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Designing Hardware for Microsoft® Operating Systems
FAT: General Overview of On-Disk Format
Version 1.02, May 5, 1999
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................................................................................. 6
General Comments (Applicable to FAT File System All Types)............................................... 6
Boot Sector and BPB ................................................................................................................ 6
FAT Data Structure ................................................................................................................. 12
FAT Type Determination......................................................................................................... 13
FAT Volume Initialization ........................................................................................................ 18
FAT32 FSInfo Sector Structure and Backup Boot Sector ...................................................... 20
FAT Directory Structure .......................................................................................................... 21
Other Notes Relating to FAT Directories ................................................................................ 24
Specification Compliance........................................................................................................ 25
Microsoft, MS_DOS, Windows, and Windows NT are trademarks or registered trademarks of Microsoft Corporation in the United States
and/or other countries. Other product and company names mentioned herein may be the trademarks of their respective owners.
© 1999 Microsoft Corporation. All rights reserved.
FAT: General Overview of On-Disk Format—Page 2
© 1999 Microsoft Corporation. All rights reserved.
2
Disclaimer
IMPORTANT—READ CAREFULLY: This Microsoft Agreement (“Agreement”) is a legal
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(a) Provided that you comply with all terms and conditions of this Agreement and subject to the limitations in
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portions of products which comply with the Specification in unmodified form; and
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offer to sell, sell and otherwise distribute and dispose of portions of products which comply with
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For purposes of the foregoing, the Specification is “unmodified” if there are no changes, additions or
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(b) The foregoing covenant not to sue shall not extend to any part or function of a product which (i) is not
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(c) The covenant not to sue described above shall be unavailable to you and shall terminate immediately if
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cause the direct of the management or policies of any entity whether through the ownership of voting securities,
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(d) The covenant not to sue described above shall not extend to your use of any portion of the Specification
for any purpose other than (a) to create portions of an operating system (i) only as necessary to adapt such
operating system so that it can directly interact with a firmware implementation of the Extensible Firmware
Initiative Specification v. 1.0 (“EFI Specification”); (ii) only as necessary to emulate an implementation of the EFI
Specification; and (b) to create firmware, applications, utilities and/or drivers that will be used and/or licensed for
only the following purposes: (i) to install, repair and maintain hardware, firmware and portions of operating
system software which are utilized in the boot process; (ii) to provide to an operating system runtime services that
are specified in the EFI Specification; (iii) to diagnose and correct failures in the hardware, firmware or operating
system software; (iv) to query for identification of a computer system (whether by serial numbers, asset tags, user
FAT: General Overview of On-Disk Format—Page 3
© 1999 Microsoft Corporation. All rights reserved.
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or otherwise); (v) to perform inventory of a computer system; and (vi) to manufacture, install and setup any
hardware, firmware or operating system software.
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other Microsoft patents, trademarks, copyrights or other intellectual property rights; or any license with
respect to any Microsoft intellectual property rights.
2. ADDITIONAL LIMITATIONS AND OBLIGATIONS.
(a) The foregoing covenant not to sue is applicable only to the version of the Specification which you are
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(b) Without prejudice to any other rights, Microsoft may terminate this Agreement if you fail to comply with the
terms and conditions of this Agreement. In such event you must destroy all copies of the Specification.
3. INTELLECTUAL PROPERTY RIGHTS. All ownership, title and intellectual property rights in and to the
Specification are owned by Microsoft or its suppliers.
4.U.S. GOVERNMENT RIGHTS. Any Specification provided to the U.S. Government pursuant to solicitations
issued on or after December 1, 1995 is provided with the commercial rights and restrictions described elsewhere
herein. Any Specification provided to the U.S. Government pursuant to solicitations issued prior to December 1,
1995 is provided with RESTRICTED RIGHTS as provided for in FAR, 48 CFR 52.227-14 (JUNE 1987) or DFAR,
48 CFR 252.227-7013 (OCT 1988), as applicable.
5.EXPORT RESTRICTIONS. Export of the Specification, any part thereof, or any process or service that is
the direct product of the Specification (the foregoing collectively referred to as the “Restricted
Components”) from the United States is regulated by the Export Administration Regulations (EAR, 15 CFR
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or general damages), the entire liability of Microsoft and any of its suppliers under any provision
of this Agreement and your exclusive remedy for all of the foregoing shall be limited to the greater
FAT: General Overview of On-Disk Format—Page 4
© 1999 Microsoft Corporation. All rights reserved.
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of the amount actually paid by you for the Specification or U.S.$5.00. The foregoing limitations,
exclusions and disclaimers shall apply to the maximum extent permitted by applicable law, even if
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10.QUESTIONS. Should you have any questions concerning this Agreement, or if you desire to contact
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11.ENTIRE AGREEMENT. This Agreement is the entire agreement between you and Microsoft relating to
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Si vous avez acquis votre produit Microsoft au CANADA, la garantie limitée suivante vous concerne :
RENONCIATION AUX GARANTIES. Dans toute la mesure permise par la législation en vigueur, Microsoft
et ses fournisseurs fournissent la Specification (et à toute propriété intellectuelle dans celle-ci) et tous
(selon le cas) les services d’assistance liés à la Specification (“Services d’assistance”) TELS QUELS ET
AVEC TOUS LEURS DÉFAUTS, et par les présentes excluent toute garantie ou condition, expresse ou
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professionnels et d’absence de négligence, le tout relativement à la Specification, à toute propriété
intellectuelle dans celle-ci et à la prestation ou à la non-prestation des Services d’assistance. DE PLUS, IL
N’Y A AUCUNE GARANTIE ET CONDITION DE TITRE, DE JOUISSANCE PAISIBLE, DE POSSESSION
PAISIBLE, DE SIMILARITÉ À LA DESCRIPTION ET D’ABSENCE DE CONTREFAÇON RELATIVEMENT À
LA SPÉCIFICATION ET À TOUTE PROPRIÉTÉ INTELLECTUELLE DANS CELLE-CI. VOUS
SUPPORTEZ TOUS LES RISQUES DÉCOULANT DE L’UTILISATION ET DE LA PERFORMANCE DE LA
SPÉCIFICATION ET DE TOUTE PROPRIÉTÉ INTELLECTUELLE DANS CELLE-CI ET CEUX
DÉCOULANT DES SERVICES D’ASSISTANCE (S’IL Y A LIEU).
EXCLUSION DES DOMMAGES INDIRECTS, ACCESSOIRES ET AUTRES. Dans toute la mesure permise
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obligation y compris les obligations de bonne foi et de diligence raisonnable, la négligence et toute autre
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ou la non-prestation des Services d’assistance ou autrement en vertu de ou relativement à toute disposition
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et ce, même si Microsoft ou l’un de ses fournisseurs a été avisé de la possibilité de tels dommages.
LIMITATION DE RESPONSABILITÉ ET RECOURS. Malgré tout dommage que vous pourriez encourir pour
quelque raison que ce soit (y compris, mais sans limitation, tous les dommages mentionnés ci-dessus et
tous les dommages directs et généraux), la seule responsabilité de Microsoft et de ses fournisseurs en vertu
de toute disposition de cette convention et votre unique recours en regard de tout ce qui précède sont
limités au plus élevé des montants suivants: soit (a) le montant que vous avez payé pour la Spécification,
soit (b) un montant équivalant à cinq dollars U.S. (5,00 $ U.S.). Les limitations, exclusions et renonciations
ci-dessus s’appliquent dans toute la mesure permise par la législation en vigueur, et ce même si leur
application a pour effet de priver un recours de son essence.
DROITS LIMITÉS DU GOUVERNEMENT AMÉRICAIN
Tout Produit Logiciel fourni au gouvernement américain conformément à des demandes émises le ou après
le 1er décembre 1995 est offert avec les restrictions et droits commerciaux décrits ailleurs dans la présente
convention. Tout Produit Logiciel fourni au gouvernement américain conformément à des demandes
émises avant le 1er décembre 1995 est offert avec des DROITS LIMITÉS tels que prévus dans le FAR,
48CFR 52.227-14 (juin 1987) ou dans le FAR, 48CFR 252.227-7013 (octobre 1988), tels qu’applicables.
5
© 1999 Microsoft Corporation. All rights reserved.
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Sauf lorsqu’expressément prohibé par la législation locale, la présente convention est régie par les lois en
vigueur dans la province d’Ontario, Canada. Pour tout différend qui pourrait découler des présentes, vous
acceptez la compétence des tribunaux fédéraux et provinciaux siégeant à Toronto, Ontario.
Si vous avez des questions concernant cette convention ou si vous désirez communiquer avec Microsoft
pour quelque raison que ce soit, veuillez contacter la succursale Microsoft desservant votre pays, ou écrire
à: Microsoft Sales Information Center, One Microsoft Way, Redmond, Washington 98052-6399.
