Each sector of the Main Extended Boot Sectors has the same structure; however, each sector may hold distinct boot-strapping instructions (see Table 6). Boot-strapping agents, such as the boot-strapping instructions in the Main Boot Sector, alternate BIOS implementations, or an embedded system’s firmware, may load these sectors and execute the instructions they contain.

The Backup Extended Boot Sectors is a backup of the Main Extended Boot Sectors and has the same structure (see Table 6).

Prior to executing the instructions of either the Main or Backup Extended Boot Sectors, implementations should verify their contents by ensuring each sector’s ExtendedBootSignature field contains its prescribed value.

While the initial format operation will initialize the contents of both the Main and Backup Extended Boot Sectors, implementations may update these sectors (and shall also update their respective Boot Checksum) as needed.

Table 6 Extended Boot Sector Structure

Field NameOffset(byte)Size(bytes)Comments
ExtendedBootCode02BytesPerSectorShift – 4This field is mandatory and Section 3.2.1 defines its contents.Note: the Main and Backup Boot Sectors both contain the BytesPerSectorShift field.
ExtendedBootSignature2BytesPerSectorShift – 44This field is mandatory and Section 3.2.2 defines its contents.Note: the Main and Backup Boot Sectors both contain the BytesPerSectorShift field.

3.2.1 ExtendedBootCode Field

The ExtendedBootCode field shall contain boot-strapping instructions. Implementations may populate this field with the CPU instructions necessary for boot-strapping a computer system. Implementations which don’t provide boot-strapping instructions shall initialize each byte in this field to 00h as part of their format operation.

3.2.2 ExtendedBootSignature Field

The ExtendedBootSignature field shall describe whether the intent of given sector is for it to be an Extended Boot Sector or not.

The valid value for this field is AA550000h. Any other value in this field invalidates its respective Main or Backup Extended Boot Sector. Implementations should verify the contents of this field prior to depending on any other field in its respective Extended Boot Sector.