10/31/00 CSE 380 1

University of Pennsylvania

Windows File System - FAT

• originally invented as a method for storing data on floppy disks.

• later used by MS-DOS with supports for a hierarchical directory structure, and fixed disks as well as floppy disks

• still used by Windows 3.x and Win95 supporting long filenames and allowing drive size to 2047 gigabytes

Boot Sector FAT file

Directory EntriesFAT ... ... file

DirectoryEntries

FAT File Structure:

FAT: File Allocation Table

...

10/31/00 CSE 380 2

University of Pennsylvania

FAT File Structure

• Cluster - a fundamental unit of disk storage (a collection of sectors)

• File Allocation Table (FAT) – used to track of the usage of a volume’s clusters

– each entry for a cluster represents whether it is used, which cluster is next if used, or whether it contains a bad sector

– FAT12/16/32 means the size of FAT entry (12/16/32 bits)

– two identical copies of FAT for recovery

...

Clusters ...

1 2 3 4 5 6 7 8 ...

FAT 0 0 0 06 -1 7 4

pointed by directory entry

10/31/00 CSE 380 3

University of Pennsylvania

FAT File Structure

•

Directory Entry

– keeps track of information about a file or directory

– short name entry

• name and extension, attributes, size, and time

• a starting cluster number

– long name entry - used by Win95 to support long filename

• sequence, type(longname entry)

• name (wchar_t [13]) : 2-byte character for international characters

– A filename can have 256 characters.

• one short name entry can have upto 20 longname entries

10/31/00 CSE 380 4

University of Pennsylvania

FAT File Structure

Example - “C:\fooDir\barLongnameFile”

– The root directory starts at #0 cluster

Clusters

0 1 2 3 4 5 ...

...

......

fooDir 2...

. . .

. . .

. . . B le1 barLongnameFibarLon~1 4 . . .

0 1 2 3 4 5

FAT -1 3 50 -1 -1 ...

10/31/00 CSE 380 5

University of Pennsylvania

FAT File Structure

Disk Size vs Cluster Size and FAT Size

FAT12 FAT16 FAT32 DISK SIZE

(Cluster Size, FAT Size)

1Giga (230) bytes (256K, 6K) (16K, 256K) (4K, 4M)

2Giga (231) bytes (512K, 6K) (32K, 256K) (4K, 8M)

512M (229) bytes (128K, 6K) ( 8K, 256K) (4K, 2M)

10/31/00 CSE 380 6

University of Pennsylvania

FAT File Structure

Disk Size, Cluster Size vs FAT Size

e.g. Disk Size = 1 Giga (230) bytes ( 1 sector = 512(29) bytes )

Cluster Size

– FAT12 : (212) clusters * (218) bytes/cluster (256K bytes/cluster)

– FAT16 : (216) clusters * (214) bytes/cluster (16K bytes/cluster)

– FAT32 : (232) clusters * (28) bytes/cluster - ??

(218) clusters * (212) bytes/cluster (4K bytes/cluster)

FAT Size

– FAT12 : (212) entry * 12 bits/entry = 3*(211) bytes = 6K bytes

– FAT16 : (216) entry * 16 bits/entry = (218) bytes = 256K bytes

– FAT32 : (218) entry * 32 bits/entry = (222) bytes = 4M bytes