BIOS

BIOS, in computing, stands for basic input/output system.

How the BIOS Boots

The BIOS runs off the onboard flash memory when the computer is powered on and it initializes the chipset and the memory subsystem. Subsequently, it uncompresses itself from flash memory into the system main memory and starts executing from there. PC BIOS code also contains diagnostics to assure critical hardware components, such as keyboard, disk drive, I/O ports etc., are operational, and properly initialized. Nearly all BIOS implementations can optionally execute a setup program interfacing the nonvolatile BIOS memory (CMOS). 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.

Related Topics:
Flash memory - PC - Disk drive - Nonvolatile BIOS memory (CMOS) - Hard drive - 80x86

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In most modern BIOS implementations, one can select what boots first: CD, hard disk, floppy disk, flash keydrive and so on. This is particularly useful for installing operating systems or booting to LiveCDs, and for selecting the order of testing for the presence of bootable media.

Related Topics:
CD - Hard disk - Floppy disk - Flash keydrive - Operating system - LiveCD

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Some BIOSes allow the user to select the operating system to load (e.g. load another OS from the second hard disk), though this is more often handled by a second-stage boot loader.

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