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The acronym BIOS stands for Basic Input/Output System. In an IBM-compatible PC, the BIOS is implemented as read-only firmware that detects the presence (or absence) of critical system components and carries out checks to ensure they are functioning correctly when the computer is first powered on (this process is often referred to as the Power On Self Test or POST). If any problems are found during the POST, either a message is displayed on the screen or a pre-defined pattern of audible beeps is produced, depending on the exact nature of the problem. The BIOS then tells the computer how to locate and execute the boot code that will load and run the main operating system software. The sequence of events that occurs, from the time the computer is first switched on until the moment the operating system has finished loading and is ready to accept user commands, is called the boot sequence.

The system BIOS

BIOS implements a number of basic commands for carrying out low-level I/O, such as handling keyboard input, outputting text to the screen, and performing disk read or write operations. In the early years of the IBM PC, the operating system employed was either IBM PC-DOS or MS-DOS. DOS used the low-level facilities provided by BIOS to control the computer's hardware. More sophisticated operating systems such as Windows 95 and its successors incorporate device driver software that can communicate directly with hardware controllers rather than relying on the services provided by the system BIOS.

During the boot sequence, the user has the opportunity to enter the BIOS setup program and configure a range of system parameters (usually by pressing one of the function keys or the DELETE key when prompted to do so by an on-screen message). The user-configurable parameters are stored in a separate part of the BIOS called the CMOS (which stands for complementary metal-oxide semiconductor), a special form of static RAM that requires very little current, and can be supplied with the power necessary to preserve its contents by a small lithium battery when the computer is powered down. The parameters that can be set by the user usually include the system date and time, various power-saving options, and the list of devices that should be searched by BIOS for boot sector code during the boot sequence (e.g. hard drive, CD-ROM drive, floppy disk drive etc.), and in what order. The BIOS setup program also allows the user to set a BIOS password to prevent unauthorised users from changing the configuration.

BIOS chips are highly specialised, and are manufactured by a relatively small number of companies (notable example include American Megatrends Inc. and Phoenix Technologies) for a number of mainboard vendors. The ability to re-write the contents of the BIOS allows mainboard manufacturers to easily customise a proprietary BIOS chip for a particular chipset, and to offer BIOS upgrades to customers to improve compatibility, or enable the use of new or updated hardware. The utilities used to upgrade the BIOS firmware (referred to as flash utilities) can usually be downloaded from the mainboard vendor's website, although care should be taken to make sure that the software used has the correct version number, as an attempt to upgrade firmware using the wrong software could render the system unusable. .

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