust_msw;       	/* high 16 bits of starting cluster (FAT32) */
	uint16_t mtime;              	/* DOS time of last file modification */
	uint16_t mdate;              	/* DOS date of last file modification */
	uint16_t st_clust;           	/* starting cluster */
	uint32_t 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 */

The FAT

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

ext2 filesystem

ISO-9660 (CD-ROM) filesystem

Overview of common filesystems

	ISO9660 (CD-ROM)	Joliet	Mac HFS+
Filename length	Level 1: 8.3 files, 8 for subdirectories Level 2/3: 31	64	255
Path length
Illegal characters in filenames	(nul) (space) ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ ` { | } ~ Extension with directory name is not allowed		(nul) :
Legal characters in filenames	0-9 A-Z _		everything but (nul) :
Limit to number of root directory entries	no ?	no	no
Maximum subdirectory depth	8	no
Directories sorted by name	yes	yes
Character set	ASCII	UCS-2 Unicode	UCS-2 Unicode
Endian	big AND little (committee invention :)		big

Filesystem code snippets

Check disk for ext2 filesystem, display info

Links

Information on FAT12/FAT16 (DOS) filesystems:

• http://home.no.net/tkos/info/fat.html
• Official MS docs for FAT: http://www.microsoft.com/hwdev/hardware/fatgendown.asp

• http://www.freesoftware.fsf.org/ext2-doc/
• http://uranus.it.swin.edu.au/~jn/explore2fs/es2fs.htm

LTOOLS; an ext2 analog of MTOOLS (lets DOS access ext2 partitions): http://www.it.fht-esslingen.de/~zimmerma/software/ltools.html

Reentrant DOS-Compatible File System, by Philip J. Erdelsky: http://www.alumni.caltech.edu/~pje/software.html
Public domain. Includes disk cache.

TO DO

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

REPORT BUGS OR ERRORS IN THIS DOCUMENT