BIOS is the software that starts your computer before Windows or any other operating system loads

BIOS stands for Basic Input/Output System. It is the first program that runs when you turn on your computer — before your operating system, before your desktop, before anything else. Think of it as the bridge between your hardware (the physical parts) and the software (the programs) that you use every day.

When you press the power button, BIOS wakes up the motherboard, checks that all your hardware is connected and working, and then hands control over to Windows, macOS, or Linux. Without BIOS, your computer would not know how to talk to your keyboard, mouse, hard drive, or any other component. It is the reason your computer can start at all.

BIOS lives on a small chip on your motherboard, not on your hard drive. That is why it survives even if you delete everything on your computer or replace your hard drive entirely. It is permanent, built into the machine itself.

Key Takeaways

  • BIOS runs before your operating system loads and tells your computer which hardware is present and working.
  • You can enter BIOS setup by pressing a key during startup — usually Delete, F2, or F12, depending on your computer brand.
  • BIOS controls boot order, which means you can tell your computer to start from a USB drive, CD, or hard drive instead of the default.
  • Modern computers use UEFI instead of older BIOS, but UEFI does the same job and works the same way from a user perspective.
  • You rarely need to change BIOS settings unless you are troubleshooting, installing a new operating system, or overclocking hardware.

What BIOS actually does when you turn on your computer

The moment you press power, BIOS runs a test called POST — Power-On Self-Test. It checks that your RAM is working, your hard drive is connected, your CPU is responding, and every other piece of hardware is in place. If something is broken or missing, BIOS will beep at you or show an error message before anything else happens.

After the POST passes, BIOS looks at the boot order — a list you can change that tells it where to look for an operating system. By default, it looks at your main hard drive first. If it finds Windows or another operating system there, it loads that system and steps out of the way. From that point forward, Windows is in charge.

BIOS also manages basic hardware settings like the date and time, the speed of your fans, and the voltage going to your CPU. It handles the keyboard and mouse before Windows is running, which is why you can use them in BIOS setup even though Windows is not loaded yet.

How to enter BIOS and what you will see

To get into BIOS, restart your computer and press a specific key while it is starting up — before Windows loads. The key varies by manufacturer. On Dell computers it is usually F2. On HP and Lenovo it is often F10 or Delete. On ASUS it is Delete or F2. On a Mac, hold Command and R during startup instead, which opens a different system called Recovery.

You have only a few seconds to press the key, so you may need to restart and try again if you miss the window. Some computers show a message on the startup screen telling you which key to press — watch for text that says "Press F2 to enter Setup" or similar.

Once you are in BIOS, you will see a menu with options like System Information, Boot Options, Security, and Power Management. The layout and colors vary widely depending on your computer brand. You navigate using arrow keys and Enter, not a mouse. Everything is text-based and plain-looking because BIOS runs before any fancy graphics are available.

Boot order and why you might need to change it

Boot order is the list of places BIOS checks for an operating system, in the order it checks them. By default, your main hard drive is first. But you can change this to make BIOS look at a USB drive first, or a CD drive, or a network location.

You would change boot order if you are installing a fresh copy of Windows from a USB drive, or if you are running a repair tool from a bootable CD. You tell BIOS to look at the USB drive first, insert the drive, restart, and BIOS will load the installation program instead of Windows. Once the installation is done, you change boot order back to the hard drive.

Some computers let you press a different key during startup — like F12 or Esc — to pick a boot device without entering BIOS setup. This is faster if you only need to boot from USB once.

BIOS versus UEFI — what changed and why it does not matter to you

UEFI is the modern replacement for BIOS. Most computers made after 2010 use UEFI instead of the older BIOS. UEFI does the same job — it starts your computer and hands off to Windows — but it is faster, more find, and can handle larger hard drives.

From a user perspective, the difference is almost invisible. You still press the same key to enter setup. You still see a menu. You still change boot order the same way. The main difference is that UEFI has a graphical interface on newer computers, so you might see a mouse cursor and buttons instead of just text and arrow keys. But the function is identical.

You do not need to do anything to switch from BIOS to UEFI. If your computer supports it, it is already running UEFI. The terms are often used interchangeably now, so when someone says "BIOS" they usually mean the startup system in general, whether it is old BIOS or new UEFI.

Common BIOS settings you might encounter

Most BIOS menus include a System Information section that shows your CPU model, RAM amount, and hard drive size. This is read-only — you cannot change it, only look at it. It is useful if you want to confirm what hardware you have without opening your computer.

Security settings let you set a password so that only you can enter BIOS setup or start the computer. Boot options control the boot order and whether your computer will boot from USB or CD. Power Management controls fan speed, sleep settings, and how much power goes to different components.

Most users never need to change any of these. BIOS comes configured correctly from the factory. The only time you typically enter BIOS is to change boot order for an installation, or if a technician tells you to adjust a specific setting to fix a problem.

When you should and should not change BIOS settings

You should change BIOS settings if you are installing a new operating system and need to boot from USB, if you are troubleshooting a hardware problem and a technician has told you what to change, or if you are overclocking your CPU or GPU and need to adjust voltage or clock speed. Beyond those situations, leave BIOS alone.

Do not change BIOS settings if you do not know what they do. Changing the wrong setting can prevent your computer from starting, force it to run slowly, or cause hardware damage if you increase voltage too high. If you make a change and something breaks, most BIOS menus have a "Load Defaults" or "Reset to Factory Settings" option that will undo your changes.

If your computer will not start and you suspect a BIOS problem, restart and enter BIOS to check that the boot order is correct and that all your hardware shows up in the System Information section. If your hard drive is missing from the list, it may be disconnected or failing. If the boot order is wrong, fix it and restart.

Frequently Asked Questions

Can I update my BIOS?

Yes. Manufacturers release BIOS updates to fix bugs, improve security, or add support for new hardware. You can check your computer manufacturer's website for updates specific to your model. Updates are usually optional unless there is a security problem. Follow the manufacturer's instructions carefully — a failed BIOS update can make your computer unbootable.

What happens if I forget my BIOS password?

On most computers, you can reset the BIOS password by removing the CMOS battery from the motherboard for a few minutes, then putting it back. This erases all BIOS settings and passwords. If you cannot open your computer, contact the manufacturer or a repair shop — they have tools to reset it without opening the case.

Why is my computer not finding my hard drive in BIOS?

The hard drive cable may be loose, the drive may be failing, or the BIOS may not recognize the connection type. Restart and enter BIOS to see if the drive appears in System Information. If it does not, try reseating the cable — turn off the computer, open the case, disconnect the cable from the hard drive, wait 10 seconds, and reconnect it firmly.

Do I need to change BIOS settings to install Windows?

Only if you are installing from a USB drive or CD. Change the boot order to make USB or CD first, insert the installation media, restart, and BIOS will load the installer. Once Windows is installed, change boot order back to your hard drive so the computer starts normally.

Is BIOS the same thing as firmware?

BIOS is a type of firmware — software that is permanently stored on a hardware chip. All firmware is not BIOS, but BIOS is firmware. The terms are sometimes used interchangeably, but firmware is the broader category that includes BIOS, UEFI, and the software that runs on other devices like printers and routers.