BIOS parameter block

Last updated July 28, 2025

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 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
`0x00B`	`0x00`	WORD	Bytes per logical sector
`0x00D`	`0x02`	BYTE	Logical sectors per cluster
`0x00E`	`0x03`	WORD	Reserved logical sectors
`0x010`	`0x05`	BYTE	Number of FATs
`0x011`	`0x06`	WORD	Root directory entries
`0x013`	`0x08`	WORD	Total logical sectors
`0x015`	`0x0A`	BYTE	Media descriptor
`0x016`	`0x0B`	WORD	Logical sectors per FAT

Not all versions of DOS 2.00 conform to this structure. IBM PC-DOS 2.00 has additional, undocumented fields in this space that seems to indicate the bootable drive and the sector from where to start reading. This does not appear in any official IBM documentation but a PC/XT system will not boot without correct values in these bytes.

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
`0x00B`	`0x00`	13 BYTEs	DOS 2.0 BPB
`0x018`	`0x0D`	WORD	Physical sectors per track
`0x01A`	`0x0F`	WORD	Number of heads
`0x01C`	`0x11`	WORD	Hidden 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
`0x00B`	`0x00`	19 BYTEs	DOS 3.0 BPB
`0x01E`	`0x13`	WORD	Total 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
`0x00B`	`0x00`	13 BYTEs	DOS 2.0 BPB
`0x018`	`0x0D`	WORD	Physical sectors per track (identical to DOS 3.0 BPB)
`0x01A`	`0x0F`	WORD	Number of heads (identical to DOS 3.0 BPB)
`0x01C`	`0x11`	DWORD	Hidden sectors (incompatible with DOS 3.0 BPB)
`0x020`	`0x15`	DWORD	Large 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
`0x00B`	`0x00`	25 BYTEs	DOS 3.31 BPB
`0x024`	`0x19`	BYTE	Physical drive number
`0x025`	`0x1A`	BYTE	Flags etc.
`0x026`	`0x1B`	BYTE	Extended boot signature (`0x28` aka "4.0") (similar to DOS 4.0 EBPB and NTFS EBPB)
`0x027`	`0x1C`	DWORD	Volume 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
`0x00B`	`0x00`	25 BYTEs	DOS 3.31 BPB
`0x024`	`0x19`	BYTE	Physical drive number (identical to DOS 3.4 EBPB)
`0x025`	`0x1A`	BYTE	Flags etc. (identical to DOS 3.4 EBPB)
`0x026`	`0x1B`	BYTE	Extended boot signature (`0x29` aka "4.1") (similar to DOS 3.4 EBPB and NTFS EBPB)
`0x027`	`0x1C`	DWORD	Volume serial number (identical to DOS 3.4 EBPB)
`0x02B`	`0x20`	11 BYTEs	Volume label
`0x036`	`0x2B`	8 BYTEs	File-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
`0x00B`	`0x00`	25 BYTEs	DOS 3.31 BPB
`0x024`	`0x19`	DWORD	Logical sectors per FAT
`0x028`	`0x1D`	WORD	Mirroring flags etc.
`0x02A`	`0x1F`	WORD	Version
`0x02C`	`0x21`	DWORD	Root directory cluster
`0x030`	`0x25`	WORD	Location of FS Information Sector
`0x032`	`0x27`	WORD	Location of backup sector(s)
`0x034`	`0x29`	12 BYTEs	Reserved (Boot file name)
`0x040`	`0x35`	BYTE	Physical drive number
`0x041`	`0x36`	BYTE	Flags etc.
`0x042`	`0x37`	BYTE	Extended boot signature (`0x28`)
`0x043`	`0x38`	DWORD	Volume serial number

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

Sector offset	BPB offset	Field length	Description
`0x00B`	`0x00`	25 BYTEs	DOS 3.31 BPB
`0x024`	`0x19`	DWORD	Logical sectors per FAT
`0x028`	`0x1D`	WORD	Mirroring flags etc.
`0x02A`	`0x1F`	WORD	Version
`0x02C`	`0x21`	DWORD	Root directory cluster
`0x030`	`0x25`	WORD	Location of FS Information Sector
`0x032`	`0x27`	WORD	Location of backup sector(s)
`0x034`	`0x29`	12 BYTEs	Reserved (Boot file name)
`0x040`	`0x35`	BYTE	Physical drive number
`0x041`	`0x36`	BYTE	Flags etc.
`0x042`	`0x37`	BYTE	Extended boot signature (`0x29`)
`0x043`	`0x38`	DWORD	Volume serial number
`0x047`	`0x3C`	11 BYTEs	Volume label
`0x052`	`0x47`	8 BYTEs	File-system type

NTFS

Format of Extended BPB for NTFS (73 bytes):

Sector offset	BPB offset	Field length	Description
`0x00B`	`0x00`	25 BYTEs	DOS 3.31 BPB
`0x024`	`0x19`	BYTE	Physical drive number (identical to DOS 3.4 EBPB)
`0x025`	`0x1A`	BYTE	Flags etc. (identical to DOS 3.4 EBPB)
`0x026`	`0x1B`	BYTE	Extended boot signature (`0x80` aka "8.0") (similar to DOS 3.4 EBPB and DOS 4.0 EBPB)
`0x027`	`0x1C`	BYTE	Reserved
`0x028`	`0x1D`	QWORD	Sectors in volume
`0x030`	`0x25`	QWORD	MFT first cluster number
`0x038`	`0x2D`	QWORD	MFT mirror first cluster number
`0x040`	`0x35`	DWORD	MFT record size
`0x044`	`0x39`	DWORD	Index block size
`0x048`	`0x3D`	QWORD	Volume serial number
`0x050`	`0x45`	DWORD	Checksum

exFAT BPB

exFAT does not use a BPB in the classic sense. Nevertheless, the volume boot record in sector 0 is organized similarly to BPBs.^[3]

Sector offset	Field length (bytes)	Description
`0x000`	3	JumpBoot. Must be 0xEB, 0x76, 0x90.
`0x003`	8	Filesystem name. Must be "EXFAT ".
`0x00B`	53	Blank (traditional BPB area). Must be all zero.

References

↑ Microsoft. Microsoft Windows 2000 Server Operations Guide. Microsoft Press.
↑ 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.)
↑ "exFAT Filesystem". elm-chan.org.

de Boyne Pollard, Jonathan. "All about BIOS Parameter Blocks". Frequently Given Answers. Archived from the original on 2017-09-09. Retrieved 2016-04-18.— a description of BPBs, from version 2.0 to version 7.0
Christopher, Ken W.; Feigenbaum, Barry A.; Saliga, Shon O. (1990). "9: DOS Disk Usage". Developing applications using DOS. Wiley. ISBN 0-471-52231-7.— In the "processing the BIOS parameter block" section the authors describe the evolution of the BIOS parameter block from the MS-DOS version 2.0 BPB to the PC DOS version 4.0 BPB, and label each field with the DOS version that introduced it.
Townsend, Carl (1989). "4: Disk organization and management". Advanced MS-DOS Expert Techniques for programmers. Howard M. Sams. ISBN 0-672-22667-7.— Figure 4.3 contains a diagram of the version 4.0 BPB and states that the layout of BPBs "is not defined by Microsoft and can vary with different vendors". At the time that the book was written, this was true. Microsoft first publicly documented the BPB structure in the OS/2 Developers' Toolkit.
Verstak, Alex (1998-03-10). "FAT Boot Sector". Archived from the original on 2016-07-30.
— Verstak reverse engineers the BIOS parameter block. The paper contains several errors. One such is its statement that "the presence of the EBPB in FAT32 is not documented by Microsoft". See:
- Microsoft. "Chapter 10 - Disks and File Systems". Microsoft Windows 98 Resource Kit. Archived from the original on 2017-09-09. Retrieved 2017-09-09.— Microsoft documents a version 4.0 BPB and a new "FAT32 BIOS Parameter Block (BPB)" (a version 7.0 BPB) for DOS-Windows 98 that is "larger than a standard BPB", has an "identical structure to a standard BPB", but that also "includes several extra fields".
- Microsoft. "Chapter 32 - Disk Concepts and Troubleshooting". Microsoft Windows 2000 Professional Resource Kit. Archived from the original on 2006-08-10. Retrieved 2006-06-01.— Microsoft documents extended BPBs on both FAT16 and FAT32 volumes. It also documents BPBs on NTFS volumes.
Microsoft. "How NTFS Works". Microsoft Windows Server 2003 NTFS Technical Reference. Archived from the original on 2016-07-30.— The table "BPB and Extended BPB Fields on NTFS Volumes" describes BPBs on NTFS volumes. The descriptions of several fields contradict those given in the Windows 2000 Resource Kit.
de Boyne Pollard, Jonathan. "The meaning and use of the OEM Name field in volume boot blocks". Frequently Given Answers. Archived from the original on 2017-09-09. Retrieved 2016-06-04.— an issue that affects BPBs
Paul, Matthias R. (2004-08-25). "NOVOLTRK.REG". www.drdos.org. Archived from the original on 2016-03-04. Retrieved 2011-12-17.
Paul, Matthias R. (2002-02-20). "Need DOS 6.22 (Not OEM)". Newsgroup: alt.msdos.programmer. Archived from the original on 2017-09-09. Retrieved 2006-10-14.— on the misuse of OEM labels and Microsoft's Volume Tracker
Paul, Matthias R. (2002-08-12). "File System ID - "The alt.os.development boot sector descriptors standards committee"". Newsgroup: alt.os.development. Archived from the original on 2022-05-29. Retrieved 2019-07-29.
Bass, Wally (1994-02-14). "Cluster Size". Newsgroup: comp.os.msdos.programmer. Archived from the original on 2017-09-09. Retrieved 2006-10-14.
Elliott, John C. (2006-04-17) [2004-06-18]. "On OEM IDs". Seasip.info. 1.3. Archived from the original on 2019-04-08. Retrieved 2019-07-29.
Dye, Charles E. (1999-03-20). "FDISK problems". delorie.com. Archived from the original on 2019-07-29. Retrieved 2019-07-29.
Anonymous (2004-06-15). "I hope this is helpful about Microsoft and Caldera DR-DOS and OEM IDs". Groklaw . Retrieved 2019-07-29.^{[ permanent dead link ]}
Kilmer, Jen (2013-05-21). "Evil :) DOS6 FAT". Newsgroup: comp.os.os2.misc. Archived from the original on 2022-05-29. Retrieved 2019-07-29.

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[Microsoft-1] Microsoft. Microsoft Windows 2000 Server Operations Guide. Microsoft Press.

[Microsoft_2014_Altos-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.)

[exFAT-3] "exFAT Filesystem". elm-chan.org.

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