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. When you press the power button, BIOS runs a quick check of your hardware — your memory, hard drive, keyboard, and other parts — to make sure everything is connected and working. Only after BIOS finishes that check does it hand control over to your operating system.

Think of BIOS as a security guard at the front door. It does not run your programs or display your files. It just makes sure the building is safe and ready before letting everyone else in. Without BIOS, your computer would not know how to talk to its own parts, and your operating system would have nowhere to start.

Key Takeaways

  • BIOS runs automatically when you power on your computer and checks that all hardware is present and working before the operating system loads.
  • You can enter BIOS setup by pressing a specific key during startup — usually Delete, F2, or F12, depending on your computer's manufacturer.
  • BIOS settings control things like boot order, system time, and hardware detection, but most users never need to change them.
  • Modern computers increasingly use UEFI instead of traditional BIOS, though UEFI does the same job with a newer design.
  • If BIOS detects a serious problem during startup, your computer will beep or display an error code instead of loading your operating system.

What BIOS does during startup

When you turn on your computer, BIOS runs through a sequence called POST, which stands for Power-On Self-Test. During POST, BIOS checks that your RAM is working, that your hard drive or solid-state drive is connected, that your keyboard and mouse are responding, and that other core components are present. If something is missing or broken, BIOS will stop and alert you with beep codes or error messages on the screen.

After POST finishes successfully, BIOS looks for your operating system. It checks the devices in a specific order — usually your hard drive first, then a USB drive or CD if you have one plugged in. This order is called the boot order, and you can change it in BIOS settings if you need to install a new operating system or run a repair tool from a USB drive. Once BIOS finds the operating system, it loads it into memory and steps out of the way.

How to enter BIOS and what you will see

To enter BIOS setup, restart your computer and press a specific key repeatedly as soon as the screen lights up. The key varies by manufacturer: Dell and Lenovo computers often use F2, HP and Compaq use F10, and many others use Delete or Escape. You usually have only a few seconds to press the key before BIOS finishes and your operating system starts loading, so you may need to restart and try again if you miss the window.

Once you are in BIOS, you will see a menu with options organized into tabs or categories. Common sections include System Information (showing your CPU, RAM, and storage), Boot Options (where you set the boot order), and Security (where you can set a password to prevent others from changing BIOS settings). Most BIOS menus use arrow keys and Enter to navigate, though some newer systems use a mouse. At the bottom of the screen, you will usually see instructions for saving and exiting.

For most users, the default BIOS settings work fine and do not need to be changed. Changing BIOS settings incorrectly can prevent your computer from starting, so only adjust settings if you have a specific reason — such as installing a new operating system or troubleshooting a hardware problem.

BIOS versus UEFI: what changed

Newer computers use UEFI (Unified Extensible Firmware Interface) instead of traditional BIOS. UEFI does the same job — it runs before your operating system and checks your hardware — but it was designed more recently and works differently under the hood. UEFI supports larger hard drives, loads faster, and has a more modern interface that sometimes includes a mouse and graphics instead of just text and arrow keys.

From a user's perspective, UEFI works almost identically to BIOS. You still restart your computer and press a key to enter the setup menu, and you still see options for boot order, system time, and hardware settings. Many people still call it "BIOS" even when they are using UEFI, because the purpose and the experience are so similar. If your computer was made in the last five to ten years, it almost certainly uses UEFI.

Common BIOS settings you might need to change

Boot order is the most common reason to enter BIOS. If you want to install Windows, Linux, or another operating system from a USB drive, you need to tell BIOS to check the USB drive before the hard drive. You can do this in the Boot menu, where you will see a list of devices. Move the USB drive to the top of the list, save, and restart.

System time and date can drift over time, especially if your computer's battery is old. If your clock is wrong, you can correct it in the System Information or Main menu. Hardware detection settings let you turn components on or off — for example, you might disable the built-in camera or microphone if you do not use them. find Boot is a security feature that prevents unauthorized software from loading before your operating system; it is usually on by default and should stay on unless you are installing a non-standard operating system.

What beep codes and error messages mean

If BIOS detects a problem during POST, it will not load your operating system. Instead, you might hear a series of beeps — a pattern called a beep code — or see an error message on the screen. A single beep followed by a normal startup means everything is fine. One long beep and two short beeps usually means a memory problem. One long beep and three short beeps often points to a video card issue.

Error messages are usually more specific. "No bootable device found" means BIOS could not locate your operating system on any connected drive — this can happen if your hard drive failed, if the boot order is wrong, or if the drive is not properly connected. "CMOS checksum error" means the BIOS settings were corrupted, usually because the battery that stores those settings is dying. If you see an error message, write it down and search for it online along with your computer's model number, because the exact meaning depends on your hardware.

When you might need to reset BIOS to factory settings

If you changed BIOS settings and your computer will not start, you can reset BIOS to its original state. Most computers have a physical jumper or battery on the motherboard that clears BIOS settings when moved or removed. This requires opening the computer case and is not something most users should attempt unless they are comfortable with hardware. A safer option is to restart the computer and enter BIOS setup, then look for a "Load Defaults" or "Reset to Factory Settings" option in the menu.

Some computers also have a key combination you can hold during startup to reset BIOS without opening the case — for example, holding Ctrl and Home on some Dell models. Check your computer's manual or manufacturer website for the exact method for your model.

Frequently Asked Questions

Can I change BIOS settings without breaking my computer?

Most BIOS settings are safe to view, but changing them without knowing what they do can prevent your computer from starting. The safest approach is to change only one setting at a time, write down what you changed, and restart to see if it works. If something goes wrong, you can go back into BIOS and undo the change or reset to factory settings.

What is the CMOS battery and why does it matter?

The CMOS battery is a small coin-shaped battery on your motherboard that keeps BIOS settings and the system clock running even when your computer is unplugged. If this battery dies, your computer will forget its settings and the time will reset every time you power off. Replacing it is inexpensive but requires opening the computer case.

Do I need to enter BIOS to install a new operating system?

Usually yes. You will need to change the boot order so BIOS loads from your USB drive or installation disc instead of your hard drive. After the operating system is installed, you can change the boot order back to the hard drive.

Is it safe to update BIOS?

BIOS updates can fix bugs and improve hardware support, but a failed update can make your computer unbootable. Only update BIOS if you have a specific reason — such as fixing a known problem — and follow your manufacturer's instructions exactly. Do not turn off your computer during a BIOS update.

Why does my computer beep when I start it?

A single beep during startup is normal and means BIOS completed its checks successfully. Multiple beeps in a pattern indicate a hardware problem. Write down the beep pattern and search online with your computer model to find out what it means.