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 any modifications 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.
Mirroring is enabled by clearing bit 0x0080 in the extdpb_flags member of a FAT32 Drive Parameter Block (DPB) structure.
When Enabled (bit 0x0080 clear)
With mirroring enabled, whenever a FAT sector is written, it will also be written to every other FAT. Also, a mirrored FAT sector can be read from any FAT. A FAT32 drive with multiple FATs will behave the same as FAT16 and FAT12 drives with multiple FATs. That is, the multiple FATs are backups of each other.
When Disabled (bit 0x0080 set)
With mirroring disabled, only one of the FATs is active. The active FAT is the one specified by bits 0 through 3 of the extdpb_flags member of DPB. The other FATs are ignored. Disabling mirroring allows better handling of a drive with a bad sector in one of the FATs. If a bad sector exists, access to the damaged FAT can be completely disabled. Then, a new FAT can be built in one of the inactive FATs and then made accessible by changing the active FAT value in extdpb_flags.
The DPB was extended to include FAT32 information. Changes are effective for Windows 95 OEM Service Release 2 and later.
DPB STRUC dpb_drive DB ? dpb_unit DB ? dpb_sector_size DW ? dpb_cluster_mask DB ? dpb_cluster_shift DB ? dpb_first_fat DW ? dpb_fat_count DB ? dpb_root_entries DW ? dpb_first_sector DW ? dpb_max_cluster DW ? dpb_fat_size DW ? dpb_dir_sector DW ? dpb_reserved2 DD ? dpb_media DB ? ifdef NOTFAT32 dpb_first_access DB ? else dpb_reserved DB ? endif dpb_reserved3 DD ? dpb_next_free DW ? dpb_free_cnt DW ? ifndef NOTFAT32 extdpb_free_cnt_hi DW ? extdpb_flags DW ? extdpb_FSInfoSec DW ? extdpb_BkUpBootSec DW ? extdpb_first_sector DD ? extdpb_max_cluster DD ? extdpb_fat_size DD ? extdpb_root_clus DD ? extdpb_next_free DD ? endif DPB ENDS
|dpb_drive||The drive number (0 = A, 1 = B, and so on)..|
|dpb_unit||Specifies the unit number. The device driver uses the unit number to distinguish the specified drive from the other drives it supports.|
|dpb_sector_size||The size of each sector, in bytes.|
|dpb_cluster_mask||The number of sectors per cluster minus 1.|
|dpb_cluster_shift||The number of sectors per cluster, expressed as a power of 2.|
|dpb_first_fat||The sector number of the first sector containing the file allocation table (FAT).|
|dpb_fat_count||The number of FATs on the drive.|
|dpb_root_entries||The number of entries in the root directory.|
|dpb_first_sector||The sector number of the first sector in the first cluster.|
|dpb_max_cluster||The number of clusters on the drive plus 1. This member is undefined for FAT32 drives.|
|dpb_fat_size||The number of sectors occupied by each FAT. The value of zero indicates a FAT32 drive. Use the value in extdpb_fat_size instead.|
|dpb_dir_sector||The sector number of the first sector containing the root directory. This member is undefined for FAT32 drives.|
|dpb_reserved2||Reserved member. Do not use.|
|dpb_media||Specifies the media descriptor for the medium in the specified drive.|
|reserved||Reserved member. Do not use.|
|dpb_first_access||Indicates whether the medium in the drive has been accessed. This member is initialized to -1 to force a media check the first time this DPB is used.|
|dpb_reserved3||Reserved member. Do not use.|
|dpb_next_free||The cluster number of the most recently allocated cluster.|
|dpb_free_cnt||The number of free clusters on the medium. This member is 0FFFFh if the number is unknown.|
|extdpb_free_cnt_hi||The high word of free count.|
|extdpb_flags||Flags describing the drive. The low 4 bits of this value contain the 0-based FAT number of the Active FAT. This member can contain a combination of the following values.|
|BGBPB_F_ActiveFATMsk (000Fh)BGBPB_F_NoFATMirror (0080h)||Mask for low four bits.|
|Do not mirror active FAT to inactive FATs.Bits 4-6 and 8-15 are reserved.|
|extdpb_FSInfoSec||The sector number of the file system information sector. 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.|
|extdpb_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.|
|extdpb_first_sector||The first sector of the first cluster.|
|extdpb_max_cluster||The number of clusters on the drive plus 1.|
|extdpb_fat_size||The number of sectors occupied by the FAT.|
|extdpb_root_clus||The cluster number of the first cluster in the root directory.|
|extdpb_next_free||The number of the cluster that was most recently allocated.|