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In computing, the Basic Input-Output System or BIOS is computer interface code that locates and loads the operating system into RAM. It provides low-level communication, operation and configuration to the hardware of a system, which at a minimum drives the keyboard and provides primitive output to a display. The BIOS is usually written in Assembly language native to the processor.

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Boot BIOS Since the introduction of the IBM PC compatible in August 1981, BIOS code issues the first commands to the system during power-up or the boot process, such as where to find and how to load the operating system software (most typically Microsoft Windows or Linux). PC BIOS code also contains diagnostics to assure critical hardware components, such as memory, keyboard, disk drive, I/O ports etc., are operational, and nearly all BIOS implementations can optionally execute a setup program interfacing the CMOS Memory[?]; this memory holds user-customizable configuration data (time, date, hard drive details, etc) accessed by BIOS code. The 80x86 source code for early PC and AT BIOS was included with the IBM Technical Reference Manual.

BIOS as Firmware BIOS is sometimes called firmware because it is an integral part of the system hardware. Before 1990 or so BIOSs were held on ROM chips that could not be altered. As their complexity and the need for updates grew, BIOS firmware is stored on EEPROM or flash memory devices that can be upgraded by the user.

Firmware on Adapter Cards A computer system may contain several BIOS firmware chips. In addition to the boot BIOS, which contains code to access fundamental hardware components such as the keyboard or the floppy drive, plug-in adapter cards such as SCSI hard disk adapters or video boards may include their own BIOS, complementing or replacing the system BIOS code for the given component.

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