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The information about primary partitions and an extended partition is contained in the Partition Table, a 64-byte data structure located in the same sector as the Master Boot Record (cylinder 0, head 0, sector 1).
The Partition Table conforms to a standard layout that is independent of the operating system. Each Partition Table entry is 16 bytes long, making a maximum of four entries available. Each entry starts at a predetermined offset from the beginning of the sector, as follows:
Partition 1 0x01BE (446)
Partition 2 0x01CE (462)
Partition 3 0x01DE (478)
Partition 4 0x01EE (494)
The last two bytes in the sector are a signature word for the sector and are always 0x55AA. The next example is a printout of the Partition Table for the disk shown in an example earlier in this chapter. When there are fewer than four partitions, the remaining fields are all zeros.
| 000001C0:01 00 06 0F 7F 96 3F 00 -00 00 51 42 06 00 00 00 | .....?...QB.... |
| 000001D0:41 97 07 0F FF 2C 90 42 -06 00 A0 3E 06 00 00 00 | A....,.B...>.... |
| 000001E0:C1 2D 05 0F FF 92 30 81 -0C 00 A0 91 01 00 00 00 | .-....0......... |
| 000001F0:C1 93 01 0F FF A6 D0 12 -0E 00 C0 4E 00 00 55 AA | ...........N..U. |
The following table describes each entry in the Partition Table. The sample values correspond to the information for partition 1.
Partition Table Fields| Byte Offset | Field Length | Sample Value | Meaning |
| 00 | BYTE | 0x80 | Boot Indicator. Indicates whether the partition is the system partition. Legal values are: 00 = Do not use for booting. 80 = System partition. |
| 01 | BYTE | 0x01 | Starting Head. |
| 02 | 6 bits | 0x01 | Starting Sector. Only bits 0-5 are used. Bits 6-7 are the upper two bits for the Starting Cylinder field. |
| 03 | 10 bits | 0x00 | Starting Cylinder. This field contains the lower 8 bits of the cylinder value. Starting cylinder is thus a 10-bit number, with a maximum value of 1023. |
| 04 | BYTE | 0x06 | System ID. This byte defines the volume type. In Windows NT, it also indicates that a partition is part of a volume that requires the use of the HKEY_LOCAL_MACHINE\SYSTEM\DISK Registry subkey. |
| 05 | BYTE | 0x0F | Ending Head. |
| 06 | 6 bits | 0x3F | Ending Sector. Only bits 0-5 are used. Bits 6-7 are the upper two bits for the Ending Cylinder field. |
| 07 | 10 bits | 0x196 | Ending Cylinder. This field contains the lower 8 bits of the cylinder value. Ending cylinder is thus a 10-bit number, with a maximum value of 1023. |
| 08 | DWORD | 3F 00 00 00 | Relative Sector. |
| 12 | DWORD | 51 42 06 00 | Total Sectors. |
The remainder of this section describes the uses of these fields. Definitions of the fields in the Partition Table is the same for primary partitions, extended partitions, and logical drives in extended partitions.
Boot Indicator FieldThe Boot Indicator field indicates whether the volume is the system partition. On x-86-based computers, only one primary partition on the disk should have this field set. This field is used only on x86-based computers. On RISC-based computers, the NVRAM contains the information for finding the files to load.
On x86-based computers, it is possible to have different operating systems and different file systems on different volumes. For example, a computer could have MS-DOS on the first primary partition and Windows 95, UNIX, OS/2, or Windows NT on the second. You control which primary partition (active partition in FDISK) to use to start the computer by setting the Boot Indicator field for that partition in the Partition Table.
For primary partitions and logical drives, the System ID field describes the file system used to format the volume. Windows NT uses this field to determine what file system device drivers to load during startup. It also identifies the extended partition, if there is one defined.
These are the values for the System ID field:
| Value | Meaning |
| 0x0112 | -bit FAT primary partition or logical drive. The number of sectors in the volume is fewer than 32680. |
| 0x0416 | -bit FAT primary partition or logical drive. The number of sectors is between 32680 and 65535. |
| 0x05 | Extended partition. See section titled "Logical Drives and Extended Partitions," |
| 0x06 | BIGDOS FAT primary partition or logical drive. |
| 0x07 | NTFS primary partition or logical drive. |
| Value | Meaning |
| 0x0B | Primary Fat32 partition, using interrupt 13 (INT 13) extensions. |
| 0x0C | Extended Fat32 partition, using INT 13 extensions. |
| 0x0E | Extended Fat16 partition, using INT 13 extensions. |
| 0x0F | Primary Fat16 partition, using INT 13 extensions. |
When a volume set or a stripe set is created, Disk Administrator sets the high bit of the System ID field for each primary partition or logical drive that is a member of the volume. For example, a FAT primary partition or logical drive that is a member of a volume set or a stripe set has a System ID value of 0x86. An NTFS primary partition or logical drive has a System ID value of 0x87.
This bit indicates that Windows NT needs to use the HKEY_LOCAL_MACHINE\SYSTEM\DISK Registry subkey to determine how the members of the volume set or stripe set relate to each other. Volumes that have the high bit set can only be accessed by Windows NT.
When a primary partition or logical drive that is a member of a volume set or a stripe set has failed due to write errors or cannot be accessed, the second most significant bit is set. The System ID byte is set to C6 in the case of a FAT volume, or C7 in the case of an NTFS volume.
Note: If you start up MS-DOS, it can only access primary partitions or logical drives that have a value of 0x01, 0x04, 0x05, or 0x06 for the System ID. However, you should be able to delete volumes that have the other values.
If you use a MS-DOS-based low-level disk editor, you can read and write any sector, including ones that are in NTFS volumes.
On Windows NT Server, mirror sets and stripe sets with parity also require the use of the Registry subkey HKEY_LOCAL_MACHINE\SYSTEM\DISK to determine how to access the disks.
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