Specifications

2.5 Interactions between CSM and Legacy BIOS

BDS and Legacy Drivers

BDS must invoke the CSM by dispatching the Legacy BIOS Protocol if either of the

following is true:

• A traditional OpROM is required.

• A traditional boot option is found in the boot sequence

2.5.1.1 Traditional OpROMs

There are two cases where traditional OpROMs are required in an EFI environment, as

follows:

No EFI driver exists

There are cases where a required device has no EFI driver but only a

traditional OpROM. The normal binding of a device and driver fails and an

attempt is made to do the binding via the EfiCompatibility code. This binding

is done in the Legacy BIOS Protocol. The thunk driver is bound by the BDS

(due to its priority) and the thunk driver calls back into the Legacy BIOS

Protocol to load the ROM.

Booting a traditional OS

All devices requiring an OpROM in a traditional BIOS boot will require

traditional OpROMs when booting a traditional OS in an EFI

environment. This requirement means that there may be an EFI driver and a

traditional OpROM for the same device. This transition from EFI to traditional

BIOS code is done in

EFI_LEGACY_BIOS_PROTOCOL.LegacyBoot().

This code disconnects all EFI drivers for traditional devices.

2.5.1.2 Determining if Traditional OS Is Present on a Boot Device

The EFI Device Path distinguishes between booting to the following:

• An EFI-aware OS (regardless if the device is EFI or traditional)

• A traditional OS

EFI_LEGACY_BIOS_PROTOCOL.LegacyBoot() is used in the latter case. It is

recommended that potential removable media boot devices are checked to see if any

media are present prior to setting boot devices. This check speeds up the boot time and

may prevent a possible system reset. A failed traditional boot will cause a system reset

any time control is passed to an OS boot loader and the loader returns back to the BIOS.

This reset occurs because EFI might have been corrupted.

2.5.1.3 Determining the Boot Sequence When Traditional OSs Are

Involved

The EFI device path determines which of the following is used:

• The normal EFI boot sequence

• The E

FI_LEGACY_BIOS_PROTOCOL.LegacyBoot() sequence