FAT32 is a derivative of the FAT file system that supports drives over 2GB. Because FAT32 drives can contain more than 65,526 clusters, smaller clusters are used than on large FAT16 drives, resulting in more efficient allocation of space on the drive. The 32-bit FAT includes four bytes per cluster within the File Allocation Table. This differs from FAT16 and FAT12 in that those files systems contain 2 bytes and 1.5 bytes within the File Allocation Table, respectively. Note that the high 4 bits of the 32-bit values in the FAT32 File Allocation Table are reserved and are not part of the cluster number. Therefore, applications which directly read a FAT32 File Allocation Table must mask off these bits, and preserve them when writing new values. File System Cluster Limits: FAT12 Less than 4086 clusters Boot Sector and Bootstrap Modifications 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 Since a FAT32 BIOS Parameter Block (BPB) is larger than a standard BPB., the boot
record on FAT32 drives is greater than 1 sector. Also, 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 field in the BPB 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 field of the BPB is ignored on a FAT32 drive. As a result, the size of the
"old" root directory (after the last FAT) is zero clusters. Sectors Per FAT The A_BF_BPB_SectorsPerFAT field in the BPB is always zero on a FAT32 drive Partition System ID's 01H - 12-bit FAT. Partition smaller than 10MB. 04H - 16-bit FAT. Partition smaller that 32MB. 05H - Extended MS-DOS Partition. 06H - 16-bit FAT. Partition larger than or equal to 32MB. 0BH - 32-bit FAT. Partition up to 2047GB. 0CH - Same as 0Bh, but uses Logical Block Address Int 13h extensions. 0EH - Same as 06h, but uses Logical Block Address Int 13h extensions. 0FH - Same as 05h, but uses Logical Block Address Int 13h extensions. FAT32 BIOS Parameter Block (BPB) 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. 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 A_BF_BPB_BytesPerSector - The number of bytes per sector.
Flags describing the drive. Bit 8 of this field indicates whether or not writes to the active FAT will be written to all copies of the FAT. The low 4 bits of this field contain the 0-based FAT number of the Active FAT, but are only meaningful if bit 8 is set. This field can contain a combination of the following values. BGBPB_F_ActiveFATMsk (000Fh) Mask for low four bits. Mask indicating FAT mirroring state. If set, 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 he major version, and the low byte represents the minor version. A_BF_BPB_RootDirStrtClus The cluster number of the first cluster of the root directory of the FAT32 drive. 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 field 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 field 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 field. Do not use. BIGFATBOOTFSINFO Structure Contains information about the file system. BIGFATBOOTFSINFO STRUC
bfFSInf_Sig DD ?
bfFSInf_free_clus_cnt DD ?
bfFSInf_next_free_clus DD ?
bfFSInf_resvd DD 3 DUP (?)
BIGFATBOOTFSINFO ENDS
bfFSInf_Sig - The file system information sector. 61417272H "rrAa" FAT Mirroring On all FAT drives, there may be multiple copies of the FAT. If an error occurs reading the primary copy, the file system will attempt to read from the backup copies. On FAT16 and FAT12 drives, the first FAT is always the primary copy and a modification will automatically be written to all copies. However, on FAT32 drives, FAT mirroring can be disabled, and a FAT other than the first one can be the primary (or "active") copy of the FAT. |
