Extended Partition Tables

This is a discussion of extended partition tables related to my web pages discussing the boot process with emphasis on the MBR and boot code for Windows 95 OSR2.

The extended partition tables below are from a PC originally formatted under WFWG 3.11 and then upgraded to Windows 95a in a separate primary partition. Thus one primary partition contains WFWG 3.11 and the other contains Windows 95a even though the MBR table below shows one of the primary partitions as being Non-DOS.

Example of Extended Partition Tables

Example of a hard drive partitioned with DOS FDISK that has two primary partitions, each with its own OS, and one extended partition with several logical volumes (partitions). Actually the second Primary partition was obtained using PartitionMagic as DOS FDISK only allows one primary and one extended. The partition tables below are displayed in the same format as used by Norton's Disk Editor in Partition Table view. Note that (MBR only) the column labeled "Relative Sectors" is equivalent to the "Starting Sector" of the partition in LBA format.

Partition Table in MBR
Physical Sector: Cyl 0, Head 0, Sector 1
System	Boot	Starting Location			Ending Location			Relative Sectors	Number of Sectors
System	Boot	Cylinder	Head	Sector	Cylinder	Head	Sector	Relative Sectors	Number of Sectors
BIGDOS	Yes	0	1	1	129	254	63	63	2,088,387
Non-DOS	No	130	0	1	254	254	63	2,088,450	2,008,125
EXT	No	255	0	1	1022	254	63	4,096,575	12,337,920
Unused	No	0	0	0	0	0	0	0	0

Notes on MBR

The MBR is the first sector on the hard drive (Cylinder 0, Side 0, Sector 1) and the remainder of the sectors on this track are unused.
The boot sector of the first primary partition is on Cylinder 0, Side 1, Sector 1 which is the beginning of the second side on Cylinder 0. Since this hard drive has 63 sectors per track and the boot sector is at the beginning of the second side, there are 63 sectors relative to the beginning of the MBR.
The Non-DOS partition (according to FDISK) is actually a Primary DOS partition but FDISK only recognizes one Primary DOS partition and one DOS Extended partition per hard drive.
The boot sector of the second primary partition is on Cylinder 130, Side 0, Sector 1 and has a value of 2,088,450 Relative Sectors, which is the number of sectors from the MBR to the boot sector of this partition.
The starting location of the Extended partition however is not a boot sector but an Extended Boot Record (EBR). This sector only contains a partition table and the signature 0xAA55. The value of the Relative Sectors is the number of sectors from the MBR to this EBR.

First partition table in Extended Partition
Physical Sector: Cyl 255, Head 0, Sector 1
System	Boot	Starting Location			Ending Location			Relative Sectors	Number of Sectors
System	Boot	Cylinder	Head	Sector	Cylinder	Head	Sector	Relative Sectors	Number of Sectors
BIGDOS	No	255	1	1	382	254	63	63	2,056,257
EXT	No	383	0	1	510	254	63	2,056,320	2,056,320

Notes on EBR

The last two entries in the partition table are all 0's.
The first entry gives the starting location of the boot sector of the first logical partition which is non-bootable.
As with the MBR, the EBR resides on the first sector of the cylinder and the remainder of the sectors on this track are unused.
Since the hard drive has 63 sectors per track and the boot sector for the first logical drive is at the beginning of the second side, there are 63 sectors relative to the beginning of the EBR.
The second entry gives the location of the EBR for the next logical partition. Note that (EBR only) the column labeled "Relative Sectors" actually refers to an offset from the "Relative Sectors" value in the EXT entry in the MBR. When these two are added together, this is equivalent to the "Starting Sector" of the partition in LBA format.
The EBR for each logical partition follows the same format. The last partition table has only a single entry for the starting location of the boot sector in the partition.

Second partition table in Extended Partition
Physical Sector: Cyl 383, Head 0, Sector 1
System	Boot	Starting Location			Ending Location			Relative Sectors	Number of Sectors
System	Boot	Cylinder	Head	Sector	Cylinder	Head	Sector	Relative Sectors	Number of Sectors
BIGDOS	No	383	1	1	510	254	63	63	2,056,257
EXT	No	511	0	1	638	254	63	4,112,640	2,056,320

Last partition table in Extended Partition
Physical Sector: Cyl 895, Head 0, Sector 1
System	Boot	Starting Location			Ending Location			Relative Sectors	Number of Sectors
System	Boot	Cylinder	Head	Sector	Cylinder	Head	Sector	Relative Sectors	Number of Sectors
BIGDOS	No	895	1	1	1022	254	63	63	2,056,257

Boot sectors in logical drives contain code that is similar to the boot code in the main boot sector, but since extended partition volumes cannot be booted, this code generally displays a message stating that an extended volume cannot be booted.This code is not discussed further. Each boot sector also contains its own BPB for the current logical volume.

Additional sources of information:

See the following documents by Hale Landis at Hale's HIW: How It Works Documents; How It Works: Partition Tables and the text files PARTTAB1.TXT and PARTTAB2.TXT in ALLHIW.ZIP.
Partitioning Primer by Mikhail Ranish is an interesting web page on hard disks and partitioning with nice graphics.
A nice glossary of computer terms by Erik van Straten can be found at Harddrive Related Terms and Tricks. His Harddrive Related Tools and Information contains some hard drive programs and drivers and includes some links for additional hard drive info.

If you have comments or suggestions, email me at Ray Knights

MBR/Boot Index

Home Page

This page was created on 18 December 2000, last updated on 13 January 2001.