Why PCIScope doesn't read Option ROM of some PCI devices?

Why PCIScope doesn't read Option ROM of some PCI devices?
PCIScope do not display Optional ROM tab if Expansion ROM register of device is not writable ROM data do not contain valid ROM header (use CheckOptROMSig option in CUSTOM.INI to switch on/off this feature) Device is located on motherboard (use MBDevicesROM option in CUSTOM.INI to switch on/off this feature; see not below) EnableOptROM option in the CUSTOM.INI is set to 0 Note 1: ROM of devices integrated to motherboard is stored in the BIOS ROM chip. In this case device chip is not wired to ROM chip, but can support sizing of Expansion ROM register. ROM read transactions need some kind of termination. Depending on implementation, values read from ROM can be 0x00, 0xFF, 0xAD... If termination is not implemented, ROM read access can cause NMI. Note 2: Actually PCIScope determines if device is located on the motherboard or not by parsing through BIOS Interrupt Routing Table. We recently encountered few systems with incomplete or incorrect BIOS Interrupt Routing Table. In such case PCIScope may fail to determine device location properly.

PCIScope do not display Optional ROM tab if

Expansion ROM register of device is not writable
ROM data do not contain valid ROM header (use CheckOptROMSig option in CUSTOM.INI to switch on/off this feature)
Device is located on motherboard (use MBDevicesROM option in CUSTOM.INI to switch on/off this feature; see not below)
EnableOptROM option in the CUSTOM.INI is set to 0

Note 1: ROM of devices integrated to motherboard is stored in the BIOS ROM chip. In this case device chip is not wired to ROM chip, but can support sizing of Expansion ROM register. ROM read transactions need some kind of termination. Depending on implementation, values read from ROM can be 0x00, 0xFF, 0xAD... If termination is not implemented, ROM read access can cause NMI.

Note 2: Actually PCIScope determines if device is located on the motherboard or not by parsing through BIOS Interrupt Routing Table. We recently encountered few systems with incomplete or incorrect BIOS Interrupt Routing Table. In such case PCIScope may fail to determine device location properly.