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An extended boot record (EBR),[1] or extended partition boot record (EPBR)[note 1], is a descriptor for a logical partition under the common DOS disk drive partitioning system.[2] 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 partitions. 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.[note 2] If another logical partition follows, then the first EBR will contain an entry pointing to the next EBR; thus, multiple EBRs form a linked list[note 3]. 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 in the given extended partition.[note 4]
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.[1] This 2-byte signature appears in a disk editor as 55h first and AAh last, because IBM-compatible PCs store hexadecimal words in little-endian order (see table below).
Offsets within EBR sectors | Contents | Size | |
---|---|---|---|
Hex | Dec | bytes | |
000 – 1BD |
000 – 445 | Generally unused; normally filled with zeroes; may contain another boot loader i.e. a partition boot record |
446 |
1BE – 1CD |
446 – 461 | Partition table's first entry | 16 |
1CE – 1DD |
462 – 477 | Partition table's second entry | 16 |
1DE – 1ED |
478 – 493 | Unused[3] third entry filled with zeroes | 16 |
1EE – 1FD |
494 – 509 | Unused[3] fourth entry filled with zeroes | 16 |
1FE - 1FF |
510 - 511 | Signature 55AAh in big-endian network order, same as little-endian 0xAA55 |
2 |
EBR, total size: 446 +(4×16) +2 = | 512 |
The IBM Boot Manager (included with OS/2 operating systems and some early versions of Partition Magic), adds at least one 9-byte entry (starting at offset 0x18A) to each EBR sector. The entry consists of a flag value byte (indicating if the partition is on the IBM Boot Manager menu) followed by an 8-byte ASCII string which is the name to be used on the menu. If the partition is not included on the boot menu (such as data only partitions), the flag byte is zero; in which case, the following 8-byte field may contain an ASCII representation of that partition's starting sector number (in hexadecimal).
Offsets | within entry | Bytes | Description |
---|---|---|---|
Hex | Dec | ||
1?E ¹ |
0 | 1 | Boot indicator (80h for active; otherwise, 00h) |
1?F - 1?1 |
1 – 3 | 3 | CHS cylinder-head-sector address: partition start |
1?2 |
4 | 1 | Partition type code |
1?3 - 1?5 |
5 – 7 | 3 | CHS cylinder-head-sector address: partition end |
1?6 - 1?9 |
8 – 11 | 4 | LBA logical block address: partition start |
1?A - 1?D |
12 – 15 | 4 | Partition size (in sectors) |
¹: For 1?E read 1BE or 1CE to get the hex. offset of the first or second entry, respectively |
The partition type of an extended partition is 0x05 (CHS addressing) or 0x0F (LBA addressing).[4] Linux supports the concept of a second extended partition chain with type 0x85 — this type is hidden (unknown) for other operating systems supporting only one chain.[5]
The CHS addresses of a partition are hard to interpret without knowledge of the (virtual) disk geometry, because CHS to LBA translations are based on the number of heads and the number of sectors per track. However, the given LBA start address and the given partition size in sectors permit to calculate a disk geometry matching the given CHS addresses where that is at all possible. CHS addressing with 24 bits always uses 6 bits for up to 63 sectors per track (1…63), and INT 13h disk access generally uses 8 bits for up to 256 heads (0…255), leaving 10 bits for up to 1024 cylinders (0…1023). ATA CHS addresses always use 4 bits for up to 16 heads (0…15), this leaves 14 bits for up to 16383 cylinders (16383+1=2**14) in ATA-5 24 bits CHS address translations.[6]
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 MS-DOS and Windows, and Linux) ignore the "partition size" value in entries which point to another EBR sector. One exception is 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:
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.
Diagram 1. What the Starting and |
Diagram 2. What the Starting and Total |
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[note 2] 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).
Linux and similar operating systems designate IDE hard disks as /dev/hda
for the first hard disk, /dev/hdb
for the second hard disk, and so on. Likewise SCSI hard disks are identified as /dev/sda
for the first SCSI disk, etc.
The up to four partitions defined in the master boot record are designated as /dev/hda1
… /dev/hda4
for /dev/hda
. The fifth partition in this scheme, e.g., /dev/hda5
, corresponds to the first logical drive. The sixth partition /dev/hda6
would then correspond to the second logical drive, or in other words, the extended partition containers are not counted. Only the outermost extended partition defined in the MBR (one of /dev/hda1
… /dev/hda4
) has a name in this scheme.[7]
This 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 following output of a command line tool shows the layout of a disk with two logical drives. Details for the FAT and NTFS partitions stripped, the line annotated with Linux is /dev/hda6
with an extended file system. The begin of /dev/hda5
shows that the involved operating systems PC DOS 7, Windows NT, and Debian do not insist on any extended partition alignment with a gap:
\\.\PHYSICALDRIVE0 (assuming geometry CHS 99999 255 63) id. [3189-3188] MBR CHS 0 0 1 at 0, end 0 0 1, size 1 unused CHS 0 0 2 at 1, end 0 0 63, size 62 1:*06: CHS 0 1 1 at 63, end 260 254 63, size 4192902 bigFAT 2: 05: CHS 261 0 1 at 4192965, end 757 254 63, size 7984305 => EXT 3: 17: CHS 758 0 1 at 12177270, end 1522 254 63, size 12289725 NTFS 4: 1C: CHS 1523 0 1 at 24466995, end 1825 254 63, size 4867695 FAT32 (extended offset 4192965) total 29334690 => EXT CHS 261 0 1 at 0, end 261 0 1, size 1 5: 06: CHS 261 0 2 at 1, end 384 254 63, size 1992059 bigFAT 6: 05: CHS 385 0 1 at 1992060, end 757 254 63, size 5992245 => EXT (extended offset 6185025) total 7984305 => EXT CHS 385 0 1 at 0, end 385 0 1, size 1 unused CHS 385 0 2 at 1, end 385 0 63, size 62 6: 83: CHS 385 1 1 at 63, end 757 254 63, size 5992182 Linux 7: 00: CHS 0 0 0 at 0, end 0 0 0, size 0 unused total 5992245 bigFAT CHS 0 1 1 at 63, end 260 254 63, size 4192902 PC DOS 7 (cluster size 64, number 65506) total 4192902 NTFS CHS 758 0 1 at 12177270, end 1522 254 63, size 12289725 [1C81-013D] (cluster size 8, number 1536215) total 12289725 FAT32 CHS 1523 0 1 at 24466995, end 1825 254 63, size 4867695 [C417-9E22] (cluster size 8, number 607271) total 4867695 bigFAT CHS 261 0 2 at 4192966, end 384 254 63, size 1992059 FAT SWAP (cluster size 32, number 62236) total 1992059
For another example see the "Linux Partition HOWTO".[8]