Specifications

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• Compatibility16 does not need to configure motherboard devices. The EFI
PEI and DXE phases configure the devices during POST and the ACPI ASL
configures devices at runtime. Plug and Play device nodes are not supported.
This assumption implies that the ACPI device configuration selections include
entries with and without IRQs.
• Because the Compatibility16 contains traditional devices that use interrupts,
it requires an interrupt vector table and interrupts located at the traditional
locations. The EFI Interrupt Descriptor Table (IDT) resides at a different
address.
• EFI selects the boot ordering, not EfiCompatibility or Compatibility16.
EfiCompatibility can change the boot priority.
• Compatibility16 owns resources below 1 MB in memory. An area needs to be
reserved for EFI double buffering usage and 0:7C00 needs to be reserved for
the boot loader.
• The CSM is invoked in BDS and dispatches the Compatibility16 code.
• If a traditional OS is booted, then all OpROMs must be traditional.
• If an EFI OS is booted, then OpROMs can be either EFI (preferred) or
traditional.
2.7 Valid EFI and Legacy Combinations
The table below lists the valid EFI and legacy code combinations:
Table 9 Valid EFI and Legacy Combinations
Video
OpROM
Other
OpROM
Boot Device
OpROM
OS Valid?
EFI EFI EFI EFI Compatibility mode
not required
EFI EFI EFI Traditional No
EFI Note1 EFI Traditional EFI Yes
EFI EFI Traditional Traditional No
EFI Note 1 Traditional EFI EFI Yes
EFI Traditional EFI Traditional No
EFI Note1 Traditional Traditional EFI Yes
EFI Note1 Traditional Traditional Traditional Yes
Traditional EFI EFI EFI Yes
Traditional EFI EFI Traditional No
Traditional EFI Traditional EFI Yes
Traditional EFI Traditional Traditional No
Traditional Traditional EFI EFI Yes
Traditional Traditional EFI Traditional No
Traditional Traditional Traditional EFI Yes
Traditional Traditional Traditional Traditional Yes
Note 1: The EFI UGA video driver must be unloaded and re-invoked in VGA mode with a VGA OpROM.