BIOS parameter block

In computing, the BIOS parameter block, often shortened to BPB, is a data structure in the volume boot record (VBR) describing the physical layout of a data storage volume. On partitioned devices, such as hard disks, the BPB describes the volume partition, whereas, on unpartitioned devices, such as floppy disks, it describes the entire medium. A basic BPB can appear and be used on any partition, including floppy disks where its presence is often necessary, however, certain filesystems also make use of it in describing basic filesystem structures. Filesystems making use of a BIOS parameter block include FAT12 (except for in DOS 1.x), FAT16, FAT32, HPFS, and NTFS. Due to different types of fields and the amount of data they contain, the length of the BPB is different for FAT16, FAT32, and NTFS boot sectors.[1] (A detailed discussion of the various FAT BPB versions and their entries can be found in the FAT article.) Combined with the 11-byte dummyJump-OEMName data structure at the very start of volume boot records immediately preceding the BPB or EBPB, this is also called FDC descriptor or extended FDC descriptor in ECMA-107 or ISO/IEC 9293 (which describes FAT as for flexible/floppy and optical disk cartridges).

FAT12 / FAT16

DOS 2.0 BPB

Format of standard DOS 2.0 BPB for FAT12 (13 bytes):

Sector offset BPB offset Field length Description
0x00B0x00WORDBytes per logical sector
0x00D0x02BYTELogical sectors per cluster
0x00E0x03WORDReserved logical sectors
0x0100x05BYTENumber of FATs
0x0110x06WORDRoot directory entries
0x0130x08WORDTotal logical sectors
0x0150x0ABYTEMedia descriptor
0x0160x0BWORDLogical sectors per FAT

DOS 3.0 BPB

Format of standard DOS 3.0 BPB for FAT12 and FAT16 (19 bytes), already supported by some versions of MS-DOS 2.11:[2]

Sector offset BPB offset Field length Description
0x00B0x0013 BYTEsDOS 2.0 BPB
0x0180x0DWORDPhysical sectors per track
0x01A0x0FWORDNumber of heads
0x01C0x11WORDHidden sectors (incompatible with DOS 3.31 BPB)

DOS 3.2 BPB

Format of standard DOS 3.2 BPB for FAT12 and FAT16 (21 bytes):

Sector offset BPB offset Field length Description
0x00B0x0019 BYTEsDOS 3.0 BPB
0x01E0x13WORDTotal sectors (incompatible with DOS 3.31 BPB)

DOS 3.31 BPB

Format of standard DOS 3.31 BPB for FAT12, FAT16 and FAT16B (25 bytes):

Sector offset BPB offset Field length Description
0x00B0x0013 BYTEsDOS 2.0 BPB
0x0180x0DWORDPhysical sectors per track (identical to DOS 3.0 BPB)
0x01A0x0FWORDNumber of heads (identical to DOS 3.0 BPB)
0x01C0x11DWORDHidden sectors (incompatible with DOS 3.0 BPB)
0x0200x15DWORDLarge total logical sectors

DOS 3.4 EBPB

Format of PC DOS 3.4 and OS/2 1.0-1.1 Extended BPB for FAT12, FAT16 and FAT16B (32 bytes):

Sector offset BPB offset Field length Description
0x00B0x0025 BYTEsDOS 3.31 BPB
0x0240x19BYTEPhysical drive number
0x0250x1ABYTEFlags etc.
0x0260x1BBYTEExtended boot signature (0x28 aka "4.0") (similar to DOS 4.0 EBPB and NTFS EBPB)
0x0270x1CDWORDVolume serial number

FAT12 / FAT16 / HPFS

DOS 4.0 EBPB

Format of DOS 4.0 and OS/2 1.2 Extended BPB for FAT12, FAT16, FAT16B and HPFS (51 bytes):

Sector offset BPB offset Field length Description
0x00B0x0025 BYTEsDOS 3.31 BPB
0x0240x19BYTEPhysical drive number (identical to DOS 3.4 EBPB)
0x0250x1ABYTEFlags etc. (identical to DOS 3.4 EBPB)
0x0260x1BBYTEExtended boot signature (0x29 aka "4.1") (similar to DOS 3.4 EBPB and NTFS EBPB)
0x0270x1CDWORDVolume serial number (identical to DOS 3.4 EBPB)
0x02B0x2011 BYTEsVolume label
0x0360x2B8 BYTEsFile-system type

FAT32

DOS 7.1 EBPB

Format of short DOS 7.1 Extended BIOS Parameter Block (60 bytes) for FAT32:

Sector offset BPB offset Field length Description
0x00B0x0025 BYTEsDOS 3.31 BPB
0x0240x19DWORDLogical sectors per FAT
0x0280x1DWORDMirroring flags etc.
0x02A0x1FWORDVersion
0x02C0x21DWORDRoot directory cluster
0x0300x25WORDLocation of FS Information Sector
0x0320x27WORDLocation of backup sector(s)
0x0340x2912 BYTEsReserved (Boot file name)
0x0400x35BYTEPhysical drive number
0x0410x36BYTEFlags etc.
0x0420x37BYTEExtended boot signature (0x28)
0x0430x38DWORDVolume serial number

Format of full DOS 7.1 Extended BIOS Parameter Block (79 bytes) for FAT32:

Sector offset BPB offset Field length Description
0x00B0x0060 BYTEsShort FAT32 BPB
0x0470x3C11 BYTEsVolume label
0x0520x478 BYTEsFile-system type

NTFS

Format of Extended BPB for NTFS (73 bytes):

Sector offset BPB offset Field length Description
0x00B0x0025 BYTEsDOS 3.31 BPB
0x0240x19BYTEPhysical drive number (identical to DOS 3.4 EBPB)
0x0250x1ABYTEFlags etc. (identical to DOS 3.4 EBPB)
0x0260x1BBYTEExtended boot signature (0x80 aka "8.0") (similar to DOS 3.4 EBPB and DOS 4.0 EBPB)
0x0270x1CBYTEReserved
0x0280x1DQWORDSectors in volume
0x0300x25QWORDMFT first cluster number
0x0380x2DQWORDMFT mirror first cluster number
0x0400x35DWORDMFT record size
0x0440x39DWORDIndex block size
0x0480x3DQWORDVolume serial number
0x0500x45DWORDChecksum

ExFAT

The exFAT EBPB is very large at 120 bytes. There is no attempt to provide any backward compatibility, with the conventional 0x00B-0x03F part filled with zero. As a part of the VBR, it is found in two repetitions as a security feature against modification.[3]

See also

References

  1. Microsoft. Microsoft Windows 2000 Server Operations Guide. Microsoft Press.
  2. Paterson, Tim; Microsoft (2013-12-19) [1983-05-17]. "Microsoft DOS V1.1 and V2.0: /msdos/v20source/SKELIO.TXT, /msdos/v20source/HRDDRV.ASM". Computer History Museum, Microsoft. Archived from the original on 2017-09-09. Retrieved 2014-03-25. (NB. While the publishers claim this would be MS-DOS 1.1 and 2.0, it actually is SCP MS-DOS 1.25 and a mixture of Altos MS-DOS 2.11 and TeleVideo PC DOS 2.11.)
  3. "exFAT file system specification - Win32 apps". docs.microsoft.com.

Further reading

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