Appendix A: Generic Option ROM Headers

Generic Option ROM Header expansion

(Offsets are all based from the beginning of the Header)

Offset	Length	Value	Description
0h	DWORD	$??? (ASCII)	Signature	Generic
04h	BYTE	Varies	Structure Revision	Generic
05h	BYTE	Varies	Length (in 16 byte increments)	Generic
06h	WORD	Varies	Offset of next Header (0000 if none)	Generic
08h	BYTE	0FFFFh	Reserved	Generic
09h	BYTE	Varies	Checksum	Generic
10h	Varies	Varies	Specific Header Type Data	Specific

Signature - All Expansion Headers will contain a unique expansion header identifier. Each different Expansion Header will have its own unique signature. Software that wishes to make use of any given Expansion Header simply traverses the linked list of Generic Expansion Headers until the Expansion Header with the desired signature is found, or the end of the list is encountered.

Example: The Plug and Play expansion header's identifier is the ASCII string "$PnP" or hex 24 50 6E 50h.

Structure Revision - This is an ordinal value that indicates the revision number of this structure only and does not imply a level of compliance with the Plug and Play BIOS version.

Length - Length of the entire Expansion Header expressed in sixteen byte blocks. The length count starts at the Signature field.

Offset of Next Header - This location contains a link to then next expansion ROM header in this Option ROM. If there are no other expansion ROM headers then this field will have a value of 0h.

Reserved - Reserved for Expansion

Checksum - Each Expansion Header is checksummed individually. This allows the software that wishes to make use of an expansion header the ability to determine if the expansion header is valid.

The system software can determine if the expansion header is valid by performing a Checksum operation. The method for validating the checksum is to add up Length bytes, including the Checksum field, into an 8-bit value. A resulting sum of zero indicates a valid checksum operation.

Specific Header data - This area is used by the specific device header and is defined uniquely for each Expansion Header.