There are two basic DOS partition types:
A primary partition, which is described by a partition record in the MBR partition table.
A logical partition, which is described by a partition record in the EBR partition table.
Every partition table has room for four partition records; however, there are a few rules that impose limits on this.
An MBR partition table can hold four primary partition records unless you also have logical partitions. In this case, one partition record is used to describe an extended partition and the start of the EBR chain that in turn describes logical partitions.
Because all logical partitions must reside in the extended partition, you cannot allocate room for a primary partition within the extended partition and you cannot allocate room for a logical partition outside or adjacent to this area.
Lastly, an EBR partition table performs two functions:
It describes a logical partition and therefore uses a partition record for this purpose.
It uses a partition record to locate the next EBR partition table.
EBR partition tables use at most two entries.
When creating a DOS partition, the options you are presented with depend on the kind of disk you are working with. However, both OS/2 disks and Linux disks require that you choose a freespace segment on the disk within which to create the new data segment. The create options are:
The size of the partition you are creating. Any adjustments that are needed for alignment are performed by the DOS plug-in and the resulting size might differ slightly from the value you enter.
Lets you skip sectors and start the new partition within the freespace area by specifying a sector offset.
Lets you enter a partition type or choose from a list of partition types; for example, native Linux.
Lets you choose between creating a primary or logical partition. Due to the rules outlined above, you might or might not have a choice. The DOS plug-in can determine if a primary or logical partition can be created in the freespace area you chose and disable this choice.
Lets you enable the sys_ind flag field in a primary partition and disable it when creating a logical partition. The sys_ind flag field identifies the active primary partition for booting.
Additional OS/2 options are the following:
An OS/2 partition can have a name, like Fred or Part1.
OS/2 partitions belong to volumes, either compatibility or logical, and volumes have names. However, because the DOS plug-in is not a logical volume manager, it cannot actually create OS/2 logical volumes.
You can specify the drive letter for an OS/2 partition, but it is not a required field. Valid drive letters are: C,D...Z.