Extended Boot Record

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An Extended Boot Record (EBR) is a descriptor for a logical partition in the common DOS disk drive partitioning system. In that system, one partition in the Master Boot Record (MBR) is designated an "extended partition," and is subdivided into logical partitions. The structure of the extended partition is described by EBRs, which are located inside the extended partition.

Unlike the primary partitions, which are all described by one partition table in the MBR, and thus limited in number, each EBR is located immediately before the logical partition it describes and points to the next one, to form a chain. That means the number of logical partitions is limited only by the amount of space on the disk.

The first EBR is located at the beginning of the extended partition.

1 EBRs structure and values
- 1.1 Structure
- 1.2 Values
2 Examples
- 2.1 Example 1
- 2.2 Example 2
3 See also

[edit] EBRs structure and values

An EBR has the same structure as the MBR but only the first 2 entries of the partition table and the signature AA55h are in use.

[edit] Structure

|===============================================================|
|   0..445 = 000h..1BDh | 446 unused bytes                      |
|===============================================================|
| 446..461 = 1BEh..1CDh | first part. table entry (16 bytes)    |
|===============================================================|
| 462..477 = 1CEh..1DDh | second part. table entry (16 bytes)   |
|===============================================================|
| 478..509 = 1DEh..1FDh | 32 unused bytes                       |
|===============================================================|
| 510..511 = 1FEh..1FFh | signature AA55h (2 bytes)             |
|===============================================================|

|===================================================================|
| Structure of MBR or EBR partition table entry (16                 |
|===================================================================|
| Offset | Byte Count | Description of contents                     |
|===================================================================|
| 0h     |     1      | Boot indicator (00h for off, 80h for on)    |
|===================================================================|
| 1h..3h |     3      | Starting head, cylinder and sector          |
|===================================================================|
| 4h     |     1      | Filesystem descriptor                       |
|===================================================================|
| 5h..7h |     3      | Ending head, cylinder and sector            |
|===================================================================|
| 8h..Bh |     4      | Starting sector                             |
|===================================================================|
| Ch..Fh |     4      | Number of sectors                           |
|===================================================================|

[edit] Values

The first entry of the EBR partition table points to the logical partition belonging to that EBR.

Starting sector = distance between the sector of this EBR and the first sector of the logical partition
Number of sectors = number of sectors of the logical partition

The second partition table entry of the EBR contains zero values if it is the last EBR in the extended partition; otherwise it points to the next EBR in the EBR chain.

Starting sector = relative address of the next EBR in the extended partition

or LBA address of the next EBR - LBA address of the beginning of the extended partition

Number of sectors = remaining sectors of the extended partition, count starts at next EBR

or 1 + LBA address of the end of the extended partition - LBA address of the next EBR

Note : Starting sector + Number of sectors equals the size of the extended partition

Remarks:
Normally, there is a distance of a few sectors between an EBR and the beginning of the logical partition belonging to this EBR because both usually are located at a sector address that is a multiple of the sectors per track value (usually 63).
There may also be a distance between a logical partition and the next EBR or between the last logical partition and the end of the extended partition e.g. because a logical partition has been deleted or shortened.

In the following pictures, the term "EMBR" is a mistake and should be read as "EBR."

values of 1st entry

values of 2nd entry

[edit] Examples

[edit] Example 1

The first example shows an extended partion with 6,000 sectors and 3 logical partitions.
Remark: Neither a tiny extended partition with only 3 MB nor a hard drive with 20 sectors per track are realistic but these values have been chosen to make this example more readable.