Concept
An extended boot record (EBR), or extended partition boot record (EPBR), is a descriptor for a logical partition under the common DOS disk drive partitioning system. 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. 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. 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. While in Windows versions up to XP logical partitions within the extended partition were aligned following conventions called "drive geometry" or "CHS", since Windows Vista they are aligned to a 1-MiB boundary. Due to this difference in alignment, the Logical Disk Manager of XP (Disk Management) may delete these extended partitions without warning. 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 at the end of the sector. This 2-byte signature appears in a disk editor as first and last, because IBM-compatible PCs store hexadecimal words in little-endian order (see table below). 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 ) to each EBR sector.