Technical information

© Jean Louis-Guérin – V1.2a – September 2014 Page 53 / 69
10.4.1 Example of layout with DOS/FAT standard partitions
In the following example we have a hard disk with 3 primary partitions. The Master Boot Record
contains 3 pointers to the 3 partitions. These partitions can be either regular or big partitions.
10.5 DOS/FAT Extended Partition
An extended partition is a special partition which contains secondary partition(s). A hard disk may
contain only one extended partition; which can then be sub-divided into many logical drives.
10.5.1 Extended Master Boot Record (EMBR)
The first sector of the Extended Partition contains an Extended Master Boot Record (EMBR). It is
very similar to the Master Boot Record.
The Extended Master Boot Record contains the following information:
Offset
Length
Description
$0000
455
Normally unused and filled with 0.
$01BE
16
Partition Table's First entry
$01CE
16
Partition Table's Second entry
$01DE
32
Unused, should be filled with zero-bytes
$01FE
2
MBR Signature $AA55 in little-endian format
Where a Partition Table entry contains:
Offset
Length
Description
Locations
0x00
1
Status (0x80 = bootable (active), 0x00 = non-bootable)
$1BE, $1CE, $1DE, $1EE
0x01
3
CHS address of first sector in partition (next 3 bytes):
$1BF, $1CF, $1DF, $1EF
0x01
1
 Head
0x02
1
 Sector is in bits 5–0; bits 9–8 of Cylinder are in bits 7–6
0x03
1
 bits 7–0 of Cylinder
0x04
1
partition type
$1C2, $1D2, $1E2, $1F2
0x05
3
CHS address of last sector in partition (next 3 bytes):
$1C3, $1D3, $1E3, $1F3
0x05
1
 Head
0x06
1
 Sector is in bits 5–0; bits 9–8 of Cylinder are in bits 7–6
0x07
1
 bits 7–0 of Cylinder
0x08
4
LBA of first sector in the partition, in little-endian format
$1C6, $1D6, $1E6, $1F6
0x0C
4
number of sectors in partition, in little-endian format
$1CA, $1DA, $1EA, $1FA