Hardware White Paper
Designing Hardware for Microsoft® Operating Systems
Microsoft Extensible Firmware Initiative
FAT32 File System Specification
FAT: General Overview of On-Disk Format
Version 1.03, December 6, 2000
Microsoft Corporation
The FAT (File Allocation Table) file system has its origins in the late 1970s and early1980s
and was the file system supported by the Microsoft® MS-DOS® operating system. It was
originally developed as a simple file system suitable for floppy disk drives less than 500K in
size. Over time it has been enhanced to support larger and larger media. Currently there are
three FAT file system types: FAT12, FAT16 and FAT32. The basic difference in these FAT
sub types, and the reason for the names, is the size, in bits, of the entries in the actual FAT
structure on the disk. There are 12 bits in a FAT12 FAT entry, 16 bits in a FAT16 FAT entry
and 32 bits in a FAT32 FAT entry.
Contents
Notational Conventions in this Document ...............................................................................................7
General Comments (Applicable to FAT File System All Types) ............................................................7
Boot Sector and BPB................................................................................................................................7
FAT Data Structure ................................................................................................................................13
FAT Type Determination .......................................................................................................................14
FAT Volume Initialization .....................................................................................................................19
FAT32 FSInfo Sector Structure and Backup Boot Sector .....................................................................21
FAT Directory Structure ........................................................................................................................22
FAT Long Directory Entries ..................................................................................................................25
Name Limits and Character Sets............................................................................................................29
Name Matching In Short & Long Names ..............................................................................................30
Naming Conventions and Long Names .................................................................................................30
Effect of Long Directory Entries on Down Level Versions of FAT......................................................32
Validating The Contents of a Directory .................................................................................................32
Other Notes Relating to FAT Directories ..............................................................................................33
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