An option ROM for the PC platform (i.e. the IBM PC and derived successor computer systems) is a piece of firmware that resides in ROM on an expansion card (or stored along with the main system BIOS), which gets executed to initialize the device and (optionally) add support for the device to the BIOS.
For this reason, it is possible (but not usual) for an option ROM to keep control and preempt the BIOS boot process.
In principle, an option ROM could provide any sort of firmware extension, such as a library of video graphics subroutines, or a set of PCM audio processing services, and cause it to be installed into the system RAM and optionally the CPU interrupt system before boot time.
A common option ROM is the video BIOS which gets loaded very early on in the boot process and hooks INT 10h so that output from the power-on self-test (POST) can be displayed.
[1] The final search address was limited to segment DFFFFh[2] or EFFFFh[3] in modern products.
The BIOS Boot Specification requires that option ROMs be aligned to 2 kB boundaries (e.g. segments C8000h, C8800h, C9000h, C9800h, etc.).
And the fourth byte is where the BIOS begins execution of the option ROM to initialize it before the system boots.
[6] After the basic POST checks are complete, the BBS specifies that the BIOS will detect and shadow all option ROMs that reside in the BIOS into the aforementioned region and it will traverse the PCI configuration space, filling in XROMBARs and copying the expansion card option ROMs from MMIO space to the region.
The BCV table is full of BCV function pointers but has a fixed entry representing legacy option ROMs which is a pointer to a BIOS routine which calls +03h in all the remaining option ROMs that don't have a PnP Expansion header.
[6] This allows it to scan the PCI configuration space to find the correct device and BARs it needs to use.
The BCV, however, hooks interrupt routines which interact with the device which are adjusted based on a base MMIO address location, disk information ascertained in the option ROM initialization routine and the current disk number in the BDA.
A SCSI controller card may hook INT 13h which is responsible for providing disk services.
In this particular case, the BIOS itself may call INT 13h to provide a list of possible boot devices to the user, and because the SCSI BIOS has hooked the interrupt the user will be able to choose not only which standard system devices to boot from, but also which SCSI disks as well.
The BBS specifies that the NIC option ROM does not hook INT 19h, but instead the BIOS 19h handler should call the BEV, which will then download the boot code.
This dual compatibility in devices can function in both legacy BIOS and modern UEFI environments.
UEFI systems can utilize legacy Option ROMs through the Compatibility Support Module (CSM).