Specifications

2.5.1.4 Traditional Installation

The traditional BIOS driver is used to abstract the traditional BIOS for EFI. BDS installing

the traditional code causes the Legacy BIOS Protocol initialization code to do the

following actions:

• Find the Legacy Region Protocol.

• Find the Legacy Interrupt Protocol.

• Find the Legacy BIOS Platform Protocol.

• Find the Legacy 8259 Protocol.

• Allocate the first 4 KB for interrupt vectors and BDA from traditional

memory.

• Allocate 0x80000–0x9FFFF for EfiCompatibility usage and for the EBDA.

• Allocate memory for thunk and reverse thunk code.

• Initialize thunk code.

• Initialize the traditional memory map.

• Allocate PMM memory between 1 MB and 16 MB.

• Initialize the BDA and EBDA.

• Locate the firmware volume from which this code was loaded and searches

that firmware volume for the Compatibility16 code.

• Determine the size of the Compatibility16 code and from the size calculates

the starting address of the Compatibility16 code.

• Make the final destination of the Compatibility16 code read-write and then

shadows the Compatibility16 code to the final destination.

• Search for and validate the

EFI_COMPATIBILITY16_TABLE, saves the

Compatibility16 entry point, and updates internal data structures.

• Using the Compatibility16 table function entry point, thunk into the

Compatibility16 bit code and request it to perform the function

Compatibility16InitializeYourself(). The traditional memory

map is passed in as a parameter.

•

EFI_COMPATIBILITY16_TABLE is read to get the Plug and Play

installation check address. The internal data structures are updated.

• The Compatibility16 code is then set to read only.

• Install the Legacy BIOS Protocol.

• The Legacy BIOS Protocol returns back to BDS.

16-Bit Traditional Code

The 16-bit traditional code consists of traditional OpROMs and Compatibility16 code. The

Compatibility16 code roughly consists of traditional BIOS minus POST and BIOS Setup.

It is assembled separately from the EFI code and is linked as a binary module just like

an EFI OpROM would be.

See Compatibility16 in CSM Architecture for details on the new Compatibility16 code that

is used to interface between the EfiCompatibility portion and the Compatibility16 portion

of the traditional BIOS.