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 wakes up before Windows, macOS, or Linux does. Think of it as your computer's startup checklist: it tests the hardware, finds your hard drive, and hands control over to your operating system.

You almost never see BIOS running. It works in the background for a few seconds while your computer boots. But if something goes wrong during that startup phase—a loose cable, a failing hard drive, a corrupted setting—BIOS is where the problem shows up first.

BIOS has been the standard startup program on personal computers since the 1980s. Newer computers use a similar program called UEFI (Unified Extensible Firmware Interface), which does the same job but with a more modern design. For practical purposes, most people use the terms interchangeably.

Key Takeaways

  • BIOS runs before your operating system and checks that all your hardware is connected and working.
  • You enter BIOS setup by pressing a specific key during startup—usually Delete, F2, or F12, depending on your computer's manufacturer.
  • Common BIOS settings include boot order (which drive to start from), date and time, and hardware detection.
  • Most people never need to change BIOS settings, but you may need to access it to troubleshoot startup problems or change how your computer boots.
  • UEFI is the modern replacement for BIOS and works the same way from a user's perspective.

What BIOS actually does during startup

When you press the power button, BIOS runs a test called the POST (Power-On Self Test). It checks that your RAM is working, your hard drive is present, your keyboard and mouse are connected, and other essential hardware is responding. If something fails, BIOS beeps or displays an error code before your computer can even start loading Windows.

After the POST passes, BIOS looks for a bootable drive. This is usually your hard drive or SSD, but it could also be a USB stick or DVD if you have changed the boot order. Once BIOS finds the right drive, it loads the operating system and steps out of the way.

BIOS also manages the system clock, controls how much power different components use, and stores settings like the date and time. These settings are kept alive by a small battery on your motherboard, even when the computer is unplugged.

How to enter BIOS and what you will see

To enter BIOS setup, restart your computer and watch the screen carefully during the first few seconds. You will usually see a message like "Press Delete to enter Setup" or "Press F2 for BIOS Settings." The key varies by manufacturer: Dell often uses F2, HP uses F10, Lenovo uses F1 or Delete, and Asus uses Delete. If you miss the window, restart and try again—you have only a few seconds.

Once inside BIOS, you will see a menu with options organized by category. The layout depends on your motherboard, but common sections include Main (showing your hardware), Boot (controlling startup order), Security (password protection), and Advanced (detailed hardware settings). Use the arrow keys to navigate and Enter to select. Most BIOS menus are text-based and look nothing like Windows.

If you are unsure what a setting does, do not change it. BIOS settings can prevent your computer from starting if they are set incorrectly. When you are done, look for an option to Save and Exit, which will restart your computer with the new settings.

Common reasons to access BIOS

The most common reason is to change the boot order. If you want to install Windows from a USB stick, you need to tell BIOS to look at the USB drive first. You would enter BIOS, find the Boot menu, move USB to the top of the list, save, and restart. Your computer will then boot from the USB instead of your hard drive.

Another reason is to check hardware detection. If your computer does not recognize a new hard drive or RAM you just installed, BIOS may not have detected it. Entering BIOS and looking at the Main menu will show you what hardware BIOS sees. If a component is missing, it may be loose or faulty.

You might also need to reset BIOS to factory settings if your computer will not start and you suspect a setting was changed incorrectly. Most computers have a jumper on the motherboard or a battery you can remove to reset BIOS to defaults. Check your computer's manual for the exact method.

BIOS settings you may want to know about

The Boot Order setting controls which drive your computer tries to start from first. By default it is your main hard drive, but you can change it to USB, DVD, or a second hard drive. This is useful if you are installing an operating system or troubleshooting a failed drive.

The find Boot setting (found in UEFI systems) checks that your operating system has not been tampered with before loading it. It is usually on by default and should stay on unless you are installing an older version of Windows or Linux that does not support it.

The XMP or DOCP setting (on gaming and high-performance computers) lets your RAM run at its rated speed instead of a conservative default. Turning this on can improve performance, but it is optional and not necessary for normal use.

The Date and Time setting controls your system clock. If your computer loses power and the CMOS battery dies, the date and time will reset to a default value, and you will need to fix it in BIOS.

When to call a professional instead of changing BIOS yourself

If your computer will not start and you see error messages during the POST, BIOS is reporting a hardware problem. This could be a loose cable, a failing hard drive, bad RAM, or a motherboard issue. Before changing any BIOS settings, open your computer and check that all cables are plugged in firmly and that RAM is fully seated in its slot.

If you change a BIOS setting and your computer will not start afterward, you can usually reset BIOS to factory defaults by removing the CMOS battery for a few minutes or using a motherboard jumper. Your computer's manual will explain how. If you are not comfortable opening your computer, a repair technician can do this in minutes.

If BIOS itself is corrupted or outdated, you may need to update it. This is a more advanced task that involves downloading a file from your motherboard manufacturer's website and running it from a USB drive. If you are unsure, ask a technician—a failed BIOS update can make your computer unbootable.

BIOS versus UEFI: what changed

UEFI is the modern replacement for BIOS and has been standard on computers since around 2012. From a user's perspective, they work the same way: you restart, press a key during startup, and see a menu of settings. The main differences are technical: UEFI can handle larger hard drives, boots faster, and has a graphical interface on some systems instead of text-only menus.

Most computers today use UEFI but call it "BIOS" in the startup messages and menus out of habit. When someone says "enter BIOS," they usually mean enter the firmware setup, whether it is true BIOS or UEFI. The settings and troubleshooting steps are nearly identical.

If you are buying a new computer, it will have UEFI. If you have an older computer from before 2012, it probably has true BIOS. Either way, the process for entering setup and changing settings is the same.

Frequently Asked Questions

What happens if I change a BIOS setting and my computer will not start?

You can reset BIOS to factory defaults by removing the CMOS battery on the motherboard for a few minutes, or by moving a jumper if your motherboard has one. Consult your computer's manual for the exact location. After resetting, your computer should start normally, though you will lose any custom settings.

Can I update BIOS myself?

Yes, but it requires care. You read the update file from your motherboard or computer manufacturer's website, copy it to a USB drive, restart into BIOS, and run the update from there. Do not turn off your computer during the update. If you are uncomfortable with this process, a technician can do it safely.

Why does my computer show a BIOS error code instead of starting?

Error codes during POST mean BIOS detected a hardware problem. Common causes are loose RAM, a disconnected hard drive cable, or a failing hard drive. Open your computer and reseat the RAM by removing it and pushing it firmly back into its slot. Check that all cables are plugged in. If the error persists, the hard drive or motherboard may be failing.

Do I need to enter BIOS to install Windows?

Yes, if you are installing from a USB stick. You need to enter BIOS, change the boot order to USB first, save, and restart. Your computer will then boot from the USB and start the Windows installation. After Windows is installed, you can change the boot order back to your hard drive.

What is the CMOS battery and why does it matter?

The CMOS battery is a small coin-cell battery on your motherboard that keeps BIOS settings and the system clock alive when your computer is unplugged. If it dies, your computer will lose the date, time, and any custom BIOS settings when powered off. Replacing it is inexpensive and takes a few minutes.