data structure (not the same as the configuration space), the option ROM code knows the device and vendor ID is at a fixed offset from RIP. The beginning of this structure is marked by the 4 byte ASCII signature PCIR and a pointer to this is stored at offset +18h as a 2 byte little endian value. This allows it to scan the PCI configuration space to find the correct device and BARs it needs to use. To prevent this scan, and in case of two identical cards in the system, the BIOS passes the PFA (bus/device/function) to the initialization routine in AX, and the card select number (CSN) for ISA option ROMs is passed in BX. It can then interact with the device using PMIO / MMIO to see how many disks it has and which ones are bootable by reading the MBR. The BIOS will have already combed the configuration space, allocated the BARs and filled in the ACPI table prior to the initialization routine call, so the option ROM would use the addresses allocated to its BARs. 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. The BIOS INT 19h procedure then uses the IPL table priority in NVRAM to decide whether to call an entry containing a boot handler which will read the MBR of 00h (floppy disk BAID; the first device in the BCV table to register disk 00h), an entry containing a boot handler which will read the MBR of 80h (the hard drive BAID; the first device in the BCV Table to register disk 80h) or one of the BEV entries in the table. A device only has a BEV or a BCV if it is a bootable device. SCSI A SCSI controller card may hook INT 13h which is responsible for providing disk services. It will do so in its BCV if it is a PnP card. 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 respond for 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