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 a small program stored on a chip inside your computer that runs the moment you press the power button — before your operating system even exists on the screen. Think of it as the computer's startup checklist: it tests the hardware, finds the hard drive, and hands control over to Windows or macOS so the rest of your computer can begin.

Without BIOS, your computer would not know what to do when you turn it on. It would not know where to find your operating system, whether your keyboard works, or whether your hard drive is connected. BIOS is the bridge between the physical machine and the software you use every day.

Key Takeaways

  • BIOS runs automatically when you start your computer and checks that all hardware is working before loading your operating system.
  • 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 may need to adjust them if you add new hardware, want to boot from a USB drive, or need to reset a forgotten password.
  • BIOS has been gradually replaced by UEFI (Unified Extensible Firmware Interface) on newer computers, which works the same way but with a more modern design.

How BIOS starts your computer

When you power on your computer, the CPU when ready runs BIOS code from a special chip on the motherboard. BIOS then performs what is called the POST — Power-On Self-Test. During this test, BIOS checks that your RAM is working, your hard drive is connected, your keyboard and mouse respond, and all other essential hardware is present and functioning.

If BIOS detects a problem during POST, it will beep or display an error message and stop. If everything passes, BIOS looks for your operating system. It searches the hard drive (or another device you have told it to search), finds the boot files, and loads them into memory. Once the operating system takes over, BIOS steps into the background and your computer operates normally.

This entire process takes only a few seconds on modern computers, but it is happening every single time you start up. BIOS is the reason you see a brief screen with your computer manufacturer's logo before Windows appears.

Entering BIOS setup to change settings

BIOS includes a setup menu where you can change how your computer behaves. To enter this menu, you must press a specific key during the startup process — before the operating system loads. The key varies by manufacturer: on Dell computers it is usually F2, on HP and Lenovo it is often Delete or F2, and on some systems it is F12 or Escape. You will usually see a message on the startup screen telling you which key to press.

Once inside BIOS setup, you will see options organized into menus. Common settings include the boot order (which device to check first for the operating system), the system date and time, hard drive detection, and security features like a startup password. The menus look different depending on your computer brand, but they all serve the same purpose: controlling how your hardware behaves before the operating system loads.

If you make a change you do not understand, you can usually press a key to restore all settings to their factory defaults. This is a safe way to undo a mistake without damaging your computer.

When you might need to change BIOS settings

Most people use their computer for years without ever opening BIOS. However, certain situations require you to enter BIOS and make a change. If you install a new hard drive or solid-state drive, BIOS may need to detect it — sometimes this happens automatically, but occasionally you must enable it in the setup menu. If you want to boot from a USB drive to install a new operating system or run a repair tool, you may need to change the boot order in BIOS so the computer checks the USB port before the hard drive.

If you forget your Windows password, some computers allow you to reset it through BIOS, though this varies by model and security settings. If your computer is running slowly or overheating, you might check BIOS to see whether a fan is disabled or a hardware setting has been changed. In rare cases, a BIOS update from the manufacturer can fix bugs or improve performance, though this should only be done if your computer is plugged in and you follow the manufacturer's instructions exactly.

BIOS versus UEFI: what changed

Newer computers no longer use traditional BIOS. Instead, they use UEFI (Unified Extensible Firmware Interface), which does the same job but with a more modern design. UEFI can handle larger hard drives, boots faster, and has a graphical interface instead of text-only menus. However, from the user's perspective, UEFI works almost identically to BIOS: you still press a key during startup to enter setup, you still change boot order and hardware settings, and it still runs before your operating system loads.

Most computers made after 2015 use UEFI, though many still call it "BIOS" out of habit. If you have a newer computer and look for "BIOS setup," you are actually entering UEFI setup. The terms are often used interchangeably, and for practical purposes, they work the same way.

What happens if BIOS fails or becomes corrupted

BIOS is stored on a chip that is designed to be very reliable, and corruption is rare. However, if BIOS becomes damaged — usually from a failed update, a power loss during an update, or a hardware fault — your computer may not start at all. You might see no display, repeated beeping, or error messages about missing hardware.

If this happens, you have limited options. Some computers allow you to reset BIOS to factory settings by removing a small battery from the motherboard for a few minutes, then reinstalling it. Others require a BIOS update from the manufacturer, which may need to be done from a USB drive or by taking the computer to a repair shop. This is one situation where calling a professional technician is the safest choice, because incorrect steps can make the problem worse.

Security features in BIOS

BIOS includes security options that can protect your computer from unauthorized use. You can set a startup password that must be entered before the computer will boot, or a setup password that prevents someone from changing BIOS settings without permission. Some computers also have a hard drive password that encrypts the drive, making it unusable if stolen.

These passwords are stored on the BIOS chip itself, not in Windows, so they remain active even if someone tries to bypass Windows security. However, if you forget a BIOS password, recovery can be difficult and may require professional help or a trip to the manufacturer. For this reason, write down any BIOS passwords you set and store them somewhere safe.

Frequently Asked Questions

Can I damage my computer by changing BIOS settings?

You can cause problems — such as the computer not starting or hardware not being recognized — but you cannot permanently damage the hardware itself by changing settings. Most BIOS menus include a "restore defaults" option that will undo your changes. If you make a change and something stops working, restart the computer, enter BIOS again, and restore the defaults.

Why does my computer take a long time to start up?

BIOS startup is usually fast, but if your computer is slow to boot, the problem is usually in Windows or your hard drive, not BIOS. However, if you see a long delay before the Windows logo appears, you can try entering BIOS and disabling hardware detection for devices you do not use, or checking whether a USB device is plugged in that BIOS is trying to boot from.

Do I need to update BIOS?

Most people do not need to update BIOS. Manufacturers release updates to fix specific bugs, add support for new hardware, or improve security. Check your computer manufacturer's website only if you are experiencing a problem that their support team says a BIOS update will fix, or if you are installing very new hardware that requires it.

What is the difference between BIOS and the CMOS battery?

BIOS is the program itself. The CMOS battery is a small coin-shaped battery on the motherboard that keeps BIOS settings and the system clock running when the computer is powered off. If the battery dies, your BIOS settings will reset to defaults and the clock will lose time, but BIOS itself will still work once you power the computer on again.

Can I access BIOS from Windows?

On newer computers with UEFI, Windows includes a restart option that boots directly into UEFI setup without requiring you to press a key during startup. Look in Windows Settings under System, then Recovery, and select "Restart now" under Advanced startup. On older computers, you must restart and press the BIOS key during the boot process.