What Is BIOS? What It Does Before Windows Starts

Updated: October 2026

Every PC runs a small program before Windows gets a say. It checks the hardware, picks a drive to start from and hands over control, and when it trips, nothing else loads. Here's what BIOS is, what it does in the seconds after you press power, and which of its settings IT teams touch.

What Is BIOS?

BIOS stands for Basic Input/Output System. It's firmware: a small program stored on a chip on the motherboard, not on the hard drive. It runs first, every time, before any operating system exists in memory.

The name is older than the PC. Gary Kildall coined it for the CP/M operating system in 1975, and the IBM PC made it the standard in 1981. Today's PCs ship with UEFI firmware instead, but the word stuck. Vendors, manuals and technicians still say "go into the BIOS" when they mean the firmware setup screen, whatever it runs underneath.

On modern boards the code sits in rewritable flash memory. That's why it can be updated, and why an interrupted update can leave a board that won't start.

What BIOS Does When You Press Power

The job fits in four steps.

First comes the power-on self-test, or POST. The firmware checks that the processor, memory and graphics respond. If something basic fails, there's no screen to show an error, so boards signal with beep codes or diagnostic LEDs instead.

Next it initializes devices: storage controllers, USB, the keyboard. Then it reads its saved settings and walks the boot order, looking for a drive with something bootable on it. Finally it hands control to the boot loader on that drive, which starts Windows.

Each step has its own failure. A dead POST means beeps or a black screen with fans spinning. A missing boot device means a "no bootable device" message. A damaged boot loader means the firmware finds the drive but Windows never starts. Knowing which step failed tells you whether to reseat RAM, check the boot order or repair Windows.

This r/buildapc thread asks the question new builders tend to have the first time they meet the setup screen:

The top reply calls it the traffic controller for your hardware, which is close enough.

Dave Plummer, a former Microsoft engineer, walks through the whole startup chain from power button to operating system:

Where the Settings Live: The Chip and the CMOS Battery

The firmware code lives in flash memory on the board. Your settings, like boot order and the clock, were historically kept in a small block of CMOS memory kept alive by a coin-cell battery.

Many modern boards store most settings in flash too, and the battery mainly keeps the real-time clock running. Either way, a dying battery shows the same symptoms. The clock resets after a power cut, the board warns about lost settings on startup, or custom settings quietly fall back to defaults.

Clearing CMOS, by jumper or by pulling the battery for a minute, resets the settings to factory defaults. It's the standard fix when a bad setting stops a PC from starting. On business laptops it rarely clears a firmware password, which is by design.

BIOS Settings IT Teams Change

Users rarely need the setup screen. IT teams open it for a short list of reasons.

Virtualization and TPM each have their own guide. Start with enabling virtualization in BIOS for Hyper-V and WSL 2.

For Windows 11 readiness and BitLocker, see enabling TPM 2.0.

Memory speed is the one people change without meaning to. If a workstation's RAM runs slower than the box says, see what XMP does before you flip it on business machines.

For fleets, set these once and lock them. A firmware password plus vendor tooling or your management platform keeps one curious user from turning off Secure Boot on a Friday.

How to Enter BIOS

There are three ways in.

The classic way is a key at startup. Microsoft lists the common ones as Esc, Delete, F1, F2, F10, F11 or F12, and the startup screen usually says which. Fast boot often shrinks that window to under a second, which is why people miss it.

From Windows 10 or 11, hold Shift while you select Restart. Then go to Troubleshoot, Advanced options, UEFI Firmware settings. This works even when the key window is too short.

From a command prompt, the shutdown command has a firmware switch. Microsoft's reference says /fw, combined with a shutdown option, makes the next restart go to the firmware screen:

code
shutdown /r /fw /t 0

That one is handy on a remote session, as long as someone is standing at the machine when it comes back up.

BIOS vs UEFI in One Minute

Legacy BIOS starts from an MBR disk and has no way to check that the boot code is trusted. UEFI replaced it. It boots from GPT disks, supports Secure Boot and usually has a mouse-driven setup screen.

Intel said in 2017 that it would drop legacy BIOS support by 2020, and new Intel-platform PCs stopped offering the legacy option from 2019. Windows 11 lists its firmware requirement as "UEFI, Secure Boot capable."

To check what a PC runs, open System Information (msinfo32) and read BIOS Mode. "UEFI" is what you want; "Legacy" means the disk is MBR. Microsoft's MBR2GPT tool converts the system disk without deleting data, after which the firmware has to be switched to UEFI mode.

Our What Is UEFI guide goes deeper on the differences, GPT and Secure Boot.

When to Update BIOS

Update when there's a reason. Security fixes, support for new hardware or Windows versions, and stability fixes for a known problem all count. "There's a newer version" alone doesn't.

2026 gave everyone a reason: Microsoft's original Secure Boot certificates expire, and PCs need firmware and database updates to keep booting new boot code. Our guide to BIOS updates covers that and how to update without bricking a board.

At fleet scale, Dell, HP and Lenovo now ship many BIOS updates through Windows Update. The admins in this r/sysadmin thread point out the catch: not every model gets them, so track versions instead of assuming.

Tracking is one line of PowerShell per machine: Get-CimInstance Win32_BIOS returns the maker, version and release date. OpenFrame can run that as a script across a client's devices and collect the output in one place, so the models Windows Update skipped stand out.

The Short Version

BIOS is the firmware that runs before Windows. It tests the hardware, finds a boot drive and hands over control. Modern PCs run UEFI, but the setup screen is still called BIOS, and it's where IT changes boot order, virtualization, TPM, Secure Boot and firmware passwords.

Enter it with the vendor key, with Shift and Restart, or with shutdown /r /fw /t 0. Update it when there's a security or compatibility reason, not by default. Next, read our guide to BIOS updates before your next firmware push.