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 your computer runs when you press the power button — before your operating system, before your desktop, before anything else. BIOS checks that your hardware is working, finds your hard drive or SSD, and hands control over to Windows, macOS, or Linux so the rest of your computer can start.

Think of BIOS as a security guard at the front door. It does a quick inspection of the building (your hardware) to make sure everything is safe and present, then lets the real staff (your operating system) come in and do their jobs. Without BIOS, your computer would not know how to talk to its own parts.

BIOS lives on a small chip on your motherboard, not on your hard drive. That is why it can run before your operating system even exists — it does not depend on anything else being loaded first.

Key Takeaways

  • BIOS runs automatically when you turn on your computer and checks that your hardware is working before your operating system starts.
  • You can enter BIOS setup by pressing a specific key during startup — usually Delete, F2, or F12, depending on your computer brand.
  • Most people never need to change BIOS settings, but you might adjust boot order if you want to start from a USB drive or change how much power your computer uses.
  • UEFI is the newer version of BIOS that works the same way but with a better interface and more features for modern computers.

What BIOS actually does when your computer starts

When you press the power button, BIOS runs a test called POST, which stands for Power-On Self-Test. This test checks that your RAM is working, your hard drive is connected, your keyboard and mouse are responding, and all the other basic hardware is present and functional. If something is broken or missing, BIOS will beep or show an error message and stop — your computer will not start.

Once POST passes, BIOS looks for your operating system. It checks the devices you have told it to check — usually your hard drive first, then a USB drive if you have one plugged in. When BIOS finds the operating system, it loads it into memory and steps out of the way. From that point forward, Windows or macOS is in charge.

BIOS also manages some basic hardware settings while your computer is running — things like the date and time, how fast your fans spin, and which USB ports are active. But most of the time, you do not see BIOS working. It is invisible background software.

How to enter BIOS and what you will see

To open BIOS, restart your computer and press a specific key repeatedly as soon as it starts — before the Windows logo appears. The key varies by manufacturer: Delete and F2 are most common on desktop computers, while F12, F10, or Escape are common on laptops. If you are not sure, look for a message on the startup screen that says "Press [key] to enter Setup" or check your computer's manual.

Once you are in BIOS, you will see a menu with options like "Boot Order," "System Time," "Fan Speed," and "Security." The layout looks different on every computer — some have a straightforward text menu, others have a graphical interface with buttons and tabs. You navigate using arrow keys and Enter, not your mouse.

Do not panic if you accidentally open BIOS. You can press Escape or select "Exit Without Saving" to leave without changing anything. BIOS changes only take effect if you explicitly save them before exiting.

Common reasons to change BIOS settings

Most people leave BIOS alone, but a few situations require you to open it. The most common is changing the boot order — the order in which BIOS checks devices for an operating system. If you want to install Windows from a USB drive, you need to tell BIOS to check the USB drive before the hard drive. After installation is done, you change it back.

You might also enter BIOS to enable or disable hardware features. For example, if you have a second hard drive that is not showing up in Windows, BIOS might have it turned off. Or if your computer is overheating, you can increase fan speed in BIOS. Some people disable USB ports they do not use, or turn off Bluetooth if they do not need it.

Security settings are another reason. BIOS can require a password before Windows starts, or it can lock down which devices are allowed to boot. This is useful if you want to prevent someone from starting your computer from a USB drive.

UEFI is the modern replacement for BIOS

Newer computers do not actually use BIOS anymore — they use UEFI, which stands for Unified Extensible Firmware Interface. UEFI does the same job as BIOS but with a better design and more features. Most importantly, UEFI has a graphical interface with a mouse and buttons instead of text menus and arrow keys.

For your purposes, UEFI works exactly like BIOS. You still press a key at startup to enter it, you still see hardware settings, and you still change boot order the same way. The main difference is that UEFI is faster, supports larger hard drives, and looks more modern. If your computer was made in the last five to ten years, it almost certainly uses UEFI, not BIOS.

People often use the word "BIOS" to refer to both BIOS and UEFI, even though they are technically different. When someone says "go into BIOS," they usually mean "go into the firmware setup screen," whether that is actual BIOS or UEFI.

What can go wrong with BIOS and how to fix it

BIOS problems are rare, but they do happen. The most common issue is a dead CMOS battery — a small coin-shaped battery on your motherboard that keeps BIOS settings and the date and time alive when your computer is off. If this battery dies, BIOS will reset to factory defaults, and your computer might forget its settings or show the wrong date.

Another problem is a corrupted BIOS update. If the power goes out while BIOS is updating, or if the update is interrupted, BIOS can become unreadable and your computer will not start. This is rare and usually requires professional repair, but it is why you should never turn off your computer during a BIOS update.

If your computer will not start and you suspect a BIOS problem, try removing the CMOS battery for a few minutes and putting it back in. This resets BIOS to factory settings and often fixes startup issues. If that does not work, contact the manufacturer or a repair shop.

Frequently Asked Questions

Can I damage my computer by changing BIOS settings?

You can cause problems if you change settings you do not understand, but you cannot permanently break anything. The worst that usually happens is your computer will not start, and you can fix it by resetting BIOS to factory defaults. If you are nervous, write down the original settings before you change anything, or take a photo of the screen.

Do I need to update BIOS?

Most people do not. BIOS updates are released to fix bugs or add support for new hardware, but they are not like Windows updates — you do not need them to stay find or keep your computer running. Only update BIOS if the manufacturer says there is a specific problem you are experiencing, or if you are installing new hardware that requires it.

What is the difference between BIOS and UEFI?

UEFI is the newer version and works the same way, but with a graphical interface and support for larger hard drives. If your computer is less than ten years old, it uses UEFI. The terms are often used interchangeably, and you do not need to know the difference to use your computer.

Why does my computer ask me to press a key during startup?

That message is telling you how to enter BIOS or UEFI setup. You only need to press that key if you want to change settings. If you do nothing, your computer will continue starting normally after a few seconds.

Can BIOS get a virus?

BIOS viruses are extremely rare because BIOS is isolated from the internet and runs before your operating system. In practice, you do not need to worry about this. Viruses target Windows, macOS, and Linux, not BIOS.