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 (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.) 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.
Contents
FAT12 / FAT16
DOS 2.0 BPB
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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
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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
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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
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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
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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
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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
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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 |
See also
References
- ↑ Microsoft. Microsoft Windows 2000 Server Operations Guide. Microsoft Press
- ↑ Lua error in package.lua at line 80: module 'strict' not found. (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.)
Further reading
- Lua error in package.lua at line 80: module 'strict' not found. — a description of BPBs, from version 2.0 to version 7.0
- Lua error in package.lua at line 80: module 'strict' not found. — 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.
- Lua error in package.lua at line 80: module 'strict' not found. — 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.
- Lua error in package.lua at line 80: module 'strict' not found. — 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:
- Lua error in package.lua at line 80: module 'strict' not found. — 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".
- Lua error in package.lua at line 80: module 'strict' not found. — Microsoft documents extended BPBs on both FAT16 and FAT32 volumes. It also documents BPBs on NTFS volumes.
- Lua error in package.lua at line 80: module 'strict' not found. — 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.
- Lua error in package.lua at line 80: module 'strict' not found. — an issue that affects BPBs
- Lua error in package.lua at line 80: module 'strict' not found.
- Lua error in package.lua at line 80: module 'strict' not found. — on the misuse of OEM labels and Microsoft's Volume Tracker
- Lua error in package.lua at line 80: module 'strict' not found.