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Extensible Firmware Interface Specification
19-2 12/01/02 Version 1.10
19.1.2 OS Independent
Option ROMs shall not require or assume the existence of a particular OS.
19.1.3 EFI Compliant
Option ROM compliance with EFI requires (but is not limited to) the following:
1. Little endian layout
2. Single-threaded model with interrupt polling if needed
3. Where EFI provides required services, EFI is used exclusively. These include:
• Console I/O
• Memory Management
• Timer services
• Global variable access
4. When an Option ROM provides EFI services, the EFI specification is strictly followed:
• Service/protocol installation
• Calling conventions
• Data structure layouts
• Guaranteed return on services
19.1.4 Coexistence of Legacy Option ROMs
The infrastructure shall support coexistent Legacy Option ROM and EBC Option ROM images.
This case would occur, for example, when a Plug and Play Card has both Legacy and EBC Option
ROM images flashed. The details of the mechanism used to select which image to load is beyond
the scope of this document. Basically, a legacy System BIOS would not recognize an EBC Option
ROM and therefore would never load it. Conversely, an EFI Firmware Boot Manager would only
load images that it supports.
The EBC Option ROM format must utilize a legacy format to the extent that a Legacy System
BIOS can:
1. Determine the type of the image, in order to ignore the image. The type must be incompatible
with currently defined types.
2. Determine the size of the image, in order to skip to the next image.
19.1.5 Relocatable Image
An EBC option ROM image shall be eligible for placement in any system memory area large
enough to accommodate it.
Current option ROM technology requires images to be shadowed in system memory address range
0xC0000 to 0xEFFFF on a 2048 byte boundary. This dependency not only limits the number of
Option ROMs, it results in unused memory fragments up to 2 KB.