BIOS

In short: “Basic Input/Output System” — the firmware that runs first when a PC is switched on, initialises the hardware, and then starts the operating system.

In more detail: The BIOS sits on a chip on the motherboard, independent of the hard drive. It checks and initialises hardware (Power-On Self-Test), looks for a bootable drive, and then hands control to the operating system’s bootloader. Modern motherboards now almost exclusively use the successor, UEFI, which replicates BIOS functions with a graphical interface and more capabilities.

In Depth

Power-On Self-Test (POST)

The Power-On Self-Test (POST) is the very first step after switching on: the BIOS checks whether basic hardware components (RAM, graphics card, keyboard) are even present and functional at all — a well-known error beep code (a different number/length of beeps encodes different error types, depending on the BIOS manufacturer) or an error screen before the actual operating system starts comes from this phase. Only once the POST completes successfully does the BIOS look for a bootable drive (via the configured boot order) and hand control to its bootloader.

The BIOS setup menu

Via the BIOS setup menu (usually accessed by pressing a key like Del, F2 or Esc right after switching on, before the operating system loads), basic settings can be configured:

• Boot order: which drive to start from first (relevant, for example, to install from a USB stick instead of the built-in hard drive).
• Overclocking: raising clock rates and voltages for the CPU/RAM beyond manufacturer specs, for more performance at the cost of stability/lifespan.
• Security features: Secure Boot (only allow digitally signed, trusted bootloaders), a BIOS password, TPM settings.
• Hardware monitoring: fan speeds, temperatures, sometimes manual fan curves.

Limits of classic BIOS

Classic BIOS is technically heavily limited: only hard drives up to 2.2 TB addressable (due to the older MBR partitioning standard), a pure text interface with no mouse control, no network access in setup, and a 16-bit execution mode that can only address modern hardware to a limited extent. These limitations are the main reason BIOS has been almost completely replaced by UEFI — UEFI supports larger hard drives (GPT partitioning), graphical mouse operation, network features directly in setup, and faster boot times.

Usage today

The term “BIOS” is still often colloquially used for the UEFI setup menu today (“go into the BIOS and change the boot order”), even though these are technically two different firmware standards — similar to how “googling” is used for web searching in general, regardless of the search engine actually used.

CMOS battery

The BIOS settings themselves are traditionally kept in a small, battery-backed memory chip (CMOS) on the motherboard, which uses a tiny coin-cell battery (usually a CR2032) even when the PC is off, so settings and the system clock aren’t lost. If this battery runs out (usual after several years), the BIOS resets to factory defaults on the next start, and the system clock starts again from a default date — a classic diagnostic sign of a dead CMOS battery.

See also: UEFI, Motherboard/Mainboard