System ID Field
Starting and Ending Head, Sector, and Cylinder Fields
Relative Sectors and Number of Sectors Fields
Logical Drives and Extended Partitions
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 an extended partition if one is defined.
These are the values for the System ID field:
| Value | Meaning |
|---|---|
| 0x01 | 12-bit FAT primary partition or logical drive. The number of sectors in the volume is fewer than 32680. |
| 0x04 | 16-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," presented later in this chapter, for more information. |
| 0x06 | BIGDOS FAT primary partition or logical drive. |
| 0x07 | NTFS primary partition or logical drive. |
A figure presented earlier in this section has examples of a BIGDOS FAT partition, a NTFS partition, an extended partition, and a 12-bit FAT partition.
If you install Windows NT on a computer that has Windows 95 preinstalled, the FAT partitions might be shown as unknown. If you want to be able to use these partitions when running Windows NT, your only option is to delete the partitions.
OEM versions of Windows 95 support the following four partition types for FAT file systems that Windows NT cannot recognize.
| 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 you create a volume set or a stripe set, 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.
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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