Editing BIOS (section)

== Hardware ==
[[File:Elitegroup 761GX-M754 - AMIBIOS (American Megatrends) in a Winbond W39V040APZ-5491.jpg|thumb|[[American Megatrends]] BIOS 686. This BIOS chip is housed in a [[plastic leaded chip carrier|PLCC]] package in a socket.]]
The original IBM PC BIOS (and cassette BASIC) was stored on mask-programmed [[read-only memory]] (ROM) chips in sockets on the motherboard. ROMs could be replaced,<ref>{{cite book | url=https://books.google.com/books?id=V48MZCwICH8C&dq=bios+eeprom&pg=PA180 | title=CompTIA A+ Certification All-In-One Desk Reference for Dummies | isbn=978-0-471-74811-3 | last1=Clarke | first1=Glen E. | last2=Tetz | first2=Edward | date=30 January 2007 | publisher=John Wiley & Sons }}</ref> but not altered, by users. To allow for updates, many compatible computers used re-programmable BIOS memory devices such as [[EPROM]], [[EEPROM]] and later [[flash memory]] (usually [[NOR flash]]) devices. According to Robert Braver, the president of the BIOS manufacturer Micro Firmware, '''Flash BIOS''' chips became common around 1995 because the electrically erasable PROM (EEPROM) chips are cheaper and easier to program than standard [[ultraviolet]] erasable PROM ([[EPROM]]) chips. Flash chips are programmed (and re-programmed) in-circuit, while EPROM chips need to be removed from the motherboard for re-programming.<ref name="smart-computing-97"/> BIOS versions are upgraded to take advantage of newer versions of hardware and to correct bugs in previous revisions of BIOSes.<ref name="smart-computing-96"/>

Beginning with the IBM AT, PCs supported a hardware clock settable through BIOS. It had a century bit which allowed for manually changing the century when the year 2000 happened. Most BIOS revisions created in 1995 and nearly all BIOS revisions in 1997 supported [[Year 2000 problem|the year 2000]] by setting the century bit automatically when the clock rolled past midnight, 31 December 1999.<ref name="smart-somputing-99"/>

The first flash chips were attached to the [[Industry Standard Architecture|ISA bus]]. Starting in 1998, the BIOS flash moved to the [[Low Pin Count|LPC]] bus, following a new standard implementation known as "firmware hub" (FWH). In 2005, the BIOS flash memory moved to the [[Serial Peripheral Interface|SPI]] bus.<ref>{{Cite web |url=https://images.slideplayer.com/26/8671671/slides/slide_5.jpg |title=BIOS Flash Location |access-date=2025-03-30 |archive-date=2021-08-18 |archive-url=https://web.archive.org/web/20210818182418/https://images.slideplayer.com/26/8671671/slides/slide_5.jpg |url-status=live }}</ref>

The size of the BIOS, and the capacity of the ROM, EEPROM, or other media it may be stored on, has increased over time as new features have been added to the code; BIOS versions now exist with sizes up to 32 megabytes. For contrast, the original IBM PC BIOS was contained in an 8&nbsp;KB mask ROM. Some modern motherboards are including even bigger NAND [[flash memory]] ICs on board which are capable of storing whole compact operating systems, such as some [[Linux distribution]]s. For example, some ASUS notebooks included [[Splashtop OS]] embedded into their NAND flash memory ICs.<ref name="geek-splashtop"/> However, the idea of including an operating system along with BIOS in the ROM of a PC is not new; in the 1980s, Microsoft offered a ROM option for MS-DOS, and it was included in the ROMs of some PC clones such as the [[Tandy 1000#Tandy 1000 HX .26 TX|Tandy 1000 HX]].

Another type of firmware chip was found on the IBM PC AT and early compatibles. In the AT, the [[keyboard controller (computing)|keyboard interface]] was controlled by a [[microcontroller]] with its own programmable memory. On the IBM AT, that was a 40-pin socketed device, while some manufacturers used an EPROM version of this chip which resembled an EPROM. This controller was also assigned the [[A20 gate]] function to manage memory above the one-megabyte range; occasionally an upgrade of this "keyboard BIOS" was necessary to take advantage of software that could use upper memory.{{citation needed|date=March 2013}} <!-- An earlier version of this article stated that the IBM PC also used a microcontroller to control the keyboard interface. This is incorrect; the PC and XT used a simple shift register and an [[8-bit computing|8-bit]] parallel port of the 8255A PPI chip for the computer end of the keyboard interface. -->

The BIOS may contain components such as the [[Memory Reference Code]] (MRC), which is responsible for the memory initialization (e.g. [[Serial presence detect|SPD]] and [[memory timings]] initialization).<ref name="Life and Times"/>{{rp|8}}<ref name="Hilber"/>

Modern BIOS<ref>{{Cite web|title=Types of BIOS|url=https://www.rompacks.com/bios-files-packs/|access-date=2021-09-20|website=rompacks.com}}</ref> includes
[[Intel Management Engine]] or [[AMD Platform Security Processor]] firmware.