Option ROM

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An Option ROM is a piece of firmware that resides in BIOS or on an expansion card, which gets shadowed into memory and executed to initialize the device and register it with the BIOS. It is essentially a driver that interfaces between BIOS services and hardware.

UEFI Option ROMs

The PCI spec allows multiple option ROM images on the same device. These option ROMs could be Legacy x86 & UEFI. If the Option ROM format is set to "UEFI Compatible" in the UEFI Setup, the DXE stage will load the newer UEFI Option ROM if one is present and the legacy Option ROM if one is not. UEFI can use legacy option ROMs when a Compatibility Support Module (CSM) is enabled. Note that when Secure Boot is enabled, execution of the Compatibility Support Module and legacy Option ROMs is prohibited because legacy firmware drivers do not support authentication, which is a security threat.

See Wikipedia for more