Editing BIOS (section)

== Extensions (option ROMs) ==
{{See also|Option ROM}}
Peripheral cards such as hard disk drive [[host bus adapter]]s and [[video card]]s have their own firmware, and BIOS extension [[option ROM]] code may be a part of the expansion card firmware; that code provides additional capabilities in the BIOS. Code in option ROMs runs before the BIOS boots the operating system from [[mass storage]]. These ROMs typically test and initialize hardware, add new BIOS services, or replace existing BIOS services with their own services. For example, a [[SCSI controller]] usually has a BIOS extension ROM that adds support for hard drives connected through that controller. An extension ROM could in principle contain operating system, or it could implement an entirely different boot process such as [[network booting]]. Operation of an IBM-compatible computer system can be completely changed by removing or inserting an adapter card (or a ROM chip) that contains a BIOS extension ROM.

The motherboard BIOS typically contains code for initializing and bootstrapping integrated display and integrated storage. The initialization process can involve the execution of code related to the device being initialized, for locating the device, verifying the type of device, then establishing base registers, setting [[Pointer (computer programming)|pointers]], establishing interrupt vector tables,<ref>{{cite book |title=System BIOS for IBM PCs, Compatibles, and EISA Computers&nbsp;— The Complete Guide to ROM-Based System Software |author=Phoenix Technologies, Ltd. |author-link=Phoenix Technologies |series=Phoenix Technical Reference Series |publisher=[[Addison Wesley Publishing Company, Inc.]] |edition=2nd |date=June 1991 |location=Amsterdam |isbn=0-201-57760-7 |url-access=registration |url=https://archive.org/details/isbn_9780201577600}}</ref> selecting paging modes which are ways for organizing available [[Processor register|registers]] in devices, setting default values for accessing software routines related to [[Interrupt#Hardware interrupts|interrupts]],<ref>{{cite book |title=System BIOS for IBM PC/XT/AT Computers and Compatibles&nbsp;— The Complete Guide to ROM-Based System Software |author=Phoenix Technologies, Ltd. |url=https://archive.org/details/System_BIOS_for_IBM_PC_XT_AT_Computers_and_Compatibles |author-link=Phoenix Technologies |series=Phoenix Technical Reference Series |publisher=[[Addison Wesley Publishing Company, Inc.]] |edition=1st |orig-date=1987 |date=1989 |isbn=0-201-51806-6}}</ref> and setting the device's configuration using default values.<ref>{{cite book | url=https://books.google.com/books?id=gZ5SDTpMR-MC&dq=hardware+initialization&pg=PA282 | title=The PC Graphics Handbook | isbn=978-0-203-01053-2 | last1=Sanchez | first1=Julio | last2=Canton | first2=Maria P. | date=26 February 2003 | publisher=CRC Press }}</ref> In addition, plug-in adapter cards such as [[SCSI]], [[RAID]], [[network interface card]]s, and [[video card]]s often include their own BIOS (e.g. [[Video BIOS]]), complementing or replacing the system BIOS code for the given component. Even devices built into the motherboard can behave in this way; their option ROMs can be a part of the motherboard BIOS.

An add-in card requires an option ROM if the card is not supported by the motherboard BIOS and the card needs to be initialized or made accessible through BIOS services before the operating system can be loaded (usually this means it is required in the boot process). An additional advantage of ROM on some early PC systems (notably including the IBM PCjr) was that ROM was faster than main system RAM. (On modern systems, the case is very much the reverse of this, and BIOS ROM code is usually copied ("shadowed") into RAM so it will run faster.)

=== Physical placement ===
[[File:Dell 310 BIOS chips.jpg|right|thumb|BIOS chips in a Dell 310 that were updated by replacing the chips]]
Option ROMs normally reside on adapter cards. However, the original PC, and perhaps also the PC XT, have a spare ROM socket on the motherboard (the "system board" in IBM's terms) into which an option ROM can be inserted, and the four ROMs that contain the BASIC interpreter can also be removed and replaced with custom ROMs which can be option ROMs. The [[IBM PCjr]] is unique among PCs in having two ROM cartridge slots on the front. Cartridges in these slots map into the same region of the upper memory area used for option ROMs, and the cartridges can contain option ROM modules that the BIOS would recognize. The cartridges can also contain other types of ROM modules, such as BASIC programs, that are handled differently. One PCjr cartridge can contain several ROM modules of different types, possibly stored together in one ROM chip.