Option ROM
An Option ROM typically consists of firmware that is called by the system BIOS. For example, an adapter card that controls a boot device might contain firmware that is used to connect the device to the system once the Option ROM is loaded.
Standard IBM PC BIOS
This section refers to how option ROMs are used with standard IBM PC compatible BIOSes (most desktop computers).
A common example of an option ROM is the Video BIOS found on all standard PC video cards. This is a special case of an option ROM, as it is loaded very early on in the boot process (otherwise nothing would appear on the screen during the POST.) It is usually easy to tell when this particular ROM is initialized, as it is the first time something appears on the monitor since the computer was switched on. Unless working with an embedded system, the video BIOS is almost always located in the C0000 memory segment (i.e. the very start of the memory area reserved for option ROMs.) Other ROMs can be located from segments C8000 all the way up to E0000. The BIOS Boot Specification (BBS) requires that option ROMs be aligned to 2kB boundaries (e.g. segments C8000, C8800, C9000, C9800, etc.).
After the basic POST checks are complete, the option ROMs are normally initialized. This gives them the chance to hook (intercept) system interrupts, in order to provide increased functionality to the system.
SCSI
For example, a SCSI controller card may hook INT 13 which is responsible for providing disk services. Once it has done this, any subsequent calls to INT 13H will be "caught" by the SCSI option ROM (or "SCSI BIOS"), allowing it to insert details about any disks that may exist on the SCSI bus. Before it had hooked the interrupt there may have been no disks on the system, but by intercepting the interrupt and altering the values returned, the SCSI BIOS can make all the disks on the SCSI bus visible to the system.
In this particular case, the BIOS itself may call INT 13 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.
Network boot ROM
Another common option ROM is a network boot ROM (e.g., a PXE option ROM). This allows a computer without any disks or persistent storage to run an operating system by downloading the necessary software over the network. Of course there needs to be some program to do this download of code, and this is what is stored in the option ROM. This option ROM works differently to the SCSI example above, as it does not provide any new devices to the system. Older ROMs hooked either INT 18H or INT 19H as these two interrupts are called when the boot process is about to begin - INT 19H is called to initiate the boot process, while INT 18H is called when the system has tried to boot from all possible devices and none were bootable. By hooking INT 18H, the system would try to boot from the network when all other boot devices (floppy drives, hard drives, etc.) had failed. By hooking INT 19H, the system would attempt to boot from the network before any other devices.
BIOS Boot Specification
Since these two methods don't offer a large amount of control over the boot process, the BIOS Boot Specification (BBS) was developed. This provided a more standardized approach where an option ROM could be initialized at the start of the boot process, it could inform the BIOS about its functionality, and then it could be called later on in the boot process depending on the order the user had selected.