BIOS
also: Basic Input/Output System, firmware, UEFI (modern replacement)
BIOS (Basic Input/Output System) is firmware that runs before the operating system loads, initializing hardware and launching the bootloader to start the kernel.
BIOS is the first software to run when you power on a computer. It performs hardware self-tests (POST), configures essential devices like storage and memory, and then hands control to the bootloader—typically GRUB on Linux systems—which loads the kernel.
BIOS operates in real mode (16-bit), which limits it to addressing 1 MB of memory. It communicates with hardware via interrupt routines and provides basic services like disk reading during boot. Most modern systems now use UEFI (Unified Extensible Firmware Interface) instead, which is more flexible and 64-bit capable.
On a Linux system, you interact with BIOS indirectly during boot: press Delete or F2 at startup to enter BIOS setup, where you can change boot order, enable/disable hardware, or adjust clock speeds.