BIOS (Basic Input/Output System) is firmware stored in ROM that initializes and tests your computer's hardware when you turn it on, then hands control to the operating system. The BIOS is the first software that runs when you press the power button, and it's responsible for the boot process described in What Happens During Startup?.
Why it matters
Without BIOS (or its modern replacement, UEFI), your computer wouldn't know how to start. The BIOS performs a Power-On Self Test (POST) that checks if essential hardware like RAM, CPU, and storage devices are working. If something fails, the BIOS produces error beeps or messages before the operating system even loads. Understanding BIOS helps troubleshoot startup problems and configure low-level hardware settings.
What BIOS does
- POST: Tests that essential hardware is present and functioning
- Initialize hardware: Sets up CPU, RAM, and basic device drivers
- Find bootloader: Locates the operating system on your SSD or HDD
- Hand off control: Transfers execution to the operating system's bootloader
BIOS vs UEFI
UEFI (Unified Extensible Firmware Interface) is the modern replacement for traditional BIOS. Most computers made after 2010 use UEFI, though people still call the setup screen "BIOS." UEFI offers faster boot times, support for larger drives (over 2TB), a graphical interface, and better security features like Secure Boot. Both serve the same fundamental purpose: getting your computer from powered-off to running an operating system.