Specifications

Design Discussion

2.5.2.1 Legacy BIOS Interface and Functions

There is a table located within the traditional BIOS in either the 0xF000:xxxx or

0xE000:xxxx physical address range. The table is located on a 16-byte boundary and

provides the physical address of the entry point for the Compatibility16 Functions

The Compatibility16 functions provide the platform-specific information that is required

by the generic EfiCompatibility code. The functions are invoked via thunking by using

EFI_LEGACY_BIOS_PROTOCOL.FarCall86() with the 32-bit physical entry point

EFI_COMPATIBILITY16_TABLE.

See Compatibility16 Code in Code Definitions for definitions of the Compatibility16

functions and the 32-bit physical entry point.

2.5.2.2 EfiCompatibility to 16-Bit Legacy Transitions

There the following two cases of transitions:

• Thunk

• Reverse thunk

A thunk is the transition from EfiCompatibility to 16-bit traditional code and back. A

reverse thunk is the transition from 16-bit traditional code to EfiCompatibility and back.

See Thunk in CSM Architecture

for additional details.

2.5.2.3 EfiCompatibility Drivers

Three drivers are needed to emulate various traditional software INTs, as follows:

• UGA emulation of INT10

• Keyboard emulation of INT16

• Block I/O emulation

The following sections provide more information on these INTs.

2.5.2.4 UGA Emulation of INT10

This situation occurs when traditional OpROMs are to be invoked. The UGA controller is

to be placed in VGA emulation mode and a VGA OpROM invoked. This driver must

translate EFI console-out data and requests into their VGA equivalent. The following

assumptions are used in this document:

• All INT10 functions, both character and dot must be supported.

• The OpROMs may access direct both VGA registers and video memory

buffers.

• UGA hardware supports a VGA mode and can be switched between UGA/VGA

modes multiple times.

2.5.2.5 Keyboard Emulation of INT16

The Compatibility16 BIOS does not take over USB emulation until the final states of

traditional boot. Until that time, INT16 requests must be converted into EFI requests,

data received and converted back into INT16 format.