BIOS parameter block

In computing, the BIOS parameter block, often shortened to BPB, is a data structure in the Volume Boot Record 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, 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] ECMA-107 or ISO/IEC 9293 (which describes FAT as for flexible/floppy and optical disk cartridges) also describes this as an FDC Descriptor or an FDC Extended Descriptor.

The standard BPB for FAT16 is as follows:

Field	Field Length	Hex offset	Description
BytesPerSector	WORD	`0x000B`	Bytes Per Sector
SectorsPerCluster	BYTE	`0x000D`	Sectors Per Cluster
ReservedSectors	WORD	`0x000E`	Reserved Sectors
FatCopies	BYTE	`0x0010`	Number of FATs
RootDirEntries	WORD	`0x0011`	Root Entries
NumSectors	WORD	`0x0013`	Small Sectors
MediaType	BYTE	`0x0015`	Media Descriptor
SectorsPerFAT	WORD	`0x0016`	Sectors Per FAT
SectorsPerTrack	WORD	`0x0018`	Sectors Per Track
NumberOfHeads	WORD	`0x001A`	Number of Heads
HiddenSectors	DWORD	`0x001C`	Hidden Sectors
SectorsBig	DWORD	`0x0020`	Large Sectors

Extended BPB for FAT16 Volumes:

Field Length	Hex offset	Description
BYTE	`0x0024`	Physical Drive Number
BYTE	`0x0025`	Reserved
BYTE	`0x0026`	Extended Boot Signature
DWORD	`0x0027`	Volume Serial Number
11 bytes	`0x002B`	Volume Label
QWORD	`0x0036`	File System Type

References

^ Microsoft. Microsoft Windows 2000 Server Operations Guide. Microsoft Press

Jonathan de Boyne Pollard. "All about BIOS Parameter Blocks". Frequently Given Answers. http://homepage.ntlworld.com./jonathan.deboynepollard/FGA/bios-parameter-block.html. — a description of BPBs, from version 2.0 to version 7.0
Ken W. Christopher, Barry A. Feigenbaum, & Shon O. Saliga (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.
Carl Townsend (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.
Alex Verstak (1998-03-10). "FAT Boot Sector". http://averstak.tripod.com/fatdox/bootsec.htm. — 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. http://www.microsoft.com/technet/archive/win98/reskit/part2/wrkc10.mspx. — 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. http://www.microsoft.com/technet/prodtechnol/Windows2000Pro/reskit/part7/proch32.mspx. — 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. http://technet.microsoft.com/en-us/library/cc781134%28WS.10%29.aspx. — 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.
Jonathan de Boyne Pollard. "The meaning and use of the OEM Name field in volume boot blocks". Frequently Given Answers. http://homepage.ntlworld.com./jonathan.deboynepollard/FGA/volume-boot-block-oem-name-field.html. — an issue that affects BPBs
Matthias Paul (2004-08-25). "NOVOLTRK.REG". www.drdos.org. http://www.ibiblio.org/pub/micro/pc-stuff/freedos/win9x/NOVOLTRK.ZIP. Retrieved 2011-12-17.
Matthias Paul (2002-02-20). "Need DOS 6.22 (Not OEM)". alt.msdos.programmer. http://groups.google.com/group/alt.msdos.programmer/msg/6b10a1ea602e61e. Retrieved 2006-10-14. — on the misuse of OEM labels and Microsoft's Volume Tracker
Wally Bass (1994-02-14). "Cluster Size". comp.os.msdos.programmer. http://groups.google.co.uk/group/comp.os.msdos.programmer/msg/79de2d76832cfbd6. Retrieved 2006-10-14.

BIOS parameter block

References

Further reading

See also