# 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. (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 |

### 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:
| 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.
| 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. |