BIOS is the first program your computer runs when you turn it on

BIOS stands for Basic Input/Output System. It is a small program stored on a chip inside your computer that runs before your operating system (Windows, macOS, or Linux) loads. Think of it as your computer's startup checklist — it tests your hardware, finds your hard drive, and hands control over to Windows or whatever system you use.

When you press the power button, BIOS wakes up first. It checks that your RAM works, finds your storage drive, and makes sure your keyboard and mouse are connected. Only after BIOS finishes this inspection does it look for your operating system and start loading it. If something is broken or missing, BIOS stops and alerts you before Windows even has a chance to start.

You almost never see BIOS running. It happens in the background in a few seconds, and then your computer continues to the login screen. But if something goes wrong during startup — like a loose cable or a failing hard drive — BIOS is what catches it and tells you.

Key Takeaways

  • BIOS runs automatically when you turn on your computer, before your operating system loads, and checks that all your hardware is working.
  • You can enter BIOS setup by pressing a specific key during startup (usually Delete, F2, or F12, depending on your computer brand) to change settings like boot order or fan speed.
  • BIOS stores settings in a small battery-powered memory chip, so your preferences stay saved even when the computer is unplugged.
  • Newer computers use UEFI instead of traditional BIOS, but UEFI does the same job and works the same way from a user's perspective.

How BIOS checks your hardware at startup

When your computer powers on, BIOS runs a test called POST, which stands for Power-On Self-Test. This test checks your RAM, your hard drive, your CPU, and other core parts to make sure they respond. If a part fails the test, BIOS displays an error code or beep pattern and stops the startup process.

For example, if your RAM is loose or failing, BIOS will beep repeatedly and show an error message instead of loading Windows. This is actually helpful — it tells you the problem is hardware, not software, so you know where to look. If BIOS did not run this test, your computer might start loading Windows, freeze halfway through, and give you no clue what went wrong.

After BIOS finishes testing, it looks for a bootable drive — usually your hard drive or SSD — and reads the first part of your operating system. Once it finds that, it hands over control and your operating system takes over.

Entering BIOS setup to change settings

You can enter BIOS setup while your computer is starting up by pressing a specific key. The key varies by manufacturer: Dell and HP computers often use F2, Lenovo uses F1 or Enter, and many others use Delete or F12. You have to press it during the first few seconds after you turn on the computer, before Windows loads.

When you enter BIOS setup, you see a menu with options to change things like the order in which your computer looks for bootable drives, the speed of your cooling fans, or the date and time. Most people never need to change these settings — they work fine at their defaults. But if you want to boot from a USB drive to install Windows, or if your computer is overheating and you want to adjust fan speed, BIOS is where you make those changes.

The BIOS menu looks different on every computer brand, but the basic layout is similar: you use arrow keys to move around, Enter to select, and Escape to go back. If you are not sure what a setting does, it is usually safe to leave it alone and just look at what you came to change.

BIOS settings that matter for most users

A few BIOS settings show up on almost every computer. Boot order tells your computer which drive to look at first when starting up — normally your main hard drive, but you can change it to USB or DVD if you want to install a new operating system. find Boot is a security feature that checks whether your operating system is legitimate before loading it. Fan speed controls how fast your cooling fans spin — higher speed means more cooling but more noise.

Most computers also have a setting to enable or disable specific hardware, like your WiFi card or USB ports. You might disable something if it is broken and you want to use a different one, or if you are troubleshooting and want to rule out a piece of hardware as the cause of a problem.

The date and time setting in BIOS is important because your computer uses it as a reference. If BIOS loses power for a long time (usually because the battery on the motherboard is dying), the date and time reset, and your computer may have trouble connecting to the internet or opening files with date-based security checks.

UEFI is the modern replacement for BIOS

Newer computers, especially those made in the last ten years, use UEFI (Unified Extensible Firmware Interface) instead of traditional BIOS. UEFI does the same job — it tests your hardware and loads your operating system — but it is faster, more flexible, and has a better interface. Some UEFI setups even show a graphical menu with a mouse instead of just text and arrow keys.

From a user's perspective, UEFI works almost exactly like BIOS. You still press a key during startup to enter setup, you still see a menu of hardware settings, and you still change things like boot order the same way. The main difference is that UEFI can handle larger hard drives and boots faster, but you do not need to know that to use your computer.

If your computer manual mentions BIOS, it probably means UEFI — manufacturers often use the term BIOS loosely to mean the firmware that runs at startup, whether it is traditional BIOS or UEFI.

What happens if BIOS gets corrupted or outdated

BIOS is stored on a chip on your motherboard, so it does not get deleted or corrupted the way files on your hard drive can. However, it can become outdated. Computer manufacturers sometimes release BIOS updates to fix bugs, improve hardware support, or patch security problems. You can check your computer brand's support website to see if an update is available.

Updating BIOS is more technical than updating Windows, and it carries a small risk — if the update fails partway through, your computer may not start. For this reason, most people do not update BIOS unless they have a specific reason to, like a hardware compatibility problem or a security warning from the manufacturer. If your computer is working fine, the BIOS version it came with is usually fine to keep.

If BIOS does become corrupted (which is rare), your computer will not start at all, and you will need to contact the manufacturer or a repair technician. This is not something you can fix yourself without special equipment.

The battery that keeps BIOS settings alive

Your computer has a small coin-shaped battery on the motherboard that powers BIOS memory even when the computer is unplugged. This battery keeps your BIOS settings, the date and time, and other information stored. A typical motherboard battery lasts five to ten years.

When the battery dies, your BIOS settings reset to factory defaults, and the date and time go back to a default value (often 2000 or 2006). This is usually just an inconvenience — you can enter BIOS setup and set the date and time again. However, if your computer is very old and the battery dies, you may have trouble booting because the date reset can interfere with security checks.

Replacing the battery is straightforward if you are comfortable opening your computer case — it is just a matter of popping out the old battery and snapping in a new one. If you are not comfortable doing that, a repair shop can do it in a few minutes for a small fee.

Frequently Asked Questions

How do I know what key to press to enter BIOS on my computer?

During startup, before Windows loads, your computer usually displays a message like "Press F2 to enter Setup" or "Press Delete to enter BIOS." If you miss it, restart your computer and watch the screen carefully during the first few seconds. If you still cannot find it, check your computer's manual or the manufacturer's website — search for your brand and model plus "enter BIOS."

Can I damage my computer by changing BIOS settings?

Most BIOS settings are safe to change, and you can always reset them to factory defaults if something goes wrong. However, changing things like voltage or clock speed can damage hardware if you do not know what you are doing. If you are not sure what a setting does, leave it alone. Stick to common changes like boot order, which are safe.

What should I do if my computer will not start and I think BIOS is the problem?

If your computer beeps or shows an error message during startup, write down the beep pattern or error code and search for it online along with your computer brand. This will usually tell you which hardware is failing. If your computer starts but gets stuck before Windows loads, try restarting and entering BIOS to check the boot order — sometimes a USB drive or DVD gets set as the first boot device by accident.

Is it safe to update BIOS?

BIOS updates from the manufacturer are generally safe if you follow their instructions carefully and do not turn off your computer during the update. However, there is a small risk of failure, so only update BIOS if the manufacturer recommends it for a specific reason — like a security patch or a hardware compatibility fix. If your computer is working fine, you do not need to update.

Why does my computer show the wrong date and time after I turn it on?

This usually means the motherboard battery is dying or dead. The battery keeps the date and time stored even when the computer is unplugged. You can enter BIOS setup and set the date and time manually, but if it keeps resetting, the battery needs to be replaced. A repair shop can do this quickly and inexpensively.