the BIOS – the BIOS knows where it is and shadows it into RAM at a fixed time. Other ROMs can be located from segments C8000h all the way up to F4000h in early PCs. The final search address was limited to segment DFFFFh or EFFFFh 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.). The first two bytes of the ROM must be 55 AA. The third byte indicates the ROM size, and the fourth byte is where the BIOS begins execution of the option ROM to initialize it before the system boots. Often this initialization is done by a 3 byte jump instruction starting with hexadecimal value E9. Original usage of Option ROMs for booting through expansion cards Prior to the development and ubiquitous adoption of the Plug and Play BIOS standard, an add-on device such as a hard disk controller or a network adapter card (NIC) was generally required to include an option ROM in order to be bootable, as the motherboard BIOS did not include any support for the device and so could not incorporate it into the BIOS's boot protocol. Such an option ROM would hook INT 19h, the BIOS boot interrupt, to preempt the BIOS boot loader and substitute their own boot loader. The boot loader on the option ROM would attempt to boot from a disk, network, or other boot program source attached to or installed on the adapter card; if that boot attempt failed, it would pass control to the previous boot loader (to which INT 19h pointed before the option ROM hooked it), allowing the system to boot from another device as a fallback strategy. Some adapters cards, such as certain SCSI adapters (e.g. some made by Adaptec), were available in versions that differed only in the presence or absence of the option ROM to enable booting from attached SCSI devices. As a result of the option ROM scanning protocol, the highest-addressed option ROM is the last one to be initialized and so the last one to hook any interrupts and the first one in those interrupt service routine (ISR) chains; thus the addresses of the option ROMs completely determine the boot priority between adapter cards that are enabled for booting, and the boot devices supported by the motherboard BIOS collectively have lowest priority, i.e. the