To create the structure for an extended volume in the extended DOS partition, FDISK determines if there is available space in the extended DOS partition and if less than 24 total devices are allocated in the system. The maximum number of block devices allowed is 26, and two are used by diskettes, A: and B:. The program then creates an extended boot record at the space located, with a partition entry filled in (with the size and location information) for that logical block device. If this is not the first extended boot record, the program backs up to the last extended boot record in the chain (as linked by the 05h entries), and creates a partition entry in that extended boot record that has the size and location data for the newly created record. This action creates the pointer required to locate the newly created boot record.

If this is the first extended boot record in the extended DOS partition only the size, type, and location of the logical block device needs to be put into a partition entry. The start of the extended DOS partition in the Master Boot Record serves as a pointer to this extended volume.