bios

In IBM PC compatible computers, the Basic Input/Output System (BIOS), also known as System BIOS, ROM BIOS or PC BIOS (/ˈbaɪ.oʊs/), is a de facto standard defining a firmware interface.[1] The name originated from the Basic Input/Output System used in the CP/M operating system in 1975.[2][3] The BIOS software is built into the PC, and is the first software run by a PC when powered on. The fundamental purposes of the BIOS are to initialize and test the system hardware components, and to load a bootloader or an operating system from a mass memory device. The BIOS additionally provides abstraction layer for the hardware, i.e. a consistent way for application programs and operating systems to interact with the keyboard, display, and other input/output devices. Variations in the system hardware are hidden by the BIOS from programs that use BIOS services instead of directly accessing the hardware. Modern operating systems ignore the abstraction layer provided by the BIOS and access the hardware components directly. The BIOS of the original IBM PC/XT had no interactive user interface. Error messages were displayed on the screen, or coded series of sounds were generated to signal errors. Options on the PC and XT were set by switches and jumpers on the main board and on peripheral cards. Modern Wintel-compatible computers provide a setup routine, accessed at system power-up by a particular key sequence. The user can configure hardware options using the keyboard and video display. BIOS software is stored on a non-volatile ROM chip on the motherboard. It is specifically designed to work with each particular model of computer, interfacing with various devices that make up the complementary chipset of the system. In modern computer systems, the BIOS contents are stored on a flash memory chip so that the contents can be rewritten without removing the chip from the motherboard. This allows BIOS software to be easily upgraded to add new features or fix bugs, but can make the computer vulnerable to BIOS rootkits. MS-DOS (PC DOS), which was the dominant PC operating system from the early 1980s until the mid 1990s, relied on BIOS services for disk, keyboard, and text display functions. MS Windows NT, Linux, and other protected mode operating systems in general do not use it after loading.

The BIOS ROM is customized to the particular manufacturer's hardware, allowing low-level services (such as reading a keystroke or writing a sector of data to diskette) to be provided in a standardized way to an operating system. For example, an IBM PC might have either a monochrome or a color display adapter (using different display memory addresses and hardware), but a single, standard, BIOS system call may be invoked to display a character at a specified position on the screen in text mode. The BIOS provides a small library of basic input/output functions to operate peripherals (such as the keyboard, rudimentary text and graphics display functions and so forth). When using MS-DOS, BIOS services could be accessed by an application program (or by MS-DOS) by executing an INT 13H interrupt instruction to access disk sectors, or one of a number of other documented BIOS interrupt calls to access video display, keyboard, cassette, and other devices. Operating systems and executive software, designed to supersede this basic firmware functionality, provide replacement software interfaces to applications. This began even in the 1980s under MS-DOS, when programmers observed that using the BIOS video services for graphics display was very slow. To increase the speed of screen output, many programs bypassed the BIOS and programmed the video display hardware directly. Since the AT-compatible BIOS ran in Intel real mode, operating systems on '286 and later processors required hardware device drivers compatible with protected mode operation to replace BIOS services. In modern personal computers the BIOS is used only during booting and initial loading of system software. Before the operating system's first graphical screen is displayed, input and output are typically handled through BIOS. A boot menu such as the textual menu of Windows that allows one to choose an operating system to boot or to boot into Safe Mode or to use the last known good configuration, is displayed and receives keyboard input through BIOS.

Historically the BIOS in the IBM PC and XT had no built-in user-interface. The BIOS versions in earlier PCs (XT-class) were not software configurable; users set options via DIP switches on the motherboard. Later computers, including all IBM-compatibles with 80286 CPUs, had a battery-backed CMOS RAM chip that held BIOS settings.[12] These settings, such as video-adapter type, memory size, and hard-disk parameters, could only be configured by running a configuration program from a disk, not built into the ROM. A special "reference diskette" was inserted in an IBM AT to configure settings such as memory size. Early BIOS versions did not have passwords or boot-device selection options. The BIOS was hard-coded to boot from the first floppy drive, or, if that failed, the first hard disk. Access control in early AT-class machines was by a physical keylock switch (which was not hard to defeat if the computer case could be opened). Anyone who could switch on the computer could boot it.[citation needed]   Award BIOS setup utility on a standard PC A modern BIOS has a menu-based user interface (UI) accessed by pressing a certain key on the keyboard when the PC starts. Usually the key is advertised for short time during the early startup, e.g. "Press F1 to enter CMOS setup". The actual key depends on your hardware. In the BIOS UI, a user can: configure hardware set the system clock enable or disable system components select which devices are potential boot devices set various password prompts, such as a password for securing access to the BIOS user interface functions itself and preventing malicious users from booting the system from unauthorized peripheral devices

Chips

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, but not altered, by users. To allow for updates, many compatible computers used re-programmable memory devices such as EEPROM and later flash memory 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.BIOS versions are upgraded to take advantage of newer versions of hardware and to correct bugs in previous revisions of BIOSes.[14] 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 the year 2000 by setting the century bit automatically when the clock rolled past midnight, December 31, 1999.[15] The first flash chips were attached to the ISA bus. Starting in 1997, the BIOS flash moved to the LPC bus, a functional replacement for ISA, following a new standard implementation known as "firmware hub" (FWH). In 2006, the first systems supporting a Serial Peripheral Interface (SPI) appeared, and the BIOS flash memory moved again.[citation needed] The size of the BIOS, and the capacities of the ROM, EEPROM and 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 16 megabytes. 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 distributions. For example, some ASUS motherboards included SplashTop Linux embedded into their NAND flash memory ICs.[16] Another type of firmware chip was found on the IBM PC and early compatibles. In the PC and AT, the keyboard interface was controlled by a microcontroller with its own programmable memory. On the IBM AT, this was a 40 pin socketed device. Some manufacturers used an EPROM version of this chip which resembled an EPROM. In the AT, this controller was also assigned the A20 gate function to manage memory above the 1 megabyte range; occasionally an upgrade of this "keyboard BIOS" was necessary to take advantage of software that could use upper memory.