Extended boot record
An Extended Master Boot Record (EMBR) is a partition addressed by the Master Boot Record (MBR) of a Computer Hard Drive which can contain one ore more logical partions. The structure of the extended partition is described by a chain of EMBRs which are located inside the extended partition. The first EMBR is located at the beginning of the extended partition.
NOTE: This article incorrectly uses the term Extended Master Boot Record (EMBR). The correct terms is EBR (Extended Boot Record). See discussion tab.
EMBRs structure and values
An EMBR has the same structure as the MBR but only the first 2 table entries and the signature AA55h are in use.
Structure
|=========================================================| | 0..445 = 000h..1BDh | 446 unused bytes | |=========================================================| | 446..461 = 1BEh..1CDh | first table entry (16 bytes) | |=========================================================| | 462..477 = 1CEh..1DDh | second table entry (16 bytes) | |=========================================================| | 478..509 = 1DEh..1EDh | 32 unused bytes | |=========================================================| | 510..511 = 1EEh..1EFh | signature AA55h (2 bytes) | |=========================================================|
|===================================================================| | Structure of (Extended) MBR table entry (16 bytes) | |===================================================================| | 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 | |===================================================================|
Values
The first entry of the EMBR points to the logical partition belonging to this EMBR.
- Starting sector = distance between the sector of this EMBR and the first sector of the logical partition
- Number of sectors = number of sectors of the logical partition
The second entry of the EMBR contains zero values if it is the last EMBR within the extended partition, otherwise it points to the next EMBR in the EMBR chain.
- Starting sector = relative address of the next EMBR within the extended partition
- or LBA address of the next EMBR - LBA address of the beginning of the extended partition
- Number of sectors = remaining sectors of the extended partition, count starts at next EMBR
- or 1 + LBA address of the end of the extended partition - LBA address of the next EMBR
- Note : Starting sector + Number of sectors equals the size of the extended partition
Remarks:
Normally, there is a distance of few sectors between an EMBR and the beginning of the logical partition belonging to this EMBR
because both usually are located at a sector address which is a multiple of the sectors per track value (mostly 63).
There may also be a distance between a logical partition and the next EMBR or
between the last logical partition and the end of the extended partition
e.g. because a logical partition has been deleted or shortened.
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Examples
Example 1
The first example shows an extended partion with 6,000 sectors and 3 logical partitions.
Remark: Neither a tiny extended partion with only 3 MB nor a hard drive with 20 sectors per track are realistic
but this values had been chosen for making this example more readable.
Example 2
It's the same extended partion as in example 1 after the 2nd logical partition was deleted and the last one was shortened.
