Specifications

Design Discussion

Table 2 EfiCompatibility Protocols

Protocol Description

Legacy BIOS Protocoll The primary protocol of the CSM. This protocol is platform and

hardware neutral.

Legacy BIOS Platform

Protocol

Provides the information that makes this platform unique compared

to another platform using the same chipset. This protocol is platform

specific.

Legacy Region Protocol Manages the hardware that allows the region from physical address

0xC0000 to 0xFFFFF to be made read only or read-write. It can

optionally manage the hardware that prevents a write in the above

region propagating to any aliased memory regions This protocol is

chipset specific.

Legacy 8259 Protocol Manages the hardware controlling the 8259 PIC in 32-bit protected

mode and in 16-bit real mode. It keeps track of the interrupt masks

and edge/level programming for both modes. This protocol is

platform and chipset neutral.

Legacy Interrupt Protocol Manages the hardware for assigning IRQs to PCI. EFI is polled rather

than interrupt driven. This protocol is chipset specific.

Each of the above protocols is described in more detail in the following sections. See

EfiCompatibility Code in Code Definitions for the definitions of these protocols.

2.4.2.1 Legacy BIOS Protocol

LegacyBios Module

The LegacyBios module constitutes the CSM skeleton. The other CSM modules are

support modules. The LegacyBios initialization code consists of the following elements:

• Code to initialize itself and to load the Compatibility16 code

• Code to determine the boot device

• Code to load other EfiCompatibility drivers

• Code to load the thunk and reverse thunk code

• Code to interface with Compatibility16 functions

• Code to load and invoke traditional OpROMs, including code to find

baseboard traditional OpROMs and to load them

• Code to load a traditional OS

The BDS code, as part of its normal functions, binds a device with a traditional OpROM,

but it uses the LegacyBios code to dispatch and initialize the traditional OpROM. The BDS

code also generates the various boot device paths, but it uses the LegacyBios code to

boot to a traditional OS. The LegacyBios code that is used to boot a traditional OS

performs the following actions:

• Extracts EFI data into standard EfiCompatibility structures

• Updates standard BDS, EBDA, and CMOS locations

• Updates hardware with traditional resources (IRQs)

The LegacyBios code parses the data hub or invokes EFI APIs to gather data that is

required by Compatibility16 and translates it into a series of standard Compatibility16

data structures. That data is used to update standard traditional data values in the BDA,