In Legacy BIOS Mode, there are PC architecture constraints that are placed on Legacy Option ROM allocation. These constraints are not placed on UEFI Option ROMs, which are often referred to as UEFI drivers.
Choose Legacy BIOS Mode to allow host bus adapters (HBAs) to use Option ROMs. Choose UEFI Boot Mode to use UEFI drivers.
The system BIOS allocates 128 Kbytes of address space for Legacy Option ROMs. This address space is shared between on-board devices and PCIe add-in cards. This fixed address space limitation is imposed by the PC architecture and not by the BIOS itself. It is possible to exhaust the available address space when installing PCIe add-in cards. When the address space is exhausted, Oracle ILOM displays an Option ROM Space Exhausted message, which means that one or more devices cannot load Option ROMs.
For example, if you install a SAS PCIe card, you might encounter the following message in the Oracle ILOM event log:
Option ROM Space Exhausted - Device XXX Disabled
By default, all on-board Legacy Options ROMs are enabled in the BIOS. However, you can disable most of these Option ROMs, unless they are required to support booting from the associated device or to provide some other boot-time function. For example, it is not necessary to load the Option ROM for the on-board network ports unless you want to boot from one or more network ports (even then, you can disable the Options ROMs for the remaining ports).
To minimize server boot time and reduce the likelihood of exhausting the available Option ROM address space, disable the Option ROMs for all devices that you do not intend to boot from. Enable Option ROMs only for those devices from which you intend to boot. If Option ROMs are enabled for more than one boot device, you might encounter an Option ROM space exhausted condition.If you encounter the Option ROM space exhausted condition even after disabling all devices from which you do not intend to boot, then disable additional Option ROMs. Under some circumstances it might be necessary to disable Option ROMs for all devices except for the primary boot device.