Boot Sector and Bootstrap Modifications

Modifications to Boot Sector

Modifications Description
Reserved Sectors FAT32 drives contain more reserved sectors than FAT16 or FAT12 drives. The number of reserved sectors is usually 32, but can vary.
Boot Sector Modifications Because a FAT32 BIOS Parameter Block (BPB), represented by the BPB structure, is larger than a standard BPB, the boot record on FAT32 drives is greater than 1 sector. In addition, there is a sector in the reserved area on FAT32 drives that contains values for the count of free clusters and the cluster number of the most recently allocated cluster. These values are members of the BIGFATBOOTFSINFO structure which is contained within this sector. These additional fields allow the system to initialize the values without having to read the entire file allocation table.
Root Directory The root directory on a FAT32 drive is not stored in a fixed location as it is on FAT16 and FAT12 drives. On FAT32 drives, the root directory is an ordinary cluster chain. The A_BF_BPB_RootDirStrtClus member in the BPB structure contains the number of the first cluster in the root directory. This allows the root directory to grow as needed. In addition, the BPB_RootEntries member of BPB is ignored on a FAT32 drive.
Sectors Per FAT The A_BF_BPB_SectorsPerFAT member of BPB is always zero on a FAT32 drive. Additionally, the A_BF_BPB_BigSectorsPerFat and A_BF_BPB_BigSectorsPerFatHi members of the updated BPB provide equivalent information for FAT32 media.


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_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 (?)

DBP Members

Member Name Description
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 (seet table below).
A_BF_BPBExtFlags Value Description
BGBPB_F_ActiveFATMsk (000Fh) Mask for low four bits.
BGBPB_F_NoFATMirror (0080h) Do not mirror active FAT to inactive FATs. 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.


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

    bfFSInf_Sig               DD    ?
    bfFSInf_free_clus_cnt     DD    ?
    bfFSInf_next_free_clus    DD    ?
    bfFSInf_resvd             DD    3 DUP (?)


Member Name Description
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.