An Extended Boot Record (EBR), or Extended Partition Boot Record (EPBR), is a descriptor for a logical partition under the common DOS disk drive partitioning system. In that system, when one (and only one) partition record entry in the Master Boot Record (MBR) is designated an "extended partition," then that partition can be subdivided into a number of logical drives. The actual structure of that extended partition is described by one or more EBRs, which are located inside the extended partition. The first (and sometimes only) EBR will always be located on the very first sector of the extended partition.
Unlike primary partitions, which are all described by a single partition table within the MBR, and thus limited in number,each EBR precedes the logical partition it describes. If another logical partition follows, then the first EBR will contain an entry pointing to the next EBR; thus, multiple EBRs form a sort of chain from the first to the next, and finally to the last one. This means the number of logical drives that can be formed within an extended partition is limited only by the amount of available disk space.
EBRs have essentially the same structure as the MBR; except only the first two entries of the partition table are supposed to be used, besides having the mandatory boot record signature (or magic number) of 0xAA55 at the end of the sector. As always, this 2-byte signature appears in a disk editor as 55h first and AAh last, because IBM-compatible PCs store hexadecimal Words in reverse order (see table below).
| Offsets ( within EBR sectors ) | Contents | Size in bytes | ||
|---|---|---|---|---|
| Hex | Dec | |||
000 - 1BD | 000 - 445 | Generally unused; normally filled with zero-bytes | 446 | |
18A - 192 | 394 - 402 | Possible IBM Boot Manager menu entry | 9 | |
1BE - 1CD | 446 - 461 | Partition Table's First entry | 16 | |
1CE - 1DD | 462 - 477 | Partition Table's Second entry | 16 | |
1DE - 1FD | 478 - 509 | Unused, but should be filled with zero-bytes | 32 | |
1FE | 510 | 55h | Boot Record signature: 0xAA55 (AA55h) | 2 |
1FF | 511 | AAh | ||
| 512 | ||||
| Offsets ( within each entry ) | Byte Count | Description |
|---|---|---|
| 0 | 1 | Boot indicator (80h for active; otherwise, 00h) |
| 1 - 3 | 3 | Starting CHS (Cylinder, Head, Sector) values |
| 4 | 1 | Partition-type descriptor |
| 5 - 7 | 3 | Ending CHS (Cylinder, Head, Sector) values |
| 8 - 11 | 4 | Starting Sector |
| 12 - 15 | 4 | Partition Size (in Sectors) |
The following are general rules that apply only to values found in the 4-byte fields of an EBR's partition table entries (cf. Tables above). These values depend upon the partitioning tool(s) used to create or alter them, and in fact, most operating systems that use the Extended partitioning scheme (including Microsoft DOS and Windows, and Linux) ignore the "Number of sectors" value in entries which point to another EBR sector; with only one exception: that value must be one or greater for Linux operating systems.
The first entry of an EBR partition table points to the logical partition belonging to that EBR:
Note: This will be the same value for each EBR on the same hard disk; usually 63.
Note: The unused sectors in the same track as the EBR, are not considered part of the logical partition for this count value.
The second entry of an EBR partition table will contain zero-bytes if it's the last EBR in the extended partition; otherwise, it points to the next EBR in the EBR chain:
or: Starting Sector = LBA address of next EBR minus LBA address of extended partition's first EBR
Note: Unlike the first entry in an EBR's partition table, this Number of Sectors count includes the next logical partition's EBR sector along with the other sectors in its otherwise unused track. (Compare Diagram 1 and 2 below.)
|
|
|
Remarks:
First, the diagrams above are not to scale: The thin white lines between each "EBR" and its logical "partition" represent the remainder of an unused area usually 63 sectors in length; including the single EBR sector (shown at a greatly exaggerated size).
Also, on some systems, a large gap of unused space may exist between the end of a logical partition and the next EBR, or between the last logical partition and the end of the whole extended partition itself, if any previously created logical partition has been deleted or resized (shrunk).
The first example shows an extended partition 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.
The same extended partition as in example 1 after the 2nd logical partition was deleted and the last one was shortened.