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

BIOS stands for Basic Input/Output System. It is the firmware — permanent software built into your motherboard — that starts before your operating system loads. When you press the power button, the BIOS runs a series of checks on your hardware, then hands control to Windows, macOS, or Linux.

Think of it as a gatekeeper. The BIOS makes sure your hard drive, RAM, keyboard, and other components are present and working before your computer tries to boot. If something is wrong, the BIOS stops and alerts you with beep codes or error messages on screen. If everything passes, the BIOS finds your operating system and starts it.

You almost never see the BIOS in normal use. It runs in the background, invisible, unless you deliberately open it or something goes wrong during startup.

Key Takeaways

  • The BIOS runs before your operating system and checks that your hardware is working before Windows or macOS loads.
  • You can enter the BIOS setup menu by pressing a specific key during startup — usually Delete, F2, or F12, depending on your computer's manufacturer.
  • The BIOS controls boot order, so you can choose whether your computer starts from the hard drive, a USB drive, or a CD.
  • Most people never need to change BIOS settings, but you may need to adjust them to install a new operating system or troubleshoot hardware problems.

What the BIOS actually does when your computer starts

The moment you turn on your computer, the BIOS takes over. It runs what is called the POST — Power-On Self-Test. This is a quick inventory of your hardware: it checks that your RAM is installed correctly, that your hard drive is connected, that your CPU is responding, and that basic devices like your keyboard and mouse are present.

If the POST finds a problem, the BIOS will not let your computer continue. Instead, it displays an error code or a series of beep sounds. A single beep usually means everything is fine. Multiple beeps or a beep pattern means something failed — for example, three beeps might mean a RAM problem, depending on your BIOS manufacturer.

Once the POST passes, the BIOS looks for your operating system. It checks the devices in a specific order — usually the hard drive first, then USB drives, then CD drives. This order is called the boot order, and you can change it in the BIOS setup menu if you need to install Windows from a USB drive or boot from an external device.

How to enter the BIOS setup menu

To change BIOS settings, you need to enter the BIOS setup menu. This happens during startup, in the few seconds after you press the power button but before Windows or macOS loads. You have to press a specific key at exactly the right moment.

The key varies by manufacturer. On Dell computers, it is usually F2. On HP and Lenovo machines, it is often Delete or F2. On Asus motherboards, it is Delete. On some older computers, it might be F1 or F10. When you first turn on your computer, watch the screen — most manufacturers display a message like "Press F2 to enter Setup" or "Press Delete to enter BIOS."

If you miss the window, restart your computer and try again. You have only a few seconds to press the key before the operating system starts loading. Once you are in the BIOS menu, you can navigate using your keyboard. The screen will show you which keys to use — usually arrow keys to move and Enter to select.

Common BIOS settings you might need to change

Boot order is the most common reason people enter the BIOS. If you are installing a fresh copy of Windows from a USB drive, you need to tell the BIOS to check the USB drive before the hard drive. You move USB Drive to the top of the boot order list, save, and restart. The BIOS will then load Windows from the USB instead of from your existing hard drive.

find Boot is a security feature that prevents unsigned software from loading during startup. It is turned on by default on most modern computers. If you are installing Linux or an older version of Windows, you may need to turn find Boot off so the BIOS will accept the installation media.

RAM speed and timing can be adjusted in the BIOS, though most computers set these automatically. If you install new RAM and your computer does not recognize it, the BIOS may have failed to detect it — restarting or resetting the BIOS to defaults sometimes fixes this.

CPU temperature and fan speed are monitored in the BIOS. If your computer is overheating, you can check these readings in the BIOS menu to see if the cooling system is working. Some BIOS versions let you adjust fan speed manually, though automatic control is usually safer.

BIOS versus UEFI — what changed

Newer computers use UEFI instead of traditional BIOS. UEFI stands for Unified Extensible Firmware Interface. It does the same job — it starts your computer and loads your operating system — but it is faster, more flexible, and has a better interface. Many UEFI systems show a graphical menu with a mouse pointer instead of text-only screens.

For practical purposes, UEFI and BIOS work the same way. You still press a key during startup to enter the setup menu, you still change boot order the same way, and you still see POST errors if something is wrong. The main difference is that UEFI can handle larger hard drives and boots faster than older BIOS systems.

If your computer was made after 2012, it almost certainly uses UEFI, though it may still call itself "BIOS" in the menus. The terms are often used interchangeably now, even though technically UEFI is the newer standard.

When you should and should not change BIOS settings

Most people should never touch the BIOS. The default settings work for almost every situation. Change the BIOS only if you have a specific reason: installing a new operating system, troubleshooting a hardware problem, or adjusting performance settings you understand.

Do not change BIOS settings randomly hoping to speed up your computer. Changing the wrong setting can prevent your computer from starting. If you do make a change and your computer will not boot, restart and enter the BIOS again, then look for a "Reset to Defaults" or "Load Defaults" option. This will undo your changes and usually get your computer working again.

If you are not sure what a setting does, leave it alone. The BIOS menu usually includes descriptions of each option — read them before making changes. If a setting has no description and you do not recognize the name, it is almost certainly not something you need to adjust.

Frequently Asked Questions

Can I damage my computer by changing BIOS settings?

You can prevent your computer from starting if you change the wrong setting, but you cannot permanently damage the hardware. If your computer will not boot after a BIOS change, restart and enter the BIOS again, then select "Reset to Defaults" or "Load Defaults." This restores the original settings and your computer should start normally again.

What does it mean if I hear beep codes during startup?

Beep codes are the BIOS telling you something failed during the POST. One beep is normal and means everything is fine. Multiple beeps indicate a hardware problem — for example, RAM not seated correctly, a loose cable, or a failing hard drive. The exact meaning depends on your BIOS manufacturer, but restarting and checking that all cables and RAM are firmly connected usually fixes the problem.

Do I need to update my BIOS?

Most people do not. BIOS updates are released to fix bugs, add support for new hardware, or improve security. Check your computer manufacturer's website if you are having startup problems or if you are installing new hardware that is not recognized. Otherwise, the BIOS that came with your computer works fine.

What happens if the power goes out while the BIOS is running?

The BIOS runs very quickly — usually in a few seconds — so a power loss during startup is unlikely. If it does happen, your computer will straightforward restart when power returns. The BIOS will run again from the beginning. You will not lose data because the operating system has not loaded yet.

Can I access the BIOS from inside Windows?

On newer computers with Windows 10 or 11, you can restart into the BIOS from the Settings menu without pressing a key during startup. Go to Settings, then System, then Recovery, and select "Restart now" under "Advanced startup." Your computer will restart and show a menu with an option to enter UEFI firmware settings. On older computers, you must press the key during the startup sequence.