Boot Sector
The boot sector, located at sector 1 of each volume, is a critical disk structure for starting your computer. It contains executable code and data required by the code, including information that the file system uses to access the volume. The boot sector is created when you format a volume. At the end of the boot sector is a two-byte structure called a signature word or end of sector marker, which is always set to 0x55AA. On computers running Windows 2000, the boot sector on the active partition loads into memory and starts Ntldr, which loads the operating system.
The Windows 2000 boot sector consists of the following elements:
- An x86-based CPU jump instruction.
- The original equipment manufacturer identification (OEM ID).
- The BIOS parameter block (BPB), a data structure.
- The extended BPB.
- The executable boot code (or bootstrap code) that starts the operating system.
Note
All Windows 2000 boot sectors contain these elements. However, the NTFS boot sector, the FAT16, and the FAT32 boot sectors are all formatted differently.
The BPB describes the physical parameters of the volume: the extended BPB begins immediately after the BPB. Due to differing types of fields and the amount of data they contain, the length of the BPB is different for FAT16, FAT32, and NTFS boot sectors.
The information in the BPB and the extended BPB is used by disk device drivers to read and configure volumes. The area following the extended BPB typically contains executable boot code, which performs the actions necessary to continue the startup process.
Boot Sector Startup Processes
Computers use the boot sector to run instructions during startup. The initial startup process is summarized in the following steps:
- The system BIOS and the CPU initiate the power-on self test (POST).
- The BIOS searches for a boot device (typically a disk).
- The BIOS loads the first physical sector of the boot device into memory and transfers CPU execution to that memory address.
If the boot device is on a hard disk, the BIOS loads the MBR. The master boot code in the MBR loads the boot sector of the active partition, and transfers CPU execution to that memory address. On computers that are running Windows 2000, the executable boot code in the boot sector finds Ntldr, loads it into memory, and transfers execution to that file.
Note
Windows 2000 cannot start up from a spanned, striped, or RAID-5 volume that is running dynamic disk. These disk structures cannot be registered into the MBR's partition table, so a system partition using these structures is not startable. Windows 2000 must be fully loaded into memory before they can be used.
If there is a floppy disk in drive A, the system BIOS loads the first sector (the boot sector) of the disk into memory. If the disk is startable — formatted by
Non-System disk or disk error
Replace and press any key when ready
Note
This error will not appear on normally functioning systems that are configured to look for the startup files on drive C first. On many computers, an option in the CMOS setup program allows the user to set the sequence of installed disks that the system searches for the startup files.
If you get similar errors when trying to start the computer from the hard disk, the boot sector might be corrupted. For more information about troubleshooting boot sector problems, see "Damaged MBRs and Boot Sectors" later in this chapter.
Initially, the startup process is independent of disk format and operating system. The unique characteristics of operating and file systems become important when the boot sector's executable boot code starts.
Components of a Boot Sector
The MBR transfers CPU execution to the boot sector, so the first three bytes of the boot sector must be valid, executable x86-based CPU instructions. This includes a jump instruction that skips the next several nonexecutable bytes.
Following the jump instruction is the 8-byte OEM ID, a string of characters that identifies the name and version number of the operating system that formatted the volume. To preserve compatibility with
Note
You may also see the OEM ID "MSWIN4.0" on disks formatted by Windows 95 and "MSWIN4.1" on disks formatted by Windows 95 OSR2 and Windows 98. Windows 2000 does not use the OEM ID field in the boot sector except for verifying NTFS volumes.
Following the OEM ID is the BPB, which provides information that enables the executable boot code to locate Ntldr. The BPB always starts at the same offset, so standard parameters are in a known location. Disk size and geometry variables are encapsulated in the BPB. Because the first part of the boot sector is an x86 jump instruction, the BPB can be extended in the future by appending new information at the end. The jump instruction needs only a minor adjustment to accommodate this change. The BPB is stored in a packed (unaligned) format.
FAT16 Boot Sector
Table 1.6 describes the boot sector of a volume formatted with the FAT16 file system.
Table 1.6 Boot Sector Sections on a FAT16 Volume
| Byte Offset | Field Length | Field Name |
|---|---|---|
| 0x00 | 3 bytes | Jump Instruction |
| 0x03 | LONGLONG | OEM ID |
| 0x0B | 25 bytes | BPB |
| 0x24 | 26 bytes | Extended BPB |
| 0x3E | 448 bytes | Bootstrap Code |
| 0x01FE | WORD | End of Sector Marker |
The following example illustrates a hexadecimal printout of the boot sector on a FAT16 volume. The printout is formatted in three sections:
- Bytes 0x00– 0x0A are the jump instruction and the OEM ID (shown in bold print).
- Bytes 0x0B– 0x3D are the BPB and the extended BPB.
- The remaining section is the bootstrap code and the end of sector marker (shown in bold print).
Physical Sector: Cyl 0, Side 1, Sector 1
00000000: EB 3C 90 4D 53 44 4F 53 - 35 2E 30 00 02 40 01 00 .<.MSDOS5.0..@..
00000010: 02 00 02 00 00 F8 FC 00 - 3F 00 40 00 3F 00 00 00 ........?.@.?...
00000020: 01 F0 3E 00 80 00 29 A8 - 8B 36 52 4E 4F 20 4E 41 ..>...)..6RNO NA
00000030: 4D 45 20 20 20 20 46 41 - 54 31 36 20 20 20 33 C0 ME FAT16 3.
00000040: 8E D0 BC 00 7C 68 C0 07 - 1F A0 10 00 F7 26 16 00 ....|h......&..
00000050: 03 06 0E 00 50 91 B8 20 - 00 F7 26 11 00 8B 1E 0B ....P.. ..&.....
00000060: 00 03 C3 48 F7 F3 03 C8 - 89 0E 08 02 68 00 10 07 ...H........h...
00000070: 33 DB 8F 06 13 02 89 1E - 15 02 0E E8 90 00 72 57 3.............rW
00000080: 33 DB 8B 0E 11 00 8B FB - 51 B9 0B 00 BE DC 01 F3 3.......Q.......
00000090: A6 59 74 05 83 C3 20 E2 - ED E3 37 26 8B 57 1A 52 .Yt... ...7&.W.R
000000A0: B8 01 00 68 00 20 07 33 - DB 0E E8 48 00 72 28 5B ...h. .3...H.r([
000000B0: 8D 36 0B 00 8D 3E 0B 02 - 1E 8F 45 02 C7 05 F5 00 .6...>....E.....
000000C0: 1E 8F 45 06 C7 45 04 0E - 01 8A 16 24 00 EA 03 00 ..E..E.....$....
000000D0: 00 20 BE 86 01 EB 03 BE - A2 01 E8 09 00 BE C1 01 . ..............
000000E0: E8 03 00 FB EB FE AC 0A - C0 74 09 B4 0E BB 07 00 .........t......
000000F0: CD 10 EB F2 C3 50 4A 4A - A0 0D 00 32 E4 F7 E2 03 .....PJJ...2....
00000100: 06 08 02 83 D2 00 A3 13 - 02 89 16 15 02 58 A2 07 .............X..
00000110: 02 A1 13 02 8B 16 15 02 - 03 06 1C 00 13 16 1E 00 ................
00000120: F7 36 18 00 FE C2 88 16 - 06 02 33 D2 F7 36 1A 00 .6........3..6..
00000130: 88 16 25 00 A3 04 02 A1 - 18 00 2A 06 06 02 40 3A ..%.......*...@:
00000140: 06 07 02 76 05 A0 07 02 - 32 E4 50 B4 02 8B 0E 04 ...v....2.P.....
00000150: 02 C0 E5 06 0A 2E 06 02 - 86 E9 8B 16 24 00 CD 13 ............$...
00000160: 0F 83 05 00 83 C4 02 F9 - CB 58 28 06 07 02 76 11 .........X(...v.
00000170: 01 06 13 02 83 16 15 02 - 00 F7 26 0B 00 03 D8 EB ..........&.....
00000180: 90 A2 07 02 F8 CB 42 4F - 4F 54 3A 20 43 6F 75 6C ......BOOT: Coul
00000190: 64 6E 27 74 20 66 69 6E - 64 20 4E 54 4C 44 52 0D dn't find NTLDR.
000001A0: 0A 00 42 4F 4F 54 3A 20 - 49 2F 4F 20 65 72 72 6F ..BOOT: I/O erro
000001B0: 72 20 72 65 61 64 69 6E - 67 20 64 69 73 6B 0D 0A r reading disk..
000001C0: 00 50 6C 65 61 73 65 20 - 69 6E 73 65 72 74 20 61 .Please insert a
000001D0: 6E 6F 74 68 65 72 20 64 - 69 73 6B 00 4E 54 4C 44 nother disk.NTLD
000001E0: 52 20 20 20 20 20 20 00 - 00 00 00 00 00 00 00 00 R .........
000001F0: 00 00 00 00 00 00 00 00 - 00 00 00 00 00 00 55 AA ..............U.
Tables 1.7 and 1.8 illustrate the layout of the BPB and the extended BPB for FAT16 volumes. The sample values correspond to the data in the preceding example.
Table 1.7 BPB Fields for FAT16 Volumes
| Byte Offset | Field Length | Value | Field Name and Definition |
|---|---|---|---|
| 0x0B | WORD | 0x0002 | Bytes Per Sector. The size of a hardware sector. Valid decimal values for this field are 512, 1024, 2048, and 4096. For most disks used in the United States, the value of this field is 512. |
| 0x0D | BYTE | 0x40 | Sectors Per Cluster. The number of sectors in a cluster. Because FAT16 can track only a limited number of clusters (up to 65,536), large volumes are supported by increasing the number of sectors per cluster. The default cluster size for a volume depends on the volume size. Valid decimal values for this field are 1, 2, 4, 8, 16, 32, 64, and 128. Values that lead to clusters larger than 32 KB (Bytes Per Sector * Sectors Per Cluster) can cause disk and software errors. |
| 0x0E | WORD | 0x0100 | Reserved Sectors. The number of sectors preceding the start of the first FAT, including the boot sector. The value of this field is always 1. |
| 0x10 | BYTE | 0x02 | Number of FATs. The number of copies of the FAT on the volume. The value of this field is always 2. |
| 0x11 | WORD | 0x0002 | Root Entries. The total number of 32-byte file and folder name entries that can be stored in the root folder of the volume. On a typical hard disk, the value of this field is 512. One entry is always used as a Volume Label, and files and folders with long names use multiple entries per file. The largest number of file and folder entries is typically 511, but entries run out before you reach that number if long file names are used. |
| 0x13 | WORD | 0x0000 | Small Sectors. The number of sectors on the volume represented in 16 bits (< 65,536). For volumes larger than 65,536 sectors, this field has a value of zero and the Large Sectors field is used instead. |
| 0x15 | BYTE | 0xF8 | Media Descriptor. Provides information about the media being used. A value of 0xF8 indicates a hard disk and 0xF0 indicates a high-density 3.5-inch floppy disk. Media descriptor entries are a legacy of |
| 0x16 | WORD | 0xFC00 | Sectors Per FAT. The number of sectors occupied by each FAT on the volume. The computer uses this number and the number of FATs and hidden sectors, to determine where the root directory begins. The computer can also determine where the user data area of the volume begins based on the number of entries in the root directory (512). |
| 0x18 | WORD | 0x3F00 | Sectors Per Track. Part of the apparent disk geometry used on a low-level formatted disk. |
| 0x1A | WORD | 0x4000 | Number of Heads. Part of the apparent disk geometry used on a low-level formatted disk. |
| 0x1C | DWORD | 0x3F000000 | Hidden Sectors. The number of sectors on the volume before the boot sector. This value is used during the boot sequence to calculate the absolute offset to the root directory and data areas. |
| 0x20 | DWORD | 0x01F03E00 | Large Sectors. If the value of the Small Sectors field is zero, this field contains the total number of sectors in the FAT16 volume. If the value of the Small Sectors field is not zero, the value of this field is zero. |
Table 1.8 Extended BPB Fields for FAT16 Volumes
| Byte Offset | Field Length | Value | Field Name and Definition |
|---|---|---|---|
| 0x24 | BYTE | 0x80 | Physical Drive Number. Related to the BIOS physical drive number. Floppy disk drives are identified as 0x00 and physical hard disks are identified as 0x80, regardless of the number of physical disk drives. Typically, this value is set prior to issuing an INT 13h BIOS call to specify the device to access. The value is only relevant if the device is a boot device. |
| 0x25 | BYTE | 0x00 | Reserved. FAT16 volumes are always set to zero. |
| 0x26 | BYTE | 0x29 | Extended Boot Signature. A field that must have the value 0x28 or 0x29 to be recognized by Windows 2000. |
| 0x27 | DWORD | 0xA88B3652 | Volume Serial Number. A random serial number created when formatting a disk, which helps to distinguish between disks. |
| 0x2B | 11 bytes | NO NAME | Volume Label. A field once used to store the volume label. The volume label is now stored as a special file in the root directory. |
| 0x36 | LONGLONG | FAT16 | File System Type. A field with a value of either FAT, FAT12 or FAT16, depending on the disk format. |
FAT32 Boot Sector
Table 1.9 describes the boot sector of a volume formatted with the FAT32 file system.
Note
The FAT32 boot sector is structurally very similar to the FAT16 boot sector, but the FAT32 BPB contains additional fields. The FAT32 extended BPB uses the same fields as FAT16, but the offset addresses of these fields within the boot sector are different than those found in FAT16 boot sectors. Drives formatted in FAT32 are not readable by operating systems that are incompatible with FAT32.
Table 1.9 Boot Sector Sections on a FAT32 Volume
| Byte Offset | Field Length | Field Name |
|---|---|---|
| 0x00 | 3 bytes | Jump Instruction |
| 0x03 | LONGLONG | OEM ID |
| 0x0B | 53 bytes | BPB |
| 0x40 | 26 bytes | Extended BPB |
| 0x5A | 420 bytes | Bootstrap Code |
| 0x01FE | WORD | End of Sector Marker |
The following example illustrates a hexadecimal printout of the boot sector on a FAT32 volume. The printout is formatted in three sections:
- Bytes 0x00– 0x0A are the jump instruction and the OEM ID (shown in bold print).
- Bytes 0x0B– 0x59 are the BPB and the extended BPB.
- The remaining section is the bootstrap code and the end of sector marker (shown in bold print).
Physical Sector: Cyl 878, Side 0, Sector 1
00000000: EB 58 90 4D 53 44 4F 53 - 35 2E 30 00 02 08 20 00 .X.MSDOS5.0... .
00000010: 02 00 00 00 00 F8 00 00 - 3F 00 FF 00 EE 39 D7 00 ........?....9..
00000020: 7F 32 4E 00 83 13 00 00 - 00 00 00 00 02 00 00 00 2N.............
00000030: 01 00 06 00 00 00 00 00 - 00 00 00 00 00 00 00 00 ................
00000040: 80 00 29 8B 93 6D 54 4E - 4F 20 4E 41 4D 45 20 20 ..)..mTNO NAME
00000050: 20 20 46 41 54 33 32 20 - 20 20 33 C9 8E D1 BC F4 FAT32 3.....
00000060: 7B 8E C1 8E D9 BD 00 7C - 88 4E 02 8A 56 40 B4 08 {......|.N..V@.. 00000070: CD 13 73 05 B9 FF FF 8A - F1 66 0F B6 C6 40 66 0F ..s......f...@f.
00000080: B6 D1 80 E2 3F F7 E2 86 - CD C0 ED 06 41 66 0F B7 ....?.......Af..
00000090: C9 66 F7 E1 66 89 46 F8 - 83 7E 16 00 75 38 83 7E .f..f.F..~..u8.~
000000A0: 2A 00 77 32 66 8B 46 1C - 66 83 C0 0C BB 00 80 B9 *.w2f.F.f.......
000000B0: 01 00 E8 2B 00 E9 48 03 - A0 FA 7D B4 7D 8B F0 AC ...+..H...}.}...
000000C0: 84 C0 74 17 3C FF 74 09 - B4 0E BB 07 00 CD 10 EB ..t.<.t.........
000000D0: EE A0 FB 7D EB E5 A0 F9 - 7D EB E0 98 CD 16 CD 19 ...}....}.......
000000E0: 66 60 66 3B 46 F8 0F 82 - 4A 00 66 6A 00 66 50 06 f`f;F...J.fj.fP.
000000F0: 53 66 68 10 00 01 00 80 - 7E 02 00 0F 85 20 00 B4 Sfh.....~.... ..
00000100: 41 BB AA 55 8A 56 40 CD - 13 0F 82 1C 00 81 FB 55 A..U.V@........U
00000110: AA 0F 85 14 00 F6 C1 01 - 0F 84 0D 00 FE 46 02 B4 .............F..
00000120: 42 8A 56 40 8B F4 CD 13 - B0 F9 66 58 66 58 66 58 B.V@......fXfXfX
00000130: 66 58 EB 2A 66 33 D2 66 - 0F B7 4E 18 66 F7 F1 FE fX.*f3.f..N.f...
00000140: C2 8A CA 66 8B D0 66 C1 - EA 10 F7 76 1A 86 D6 8A ...f..f....v....
00000150: 56 40 8A E8 C0 E4 06 0A - CC B8 01 02 CD 13 66 61 V@............fa
00000160: 0F 82 54 FF 81 C3 00 02 - 66 40 49 0F 85 71 FF C3 ..T.....f@I..q..
00000170: 4E 54 4C 44 52 20 20 20 - 20 20 20 0D 0A 4E 54 4C NTLDR ..NTL
00000180: 44 52 20 69 73 20 6D 69 - 73 73 69 6E 67 FF 0D 0A DR is missing...
00000190: 44 69 73 6B 20 65 72 72 - 6F 72 FF 0D 0A 50 72 65 Disk error...Pre
000001A0: 73 73 20 61 6E 79 20 6B - 65 79 20 74 6F 20 72 65 ss any key to re
000001B0: 73 74 61 72 74 0D 0A 00 - 00 00 00 00 00 00 00 00 start...........
000001C0: 00 00 00 00 00 00 00 00 - 00 00 00 00 00 00 00 00 ................
000001D0: 00 00 00 00 00 00 00 00 - 00 00 00 00 00 00 00 00 ................
000001E0: 00 00 00 00 00 00 00 00 - 00 00 00 00 00 00 00 00 ................
000001F0: 00 00 00 00 00 00 00 00 - 00 7B 8E 9B 00 00 55 AA .........{....U. Tables 1.10 and 1.11 illustrate the layout of the BPB and the extended BPB for FAT32 volumes. The sample values correspond to the data in the preceding example.
Table 1.10 BPB Fields for FAT32 Volumes
| Byte Offset | Field Length | Value | Field Name and Definition |
|---|---|---|---|
| 0x0B | WORD | 0x0002 | Bytes Per Sector. The size of a hardware sector. Valid decimal values for this field are 512, 1024, 2048, and 4096. For most disks used in the United States, the value of this field is 512. |
| 0x0D | BYTE | 0x08 | Sectors Per Cluster. The number of sectors in a cluster. Because FAT32 can only track a finite number of clusters (up to 4,294,967,296), extremely large volumes are supported by increasing the number of sectors per cluster. The default cluster size for a volume depends on the volume size. Valid decimal values for this field are 1, 2, 4, 8, 16, 32, 64, and 128. The Windows 2000 implementation of FAT32 allows for the creation of volumes only up to a maximum of 32 GB. However, larger volumes created by other operating systems (Windows 95 OSR2 and later) are accessible in Windows 2000. |
| 0x0E | WORD | 0x0200 | Reserved Sectors. The number of sectors preceding the start of the first FAT, including the boot sector. The decimal value of this field is typically 32. |
| 0x10 | BYTE | 0x02 | Number of FATs. The number of copies of the FAT on the volume. The value of this field is always 2. |
| 0x11 | WORD | 0x0000 | Root Entries (FAT12/FAT16 only). For FAT32 volumes, this field must be set to zero. |
| 0x13 | WORD | 0x0000 | Small Sectors (FAT12/FAT16 only). For FAT32 volumes, this field must be set to zero. |
| 0x15 | BYTE | 0xF8 | Media Descriptor. Provides information about the media being used. A value of 0xF8 indicates a hard disk and 0xF0 indicates a high-density 3.5-inch floppy disk. Media descriptor entries are a legacy of |
| 0x16 | WORD | 0x0000 | Sectors Per FAT (FAT12/FAT16 only). For FAT32 volumes, this field must be set to zero. |
| 0x18 | WORD | 0x3F00 | Sectors Per Track. Contains the "sectors per track" geometry value for disks that use INT 13h. The volume is broken down into tracks by multiple heads and cylinders. |
| 0x1A | WORD | 0xFF00 | Number of Heads. Contains the "count of heads" geometry value for disks that use INT 13h. For example, on a 1.44-MB, 3.5-inch floppy disk this value is 2. |
| 0x1C | DWORD | 0xEE39D700 | Hidden Sectors. The number of sectors on the volume before the boot sector. This value is used during the boot sequence to calculate the absolute offset to the root directory and data areas. This field is generally only relevant for media that are visible on interrupt 13h. It must always be zero on media that are not partitioned. |
| 0x20 | DWORD | 0x7F324E00 | Large Sectors. Contains the total number of sectors in the FAT32 volume. |
| 0x24 | DWORD | 0x83130000 | Sectors Per FAT (FAT32 only). The number of sectors occupied by each FAT on the volume. The computer uses this number and the number of FATs and hidden sectors (described in this table), to determine where the root directory begins. The computer can also determine where the user data area of the volume begins based on the number of entries in the root directory. |
| 0x28 | WORD | 0x0000 | Extended Flags (FAT32 only). The value of the bits in this two-byte structure are: Bits 0–3: Number of the active FAT (starting count at 0, not 1). It is only valid if mirroring is disabled. Bits 4–6: Reserved. Bit 7: A value of 0 means the FAT is mirrored at run time into all FATs. A value of 1 means only one FAT is active (referenced in bits 0-3). Bits 8–15: Reserved. |
| 0x2A | WORD | 0x0000 | File System Version (FAT32 only). The high byte is the major revision number, whereas the low byte is the minor revision number. This field supports the ability to extend the FAT32 media type in the future with concern for old FAT32 drivers mounting the volume. If the field is non-zero, back-level Windows versions will not mount the volume. |
| 0x2C | DWORD | 0x02000000 | Root Cluster Number (FAT32 only). The cluster number of the first cluster of the root directory. This value is typically, but not always, 2. |
| 0x30 | WORD | 0x0100 | File System Information Sector Number (FAT32 only). The sector number of the File System Information (FSINFO) structure in the reserved area of the FAT32 volume. The value is typically 1. A copy of the FSINFO structure is kept in the Backup Boot Sector, but it is not kept up-to-date. |
| 0x34 | WORD | 0x0600 | Backup Boot Sector (FAT32 only). A non-zero value indicates the sector number in the reserved area of the volume in which a copy of the boot sector is stored. The value of this field is typically 6. No other value is recommended. |
| 0x36 | 12 bytes | 0x000000000000000000000000 | Reserved (FAT32 only). Reserved space for future expansion. The value of this field should always be zero. |
Table 1.11 Extended BPB Fields for FAT32 Volumes
| Byte Offset | Field Length | Value | Field Name and Definition |
|---|---|---|---|
| 0x40 | BYTE | 0x80 | Physical Drive Number. Related to the BIOS physical drive number. Floppy disk drives are identified as 0x00 and physical hard disks are identified as 0x80, regardless of the number of physical disk drives. Typically, this value is set prior to issuing an INT 13h BIOS call to specify the device to access. It is only relevant if the device is a boot device. |
| 0x41 | BYTE | 0x00 | Reserved. FAT32 volumes are always set to zero. |
| 0x42 | BYTE | 0x29 | Extended Boot Signature. A field that must have the value 0x28 or 0x29 to be recognized by Windows 2000. |
| 0x43 | DWORD | 0xA88B3652 | Volume Serial Number. A random serial number created when formatting a disk, which helps to distinguish between disks. |
| 0x47 | 11 bytes | NO NAME | Volume Label. A field once used to store the volume label. The volume label is now stored as a special file in the root directory. |
| 0x52 | LONGLONG | FAT32 | System ID. A text field with a value of FAT32. |
NTFS Boot Sector
Table 1.12 describes the boot sector of a volume formatted with NTFS. The bootstrap code for an NTFS volume is longer than the 426 bytes, as shown in Table 1.12. When you format an NTFS volume, the format program allocates the first 16 sectors for the boot sector and the bootstrap code.
Table 1.12 Boot Sector Sections on an NTFS Volume
| Byte Offset | Field Length | Field Name |
|---|---|---|
| 0x00 | 3 bytes | Jump Instruction |
| 0x03 | LONGLONG | OEM ID |
| 0x0B | 25 bytes | BPB |
| 0x24 | 48 bytes | Extended BPB |
| 0x54 | 426 bytes | Bootstrap Code |
| 0x01FE | WORD | End of Sector Marker |
On NTFS volumes, the data fields that follow the BPB form an extended BPB. The data in these fields enables Ntldr to find the master file table (MFT) during startup. On NTFS volumes, the MFT is not located in a predefined sector, as on FAT16 and FAT32 volumes. For this reason, the MFT can be moved if there is a bad sector in its normal location. However, if the data is corrupted, the MFT cannot be located, and Windows 2000 assumes that the volume has not been formatted.
The following example illustrates the boot sector of an NTFS volume formatted while running Windows 2000. The printout is formatted in three sections:
- Bytes 0x00– 0x0A are the jump instruction and the OEM ID (shown in bold print).
- Bytes 0x0B–0x53 are the BPB and the extended BPB.
- The remaining code is the bootstrap code and the end of sector marker (shown in bold print).
Physical Sector: Cyl 0, Side 1, Sector 1
00000000: EB 52 90 4E 54 46 53 20 - 20 20 20 00 02 08 00 00 .R.NTFS .....
00000010: 00 00 00 00 00 F8 00 00 - 3F 00 FF 00 3F 00 00 00 ........?...?...
00000020: 00 00 00 00 80 00 80 00 - 4A F5 7F 00 00 00 00 00 ........J......
00000030: 04 00 00 00 00 00 00 00 - 54 FF 07 00 00 00 00 00 ........T.......
00000040: F6 00 00 00 01 00 00 00 - 14 A5 1B 74 C9 1B 74 1C ...........t..t.
00000050: 00 00 00 00 FA 33 C0 8E - D0 BC 00 7C FB B8 C0 07 .....3.....|....
00000060: 8E D8 E8 16 00 B8 00 0D - 8E C0 33 DB C6 06 0E 00 ..........3.....
00000070: 10 E8 53 00 68 00 0D 68 - 6A 02 CB 8A 16 24 00 B4 ..S.h..hj....$..
00000080: 08 CD 13 73 05 B9 FF FF - 8A F1 66 0F B6 C6 40 66 ...s......f...@f
00000090: 0F B6 D1 80 E2 3F F7 E2 - 86 CD C0 ED 06 41 66 0F .....?.......Af.
000000A0: B7 C9 66 F7 E1 66 A3 20 - 00 C3 B4 41 BB AA 55 8A ..f..f. ...A..U.
000000B0: 16 24 00 CD 13 72 0F 81 - FB 55 AA 75 09 F6 C1 01 .$...r...U.u....
000000C0: 74 04 FE 06 14 00 C3 66 - 60 1E 06 66 A1 10 00 66 t......f`..f...f
000000D0: 03 06 1C 00 66 3B 06 20 - 00 0F 82 3A 00 1E 66 6A ....f;. ...:..fj
000000E0: 00 66 50 06 53 66 68 10 - 00 01 00 80 3E 14 00 00 .fP.Sfh.....>...
000000F0: 0F 85 0C 00 E8 B3 FF 80 - 3E 14 00 00 0F 84 61 00 ........>.....a.
00000100: B4 42 8A 16 24 00 16 1F - 8B F4 CD 13 66 58 5B 07 .B..$......fX[.
00000110: 66 58 66 58 1F EB 2D 66 - 33 D2 66 0F B7 0E 18 00 fXfX.-f3.f.....
00000120: 66 F7 F1 FE C2 8A CA 66 - 8B D0 66 C1 EA 10 F7 36 f......f..f....6
00000130: 1A 00 86 D6 8A 16 24 00 - 8A E8 C0 E4 06 0A CC B8 ......$.........
00000140: 01 02 CD 13 0F 82 19 00 - 8C C0 05 20 00 8E C0 66 ........... ...f
00000150: FF 06 10 00 FF 0E 0E 00 - 0F 85 6F FF 07 1F 66 61 ..........o..fa
00000160: C3 A0 F8 01 E8 09 00 A0 - FB 01 E8 03 00 FB EB FE ................
00000170: B4 01 8B F0 AC 3C 00 74 - 09 B4 0E BB 07 00 CD 10 .....<.t........
00000180: EB F2 C3 0D 0A 41 20 64 - 69 73 6B 20 72 65 61 64 .....A disk read
00000190: 20 65 72 72 6F 72 20 6F - 63 63 75 72 72 65 64 00 error occurred.
000001A0: 0D 0A 4E 54 4C 44 52 20 - 69 73 20 6D 69 73 73 69 ..NTLDR is missi
000001B0: 6E 67 00 0D 0A 4E 54 4C - 44 52 20 69 73 20 63 6F ng...NTLDR is co
000001C0: 6D 70 72 65 73 73 65 64 - 00 0D 0A 50 72 65 73 73 mpressed...Press
000001D0: 20 43 74 72 6C 2B 41 6C - 74 2B 44 65 6C 20 74 6F Ctrl+Alt+Del to
000001E0: 20 72 65 73 74 61 72 74 - 0D 0A 00 00 00 00 00 00 restart........
000001F0: 00 00 00 00 00 00 00 00 - 83 A0 B3 C9 00 00 55 AA ..............U.
Table 1.13 describes the fields in the BPB and the extended BPB on NTFS volumes. The fields starting at 0x0B, 0x0D, 0x15, 0x18, 0x1A, and 0x1C match those on FAT16 and FAT32 volumes. The sample values correspond to the data in the preceding example.
Table 1.13 BPB and Extended BPB Fields on NTFS Volumes
| Byte Offset | Field Length | Sample Value | Field Name |
|---|---|---|---|
| 0x0B | WORD | 0x0002 | Bytes Per Sector |
| 0x0D | BYTE | 0x08 | Sectors Per Cluster |
| 0x0E | WORD | 0x0000 | Reserved Sectors |
| 0x10 | 3 BYTES | 0x000000 | always 0 |
| 0x13 | WORD | 0x0000 | not used by NTFS |
| 0x15 | BYTE | 0xF8 | Media Descriptor |
| 0x16 | WORD | 0x0000 | always 0 |
| 0x18 | WORD | 0x3F00 | Sectors Per Track |
| 0x1A | WORD | 0xFF00 | Number Of Heads |
| 0x1C | DWORD | 0x3F000000 | Hidden Sectors |
| 0x20 | DWORD | 0x00000000 | not used by NTFS |
| 0x24 | DWORD | 0x80008000 | not used by NTFS |
| 0x28 | LONGLONG | 0x4AF57F0000000000 | Total Sectors |
| 0x30 | LONGLONG | 0x0400000000000000 | Logical Cluster Number for the file $MFT |
| 0x38 | LONGLONG | 0x54FF070000000000 | Logical Cluster Number for the file $MFTMirr |
| 0x40 | DWORD | 0xF6000000 | Clusters Per File Record Segment |
| 0x44 | DWORD | 0x01000000 | Clusters Per Index Block |
| 0x48 | LONGLONG | 0x14A51B74C91B741C | Volume Serial Number |
| 0x50 | DWORD | 0x00000000 | Checksum |
Protecting the Boot Sector
Because a normally functioning system relies on the boot sector to access a volume, it is highly recommended that you run disk scanning tools such as Chkdsk regularly, as well as back up all of your data files to protect against data loss if you lose access to a volume.
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