1. UMBC CMSC 421
Spring 2017
The FAT Filesystem

2. Overview Definitions What is a File Allocation Table?
History of FAT/FAT Versions
Basic structures
File organization

3. Definitions Endianness – Byte ordering of multi-byte integers
FAT follows the Little Endian byte ordering, placing the least significant byte first on disk (like x86 processors do for memory)
Multi-byte integer – Numeric data type that takes up more than one byte.
Common types: word (16 bits/2 bytes), double-word (32 bits/4 bytes), and quad- word (64 bits/8 bytes)
Logical block – A series of one or more consecutive disk sectors. Usually contains a power-of-two number of raw disk sectors.
In FAT, usually a logical block contains a single 512 byte disk sector.
Cluster – A series of consecutive filesystem logical blocks. Usually contains a power-of-two number of blocks.

4. What is a File Allocation Table?
A File Allocation Table is a simple disk structure for finding the disk blocks allocated to a file
Contains one entry per data cluster on the filesystem
Entries form linked lists to where the data is for each file on the disk
For instance, if a file begins in data cluster 10, then conceptually FAT[10] will tell us the next cluster in the file, unless the file fits in one cluster on the disk
Special markers for end of chain, unallocated space, bad clusters (a cluster that contains one or more bad disk sectors)
The directory structure will tell us where the first cluster is in a file

5. History of FAT FAT8 – 1977-1979 FAT12 – 1980-1982
Originally written by Marc McDonald
Designed for 8” floppy disks used on the NCR 7200 Model VI terminal
Used an 8 bit file allocation table, 9 byte file names
Directory entries originally 16 bytes long, extended later to 32 bytes (which is still the case to this day)
FAT12 –
Designed for Seattle Computer Products (SCP) QDOS/86-DOS operating system, based on the original 8 bit FAT filesystem by Tim Paterson
Again, designed for floppy disks
Used a 12 bit file allocation table, 11 byte (8.3) file names
Acquired by Microsoft when purchasing the rights to 86-DOS, which was the basis for MS- DOS and IBM PC-DOS
Modified slightly several times to adapt to newer disk technologies and to add minor features - still used on floppy disks (not that we use those all that much anymore)
Maximum volume size of approximately 16 or 32MiB

6. History of FAT FAT16 – 1984 FAT16B – 1987
Further extended the file allocation table size to 16 bits
Designed for early PC hard drives (which were 20MB in size)
Maximum volume size of 32MiB ( byte disk sectors)
Logical sectored versions exist, which extend the maximum volume size
FAT16B – 1987
What is commonly referred to as FAT16 today
Extended the sector count to be a 32-bit value, vastly expanding the maximum volume size to up to 16GiB on some systems (like Windows NT4)
Common maximum volume size limit is 2GiB

7. History of FAT FAT32 - 1996 Other variants
Introduced with Windows 95 OSR2
Extended the FAT entry size to 28 bits
Each entry is actually 32 bits in length, but the top 4 bits are reserved
Maximum volume size of 2TiB (4,294,967, byte sectors)
Can use a logical-sectored variant for a larger maximum volume size
Maximum file size of 2 or 4 GiB, depending on filesystem variant
Other variants
FATX – 2001, used on the Xbox and Xbox 360 video game consoles
exFAT – 2006, used commonly on flash media such as SDXC cards

8. Basic FAT Filesystem Structure
FAT filesystems follow a specific structure, placing important filesystem metadata at the beginning of the volume:
Sector 0: Boot Sector/Volume Boot Record (VBR)
Contains the BIOS Parameter Block (BPB) which defines the basic characteristics of the volume
After the BPB (and any extended BPBs), contains boot code for the OS if the volume is bootable
Sector 1 (FAT32 only): Filesystem Information Sector
Contains information intended to speed up certain common disk operations, such as the number of free clusters and the last allocated cluster
Reserved Sectors (if any) immediately follow the VBR (FAT12/FAT16) or FS Info Sector (FAT32)

9. Basic FAT Filesystem Structure
Immediately after the reserved sectors, one or more copies of the File Allocation Table itself are present
Contains one entry (either 12, 16, or 32 bits in length) per data cluster of the volume
First two entries of the FAT are reserved (entries 0 and 1), meaning the first data cluster is 2
FAT12 and FAT16 place the root directory at the beginning of the volume as well, pre-allocating some number of sectors for it
FAT32 places the root directory in the data area of the volume, usually (but not necessarily) starting in cluster 2
After the root directory (FAT12/FAT16) or the File Allocation Table (FAT32), the data area of the volume begins with cluster 2

10. BIOS Parameter Block
The BPB/EBPB of a volume contains the basic parameters of the volume itself including:
Bytes per logical block
Blocks per cluster
Number of logical blocks
Length of the FATs
Volume ID/Name

11. Directory Entries
Directory entries (either in the root directory or any other subdirectory) follow a very simple format
Name of the file (11 bytes) - Divided into an 8 byte file name and a 3 byte extension
File attributes (1 byte) - such as read only/hidden/system status)
Reserved (1 byte) - used by some OSes for various extended attributes
Creation timestamp (3 bytes) and date (2 bytes)
Access date (2 bytes)
High-order 16-bits of the starting cluster of the file (2 bytes)
Modification timestamp (2 bytes) and date (2 bytes)
Low-order 16-bits of the starting cluster of the file (2 bytes)
File size (4 bytes)

12. Long File Names
Long file name support uses directory entries in a specific format so that non-LFN- aware systems will ignore them
Sequence Number (1 byte)
5 characters of the filename in UCS-2 (10 bytes)
0x0F – attributes in a normal directory entry (1 byte)
0x00 – LFN type, reserved byte in a normal entry (1 byte)
Short filename checksum (1 byte)
6 characters of the filename in UCS-2 (12 bytes)
0x0000 – first cluster of file in a normal entry (2 bytes)
2 characters of the filename in UCS-2 (4 bytes)
Entries are chained together by way of the sequence number field
Maximum file name length is 256 UCS-2 characters
LFN entries directly proceed the basic directory entry for the file and are organized such that the last characters of the filename appear first in the directory structure

13. FAT structure First two entries of the FAT are reserved:
FAT[0] = FAT ID (always 0xF0) in bits 0-7, all other bits set to 1
FAT[1] = End of Chain marker (typically all bits set, but values with none of the low order 4 bits set are also valid)
Each allocated entry of the File Allocation Table is a next cluster pointer
End of Chain marker used to tell when a file/directory ends
Non-allocated (and non-reserved) clusters contain 0

14. Resources Wikipedia (yes, I will tell you to look at Wikipedia here)
Microsoft’s FAT32 Specification
843a f3456c/fatgen103.doc
Other Microsoft Articles