The 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 runs before your operating system (Windows, macOS, Linux) even starts. Think of it as your computer's startup checklist — it wakes up all the hardware, tests that everything is connected and working, and then hands control over to Windows or whatever system you use.

The BIOS is not something you interact with during normal computer use. You only see it if you restart your computer and press a specific key during startup — usually Delete, F2, or F12, depending on your computer's manufacturer. Most people never need to open it.

Key Takeaways

  • The BIOS runs automatically when you power on your computer, before Windows or any other operating system loads.
  • It checks that your hardware (hard drive, RAM, keyboard, mouse) is connected and working properly.
  • You can change BIOS settings by restarting and pressing a key like Delete or F2 during startup, but most users never need to.
  • Modern computers use UEFI instead of traditional BIOS, though the name BIOS is still commonly used for both.

What the BIOS actually does when you turn on your computer

When you press the power button, the BIOS wakes up first. It runs through a series of checks called the POST (Power-On Self Test). During this test, the BIOS confirms that your RAM is working, your hard drive is connected, your keyboard and mouse respond, and other essential parts are present and functional.

If something is wrong — say, your RAM is loose or your hard drive is disconnected — the BIOS will beep at you or display an error message on the screen. This happens before Windows even tries to load, so you know the problem is with the hardware itself, not with Windows.

Once the POST passes, the BIOS looks for your operating system. It finds the hard drive or SSD where Windows (or Linux, or macOS) is installed, loads the first part of that system into memory, and then steps aside. From that point on, Windows is in charge.

Where the BIOS lives and how it stays there

The BIOS is stored on a small chip called a flash ROM (read-only memory) soldered to your motherboard. Because it is on a chip, it survives even when your computer is completely powered off and unplugged. The chip has a tiny battery on the motherboard that keeps the BIOS settings saved, even during a power outage.

This is why your computer can remember things like the date and time, or custom settings you changed in the BIOS, even after you unplug it for weeks. The battery is usually a coin-sized cell that lasts several years.

How to open the BIOS and what you might change there

To enter the BIOS, restart your computer and watch the screen carefully during startup. You will see a message that says something like "Press Delete to enter Setup" or "Press F2 for BIOS Settings." The exact key varies by manufacturer — Dell often uses F2, HP uses Delete, Lenovo uses F1 or Enter. You have only a few seconds to press it before Windows starts loading.

Once inside, you will see a menu with options like boot order (which device to start from), date and time, hard drive detection, and security settings. Most of these you should leave alone. Common reasons people open the BIOS are to change the boot order (to start from a USB drive instead of the hard drive), to enable or disable hardware features, or to check that the BIOS recognizes all your RAM.

If you are not sure what a setting does, do not change it. Incorrect BIOS settings can prevent your computer from starting. If you do make a mistake, most BIOS menus have a "Load Defaults" or "Reset to Factory Settings" option that will undo your changes.

UEFI is the modern replacement for BIOS, but the name stuck

Computers made in the last ten years or so do not actually use traditional BIOS anymore. They use UEFI (Unified Extensible Firmware Interface), which is a newer, more powerful system that does the same job but with more features and a better interface. However, most people still call it "the BIOS" out of habit, and even manufacturers often label it that way.

UEFI can display graphics and use your mouse, whereas traditional BIOS was text-only and keyboard-only. UEFI also supports larger hard drives and boots faster. But from the user's perspective, opening UEFI looks and feels almost identical to opening old BIOS — you restart, press a key, and see a menu of settings.

When you actually need to know about the BIOS

Most computer users never need to think about the BIOS. It does its job silently in the background. You only need to know about it in a few situations: if your computer will not start and you see error messages during startup; if you want to boot from a USB drive (like when installing Windows); if you need to enable a hardware feature that is currently disabled; or if you are troubleshooting a hardware problem and need to check whether the BIOS recognizes all your components.

If your computer is running normally and you have no reason to change anything, you can safely ignore the BIOS entirely. It will keep working without any input from you.

Frequently Asked Questions

Can I damage my computer by changing BIOS settings?

Yes, incorrect settings can prevent your computer from starting. However, most BIOS menus have a "Load Defaults" or "Reset to Factory Settings" option that will undo your changes and get your computer working again. If you are unsure what a setting does, leave it alone.

Why does my computer beep when I turn it on?

A beep during startup usually means the BIOS detected a problem during the POST. One beep is often normal, but repeated beeps or a long beep followed by short beeps usually signals a hardware issue — often loose RAM or a disconnected hard drive. Check that all internal components are seated firmly.

Is the BIOS password the same as my Windows password?

No. A BIOS password is separate and protects access to the BIOS settings themselves. A Windows password protects your user account and files. You can have both, one, or neither. A BIOS password is useful if you want to prevent others from changing hardware settings, but it is not required for normal use.

What happens if the BIOS battery dies?

Your computer will still start and work normally. You will lose any custom BIOS settings you changed, and the date and time will reset to a default value. The BIOS will revert to factory defaults. You can replace the battery yourself if you are comfortable opening your computer, or take it to a repair shop.

Do I need to update my BIOS?

Usually no. BIOS updates are released to fix bugs, improve hardware support, or patch security issues. Most users never need to update. If your computer is working fine, leave it alone. Only update if the manufacturer recommends it for a specific problem you are experiencing.