Guide >> Concepts >> Understanding FAT File System >> FAT32 features

• FAT Partition Boot Sector
• FAT File Allocation Table
• FAT Root Folder
• FAT Folder Structure
• FAT32 Features

FAT32 features

The following topics describe the FAT32 file system.

• File System Specifications
• Boot Sector and Bootstrap Modifications
• FAT Mirroring
• Partition Types

Boot Sector and Bootstrap Modifications

BPB (FAT32)

The BPB for FAT32 drives is an extended version of the FAT16/FAT12 BPB. It contains identical information to a standard BPB, but also includes several extra fields for FAT32 specific information.

This structure is implemented in Windows OEM Service Release 2 and later.

A_BF_BPB    STRUC
    A_BF_BPB_BytesPerSector       DW    ?
    A_BF_BPB_SectorsPerCluster    DB    ?
    A_BF_BPB_ReservedSectors      DW    ?
    A_BF_BPB_NumberOfFATs         DB    ?
    A_BF_BPB_RootEntries          DW    ?
    A_BF_BPB_TotalSectors         DW    ?
    A_BF_BPB_MediaDescriptor      DB    ?
    A_BF_BPB_SectorsPerFAT        DW    ?
    A_BF_BPB_SectorsPerTrack      DW    ?
    A_BF_BPB_Heads                DW    ?
    A_BF_BPB_HiddenSectors        DW    ?
    A_BF_BPB_HiddenSectorsHigh    DW    ?
    A_BF_BPB_BigTotalSectors      DW    ?
    A_BF_BPB_BigTotalSectorsHigh  DW    ?
    A_BF_BPB_BigSectorsPerFat     DW    ?
    A_BF_BPB_BigSectorsPerFatHi   DW    ?
    A_BF_BPB_ExtFlags             DW    ?
    A_BF_BPB_FS_Version           DW    ?
    A_BF_BPB_RootDirStrtClus      DW    ?
    A_BF_BPB_RootDirStrtClusHi    DW    ?
    A_BF_BPB_FSInfoSec            DW    ?
    A_BF_BPB_BkUpBootSec          DW    ?
    A_BF_BPB_Reserved             DW    6 DUP (?)
A_BF_BPB    ENDS

Members

A_BF_BPB_BytesPerSector

The number of bytes per sector.

A_BF_BPB_SectorsPerCluster

The number of sectors per cluster.

A_BF_BPB_ReservedSectors

The number of reserved sectors, beginning with sector 0.

A_BF_BPB_NumberOfFATs

The number of File Allocation Tables.

A_BF_BPB_RootEntries

This member is ignored on FAT32 drives.

A_BF_BPB_TotalSectors

The size of the partition, in sectors.

A_BF_BPB_MediaDescriptor

The media descriptor. Values in this member are identical to standard BPB.

A_BF_BPB_SectorsPerFAT

The number of sectors per FAT.

Note: This member will always be zero in a FAT32 BPB. Use the values from A_BF_BPB_BigSectorsPerFat and A_BF_BPB_BigSectorsPerFatHi for FAT32 media.

A_BF_BPB_SectorsPerTrack

The number of sectors per track.

A_BF_BPB_Heads

The number of read/write heads on the drive.

A_BF_BPB_HiddenSectors

The number of hidden sectors on the drive.

A_BF_BPB_HiddenSectorsHigh

The high word of the hidden sectors value.

A_BF_BPB_BigTotalSectors

The total number of sectors on the FAT32 drive.

A_BF_BPB_BigTotalSectorsHigh

The high word of the FAT32 total sectors value.

A_BF_BPB_BigSectorsPerFat

The number of sectors per FAT on the FAT32 drive.

A_BF_BPB_BigSectorsPerFatHi

The high word of the FAT32 sectors per FAT value.

A_BF_BPBExtFlags

Flags describing the drive. Bit 8 of this value indicates whether or not information written to the active FAT will be written to all copies of the FAT. The low 4 bits of this value contain the 0-based FAT number of the Active FAT, but are only meaningful if bit 8 is set. This member can contain a combination of the following values.

Value	Description
BGBPB_F_ActiveFATMsk (000Fh)	Mask for low four bits.
BGBPB_F_NoFATMirror (0080h)	Mask indicating FAT mirroring state. If set, FAT mirroring is disabled. If clear, FAT mirroring is enabled.

Bits 4-6 and 8-15 are reserved.

A_BF_BPB_FS_Version

The file system version number of the FAT32 drive. The high byte represents the major version, and the low byte represents the minor version.

A_BF_BPB_RootDirStrtClus

The cluster number of the first cluster in the FAT32 drive's root directory.

A_BF_BPB_RootDirStrtClusHi

The high word of the FAT32 starting cluster number.

A_BF_BPB_FSInfoSec

The sector number of the file system information sector. The file system info sector contains a BIGFATBOOTFSINFO structure. This member is set to 0FFFFh if there is no FSINFO sector. Otherwise, this value must be non-zero and less than the reserved sector count.

A_BF_BPB_BkUpBootSec

The sector number of the backup boot sector. This member is set to 0FFFFh if there is no backup boot sector. Otherwise, this value must be non-zero and less than the reserved sector count.

A_BF_BPB_Reserved

Reserved member.

BIGFATBOOTFSINFO (FAT32)

Contains information about the file system on a FAT32 volume. This structure is implemented in Windows OEM Service Release 2 and later.

BIGFATBOOTFSINFO STRUC
    bfFSInf_Sig               DD    ?
    bfFSInf_free_clus_cnt     DD    ?
    bfFSInf_next_free_clus    DD    ?
    bfFSInf_resvd             DD    3 DUP (?)
BIGFATBOOTFSINFO ENDS

Members

bfFSInf_Sig

The signature of the file system information sector. The value in this member is FSINFOSIG (0x61417272L).

bfFSInf_free_clus_cnt

The count of free clusters on the drive. Set to -1 when the count is unknown.

bfFSInf_next_free_clus

The cluster number of the cluster that was most recently allocated.

bfFSInf_resvd

Reserved member.

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