Every PC has a tiny operating system that runs before Windows or Linux even loads. It checks that your hardware is working, sets up the boot sequence, and hands control to your real OS. That firmware is called BIOS — or these days, UEFI, which is the modern replacement. If you build your own PC, you’ll need to spend some time in here at least once.

BIOS vs UEFI

Technically, BIOS (Basic Input/Output System) is the old standard from the 80s. UEFI (Unified Extensible Firmware Interface) replaced it starting in the late 2000s. Today, every modern motherboard uses UEFI, but people still call it BIOS out of habit. Most motherboard manufacturers even label their UEFI screens “BIOS Setup.”

UEFI is faster, supports much larger drives, has a real graphical interface (mouse and all), and is required for features like Secure Boot and TPM that Windows 11 needs.

How to enter the BIOS/UEFI

During boot, immediately after powering on, press a specific key before the OS loads. The exact key varies:

  • Most motherboards: Delete key.
  • Some boards: F2.
  • Laptops: often F10 or F12.

The boot screen usually flashes the right key. Press it repeatedly during startup until the UEFI screen appears.

If Windows is already running and you want to get into UEFI: Settings → System → Recovery → Advanced Startup → Restart Now. After it reboots into the recovery menu, choose Troubleshoot → Advanced Options → UEFI Firmware Settings.

What you’ll see inside

Modern UEFI interfaces have an “Easy” or “EZ” mode showing basic info, and an “Advanced” mode with everything. The key sections you’ll use:

Boot order

Tells the system which drive to boot from first. If you install a new SSD and Windows won’t load, check that the new drive is selected as the primary boot device.

Memory / RAM settings

This is where you enable XMP (Intel) or EXPO (AMD) profiles to get your RAM running at its rated speed. Without enabling one of these, your fancy DDR5-6000 kit runs at the default 4800 MT/s.

Fan control

Set fan curves so they spin slower when the system is cool and ramp up under load. Most UEFIs include a graphical fan curve editor.

CPU settings

Voltage, clock multipliers, power limits. Don’t touch these unless you know what you’re doing — but they’re where overclocking happens.