BIOS is the software that runs before your operating system starts

BIOS stands for Basic Input/Output System. It is the first program your computer runs when you press the power button — before Windows, macOS, or Linux loads. Think of it as the computer's startup routine: it checks that your hardware is working, finds your hard drive, and hands control over to your operating system.

Every desktop and laptop has a BIOS. You rarely see it because it runs in the background in a few seconds. But if something goes wrong during startup — if your computer won't turn on, won't recognize a new hard drive, or won't boot from a USB stick — you may need to enter the BIOS to fix it.

The BIOS lives on a small chip on your motherboard, separate from your main storage. This is why it survives even if you wipe your hard drive or replace it entirely. The chip has its own battery, which is why your computer remembers the date and time even when unplugged.

Key Takeaways

  • BIOS runs before your operating system and checks that your hardware is working correctly before handing control to Windows or macOS.
  • You access BIOS by pressing a specific key during startup — usually Delete, F2, F10, or F12, depending on your computer's manufacturer.
  • Common reasons to enter BIOS include changing boot order, enabling or disabling hardware, updating drivers, or resetting a forgotten password.
  • Modern computers increasingly use UEFI instead of traditional BIOS, which offers more features but works the same way from a user's perspective.
  • Changing BIOS settings incorrectly can prevent your computer from starting, so write down any changes you make before saving.

How BIOS starts your computer

When you power on your computer, the BIOS when ready runs a test called POST — Power-On Self-Test. It checks that your RAM is working, that your hard drive is connected, that your keyboard and mouse respond, and that other essential hardware is present. If something fails, you hear beep codes or see error messages before the operating system even loads.

Once POST finishes, the BIOS looks for a bootable drive. By default, it checks your main hard drive first. If it finds a valid operating system there, it loads the first part of that operating system into memory and steps out of the way. From that point on, Windows or macOS takes over, and you see the login screen.

If the BIOS cannot find a bootable drive on your hard drive, it moves to the next device in the boot order — perhaps a USB stick, a CD drive, or a network location. This is why you can boot from a USB stick to install a fresh operating system or repair an existing one.

How to enter BIOS and what you see there

To enter BIOS, restart your computer and press a specific key repeatedly as it starts up, before the operating system loads. The key varies by manufacturer: Dell and HP often use F2, Lenovo uses F1 or Enter, Asus uses Delete, and some older systems use F10. Your computer usually displays the correct key on the startup screen for a few seconds — watch for text like "Press F2 to enter Setup" or "Press Delete for BIOS."

Once inside, you see a menu with options organized by category. The layout and colors vary, but the structure is similar across most computers. You navigate using arrow keys and Enter, not a mouse. Common sections include System Information (showing your CPU, RAM, and storage), Boot Order (which drive to check first), Security (password protection), and Advanced (hardware-specific settings like fan speed or USB power).

Most users never need to change anything in BIOS. The default settings work for almost all situations. But if you need to boot from a USB stick, disable a piece of hardware, or reset a forgotten password, BIOS is where you do it.

Common reasons to change BIOS settings

The most frequent reason to enter BIOS is to change the boot order. If you want to install Windows from a USB stick, you need to tell BIOS to check the USB drive before the hard drive. You enter BIOS, find the Boot Order section, move USB to the top of the list, save, and restart. Your computer then boots from the USB stick instead of the hard drive.

Another common task is disabling hardware. If a USB port is broken and causing errors, or if you want to turn off the built-in camera for privacy, you can disable it in BIOS rather than physically removing it. This is also useful if you have an extra hard drive you do not want the system to recognize yet.

Some users enter BIOS to update the BIOS itself. Manufacturers release BIOS updates to fix bugs, improve performance, or add support for new hardware. The update process varies by computer, but it usually involves downloading a file from the manufacturer's website, putting it on a USB stick, and running it from within BIOS.

If you forget your Windows password, some computers allow you to reset it from BIOS, though this depends on your security settings. You can also clear the CMOS battery to reset BIOS to factory defaults, which erases any custom settings you made — useful if a change has locked you out.

BIOS versus UEFI: what changed

UEFI (Unified Extensible Firmware Interface) is the modern replacement for traditional BIOS. Most computers made after 2010 use UEFI, though they often still call it "BIOS" in menus and documentation. From a user's perspective, they work almost identically: you press the same keys to enter, you see similar menus, and you make the same kinds of changes.

The main differences are technical. UEFI can handle larger hard drives (over 2 terabytes), supports a mouse in the menu, and includes better security features like find Boot, which prevents unauthorized software from loading before your operating system. UEFI also loads faster than traditional BIOS.

You do not need to do anything differently. If your computer is recent, it uses UEFI. If it is older, it uses traditional BIOS. The instructions for entering and changing settings are the same either way.

What can go wrong if you change BIOS settings

The biggest risk is disabling something your computer needs to start. If you disable the hard drive by accident, or set the boot order so that no bootable device is first, your computer will not start. You will see an error message like "No bootable device found" and a blank screen.

The good news is that this is almost always fixable. You restart, enter BIOS again, and undo the change. Write down what you changed before you save, so you know how to reverse it. If you cannot remember, you can usually reset BIOS to factory defaults by clearing the CMOS battery — a small coin-shaped battery on the motherboard that you remove for a few seconds.

Overclocking — pushing your CPU or GPU faster than the manufacturer intended — is possible in BIOS but carries real risk. It can make your computer unstable, cause crashes, or shorten the lifespan of your hardware. Unless you know what you are doing, leave overclocking settings alone.

When you should and should not change BIOS

Change BIOS when you have a specific problem to solve: you need to boot from USB, you want to disable a broken device, or you are following instructions from a technician or manufacturer. Do not change BIOS out of curiosity or because you read that a setting might improve performance. Most performance gains from BIOS tweaks are tiny and not worth the risk.

If your computer is working fine, leave BIOS alone. The default settings are chosen to be safe and reliable. Manufacturers test them extensively before shipping the computer. Any change you make is a departure from that tested configuration.

If you do need to change something, take a photo of the current settings with your phone before you make any changes. That way, if something goes wrong, you have a record of what to change back.

Frequently Asked Questions

Can I damage my computer by changing BIOS settings?

You can make it unable to start, but you cannot permanently damage the hardware by changing BIOS settings alone. If you disable something critical, you can always reset BIOS to factory defaults by removing the CMOS battery for a few seconds, or by using the "Load Defaults" option in BIOS itself. Once reset, your computer will start again.

What if I cannot remember which key to press to enter BIOS?

Restart your computer and watch the startup screen carefully. Most computers display the correct key for a few seconds — look for text like "Press F2 to enter Setup" or "Press Delete for BIOS." If you miss it, restart and try again. If you still cannot find it, search online for your computer's brand and model plus "enter BIOS" — the answer is almost always available.

Is BIOS the same as the CMOS battery?

No. BIOS is the software; the CMOS battery is the small coin-shaped battery on the motherboard that keeps BIOS settings and the system clock alive when the computer is unplugged. Removing the battery for a few seconds clears all BIOS settings and resets them to factory defaults. This is useful if you forget a BIOS password or make a change that prevents the computer from starting.

Do I need to update BIOS?

Usually no. Manufacturers release BIOS updates to fix bugs or add support for new hardware, but most users never need them. If your computer is working fine, there is no reason to update. If you are having a specific problem — like a new hard drive not being recognized — check the manufacturer's website to see if a BIOS update might help.

Can I change BIOS settings from within Windows?

Not in traditional BIOS, but some newer computers with UEFI allow you to restart into BIOS settings from within Windows. Look in Settings under System or Recovery. If the option is not there, you must restart and press the BIOS key during startup, as described above.