Your BIOS is the software that starts your computer before Windows or Mac loads

BIOS stands for Basic Input/Output System. It is the first program that runs when you turn on your computer — before your operating system, before any icons appear on your screen. The BIOS wakes up your hardware, checks that everything is connected and working, and then hands control over to Windows, macOS, or Linux so you can use your computer normally.

Think of it as the computer's startup checklist. When you press the power button, the BIOS runs through a series of tests to make sure your hard drive, memory, keyboard, and other parts are present and functioning. If something is wrong, it will beep or display an error message before your operating system even loads. If everything passes, the BIOS finds your operating system on the hard drive and launches it.

You almost never see the BIOS while using your computer. It does its job in the background, usually in less than a minute. But it is always there, and occasionally you may need to enter it to change a setting or troubleshoot a problem.

Key Takeaways

  • The BIOS runs before your operating system loads and checks that your hardware is working properly.
  • You can enter the BIOS setup screen by pressing a specific key (often Delete, F2, or F12) when ready after turning on your computer.
  • Common BIOS settings include the boot order (which device starts your operating system), date and time, and hardware detection.
  • Most computers have a newer version called UEFI that works the same way but with a more modern design and additional security features.
  • You should only change BIOS settings if you know what you are doing or are following specific troubleshooting steps, as incorrect changes can prevent your computer from starting.

How to enter the BIOS on your computer

To access the BIOS, restart your computer and watch the screen carefully during startup. Within the first few seconds, before Windows or macOS appears, you will see a message telling you which key to press. The key varies by manufacturer: on Dell and HP computers it is often F2 or Delete; on Lenovo it is usually F1 or Enter; on Asus it is often Delete or F2. On a Mac, hold down the Command and R keys during startup to reach a different system called Recovery Mode, which serves a similar purpose.

You must press the key quickly — usually within two or three seconds of the startup message appearing. If you miss the window, your computer will continue loading normally and you will have to restart and try again. Some computers also display the key to press on the initial splash screen, so watch for that message.

Once you are in the BIOS, you will see a text-based menu (or on newer computers, a graphical interface). Use your keyboard arrow keys to navigate. Do not use your mouse — the BIOS runs before your operating system, so the mouse may not work. Read any on-screen instructions carefully before making changes.

Common BIOS settings and what they control

The Boot Order setting tells your computer which device to check first for an operating system. Normally this is your hard drive or solid-state drive. If you are trying to install Windows from a USB drive, you would change the boot order to make the USB drive first, so the computer starts from the installation media instead of looking for Windows on the hard drive.

The Date and Time setting lets you correct the system clock if it has drifted. Some computers lose track of time if the internal battery (called the CMOS battery) is failing, and the BIOS is where you fix it.

Hardware Detection shows you what devices the BIOS found connected to your computer — your hard drive, memory, USB ports, and so on. If a piece of hardware is missing from this list, it may not be properly connected or may be failing.

The Security settings control features like find Boot (which prevents unauthorized software from loading during startup) and password protection for the BIOS itself. Changing these settings can affect whether your computer will start or whether you can install certain operating systems.

BIOS versus UEFI: what is the difference

Most computers made in the last ten years use UEFI (Unified Extensible Firmware Interface) instead of the older BIOS. UEFI does the same job — it starts your hardware and loads your operating system — but it is built on newer technology. UEFI supports larger hard drives, starts faster, and has better security features. It also usually has a graphical interface with a mouse and menus instead of text-only navigation.

From a user's perspective, the difference is minimal. You still restart your computer and press a key to enter the setup screen. You still see similar settings for boot order, date and time, and hardware detection. The main practical difference is that UEFI is more find by default and can handle modern storage devices better than the older BIOS could.

If you are not sure whether your computer uses BIOS or UEFI, you can check in Windows by opening the System Information utility (search for "System Information" in the Start menu) and looking for "BIOS Mode" or "Firmware Type" in the list. On a Mac, this information is less commonly needed because Apple handles firmware updates automatically.

When you might need to change BIOS settings

Most people never need to open the BIOS. Your computer comes with sensible default settings that work for everyday use. However, there are a few situations where you might need to enter it.

If you are installing a fresh copy of Windows or another operating system, you may need to change the boot order so your computer starts from the installation USB drive instead of the hard drive. If your computer is not recognizing a new hard drive or other hardware, you might check the BIOS to see if it detects the device. If your computer will not start at all, a technician might ask you to check the BIOS to see if a component has failed or become disconnected.

Some advanced users change BIOS settings to improve performance or enable specific features, but this requires knowledge of your particular computer model and should only be done if you understand the consequences. Incorrect BIOS changes can prevent your computer from starting, and you may need professional help to fix them.

What to do if you accidentally change the wrong BIOS setting

Most BIOS menus have an option to Reset to Defaults or Load Defaults. If you have made a change and your computer will not start, restart and enter the BIOS again, then look for this option. It will restore all settings to the factory defaults, which almost always allows your computer to start again.

If your computer will not start even before the BIOS loads, or if you cannot enter the BIOS at all, the problem may be hardware-related rather than a settings issue. In that case, contact the computer manufacturer's support line or take the computer to a repair shop. Describe what happened — that you were in the BIOS and made a change — so the technician knows where to look.

Frequently Asked Questions

Can I damage my computer by changing BIOS settings?

You can prevent your computer from starting by changing certain settings, but you cannot physically damage the hardware through the BIOS. If something goes wrong, you can usually reset the BIOS to its default settings and your computer will start again. If you are unsure about a setting, do not change it.

Why does my computer beep when I turn it on?

Beeping during startup is usually the BIOS running its hardware check. A single beep means everything is fine. Multiple beeps or a continuous beep pattern indicates the BIOS found a problem — often a memory module that is not seated properly or a hard drive that is not detected. Consult your computer's manual for the specific beep code.

Is the BIOS password the same as my Windows password?

No. A BIOS password is separate and protects access to the BIOS setup screen itself. Your Windows password protects your user account and files. You can set a BIOS password to prevent others from changing startup settings, but it does not affect your Windows login.

Do I need to update my BIOS?

Most users do not need to update the BIOS. Manufacturers release updates to fix bugs, improve hardware support, or patch security issues. Check your computer manufacturer's website if you are experiencing a specific problem or if you have installed new hardware that is not being recognized. Do not update the BIOS unless you have a reason to do so, as an interrupted update can cause serious problems.

What is the CMOS battery and why does it matter?

The CMOS battery is a small coin-shaped battery inside your computer that keeps the BIOS settings and system clock running when the computer is powered off. If this battery fails, your computer may lose its date and time settings or forget other BIOS changes. Replacing it is a straightforward task for a technician, but not something most users do themselves.