BIOS is the first software 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 tests your hardware, loads settings you have configured, and then hands control over to Windows or whatever operating system you use.

When you press the power button, BIOS is what runs first. It checks that your RAM is working, finds your hard drive, detects your keyboard and mouse, and makes sure the basic pieces are present and functional. Only after BIOS finishes this work does your computer load Windows or another operating system from the hard drive.

Most people never see BIOS because it works invisibly in the background. But if something goes wrong during startup—if your computer beeps instead of booting, or if it cannot find your hard drive—BIOS is often where the problem lives.

Key Takeaways

  • BIOS runs before your operating system and checks that your hardware is working before Windows or another OS loads.
  • You can enter BIOS setup by pressing a key during startup (usually Delete, F2, or F12, depending on your computer brand) to change settings like boot order or enable hardware features.
  • BIOS stores settings on a small battery-powered chip, so if that battery dies, your settings reset to factory defaults.
  • Modern computers increasingly use UEFI instead of traditional BIOS, which offers more features and faster startup, but works the same way from a user's perspective.

What BIOS actually does when your computer starts

The moment you turn on your computer, BIOS begins a process called the Power-On Self Test, or POST. This is a series of checks that verify your hardware is present and working. BIOS tests your RAM by writing data to it and reading it back, checks that your hard drive is connected and readable, and confirms that your keyboard and mouse are responding.

If BIOS finds a problem during POST, it stops and alerts you. A single beep usually means everything is fine. Multiple beeps, a long beep followed by short beeps, or no beep at all signals a specific hardware problem—a missing RAM stick, a disconnected hard drive, or a failed component. Different beep patterns mean different things, and your computer's manual or manufacturer website can decode them.

After POST passes, BIOS looks for a bootable device. By default, it searches your hard drive for the operating system and loads it. If your hard drive is missing or corrupted, BIOS can be configured to look elsewhere—a USB drive, a CD, or a network location—which is how you install Windows or recover a broken system.

How to access BIOS and what you can change there

You enter BIOS setup by pressing a specific key during the startup sequence, before Windows loads. The key varies by manufacturer: Dell and HP computers often use F2 or Delete, Lenovo uses F1 or Enter, and some Asus machines use Delete or F2. Your computer usually displays a message on the startup screen saying "Press [key] to enter Setup" or "Press [key] to enter BIOS."

Once inside BIOS, you can change settings that affect how your computer behaves. The most common changes are: adjusting the boot order (which device the computer tries to start from first), enabling or disabling hardware features like USB ports or the built-in camera, setting the system time and date, and adjusting power settings like whether the computer wakes from sleep when you move the mouse.

You can also set a password so only you can enter BIOS, or enable security features like find Boot, which prevents unauthorized software from loading during startup. Most users never need to change BIOS settings, but technicians and power users adjust them regularly to troubleshoot problems or optimize performance.

BIOS settings are stored on a battery-powered chip

Your BIOS settings do not live on your hard drive. Instead, they are stored on a small chip called the CMOS (Complementary Metal-Oxide-Semiconductor), which is powered by a coin-sized battery on your motherboard. This battery keeps the settings alive even when your computer is unplugged.

If that battery dies—which can take years, but eventually happens—your BIOS settings reset to factory defaults. Your computer will still work, but custom settings like boot order or disabled hardware features revert. You will see this if your computer suddenly forgets the time and date, or if it tries to boot from an unexpected device.

Replacing the CMOS battery is a straightforward task if you are comfortable opening your computer case. The battery is straightforward to locate and pops out like a watch battery. If you are not comfortable opening your computer, a technician can do it in minutes.

UEFI is the modern replacement for traditional BIOS

Newer computers, especially those built in the last ten years, use UEFI (Unified Extensible Firmware Interface) instead of traditional BIOS. UEFI does the same job—it initializes hardware and loads your operating system—but it is faster, supports larger hard drives, and has a more user-friendly interface with a mouse and graphics instead of text menus.

From a user's perspective, UEFI works identically to BIOS. You still press a key during startup to enter setup, you still configure boot order and hardware settings, and you still see a startup sequence before Windows loads. The difference is mostly behind the scenes: UEFI is more modern and efficient, but the fundamental purpose is unchanged.

If your computer is less than five years old, it almost certainly uses UEFI. If it is older, it probably uses traditional BIOS. Both are still in use, and both work reliably. Windows runs on both without any difference in how your computer behaves day to day.

When BIOS problems cause startup failures

If your computer will not start, BIOS is often the culprit. Common BIOS-related startup problems include: the hard drive not being detected (usually because a cable came loose), the boot order being set incorrectly (so the computer looks for Windows on a USB drive instead of the hard drive), or a corrupted BIOS that prevents the computer from initializing at all.

If your computer beeps repeatedly or displays an error message before Windows loads, the problem is almost certainly in BIOS or hardware, not in Windows itself. Restarting the computer, checking that all cables are firmly connected, and resetting BIOS to factory defaults (usually done by removing the CMOS battery for a few minutes) solve most of these problems.

If BIOS itself becomes corrupted—which is rare—you may need to reflash it, a process that rewrites the BIOS code from a file on a USB drive. Most manufacturers provide tools and instructions for this, but it is a more advanced task. If you are not comfortable with it, a technician can handle it quickly.

BIOS security and why it matters

BIOS controls what software your computer is allowed to run during startup. A feature called find Boot checks that your operating system and startup files have not been modified or replaced by malware. If find Boot detects a problem, it stops the startup process and prevents the compromised software from loading.

You can also set a BIOS password to prevent someone from changing these security settings or accessing BIOS setup without permission. This is useful if your computer is shared or if you want to prevent accidental changes to critical settings.

For most home users, the default BIOS settings are find enough. But if you work with sensitive data or use your computer in an environment where physical security is a concern, spending time in BIOS to understand and configure these features is worthwhile.

Frequently Asked Questions

What happens if I change the wrong setting in BIOS?

Most BIOS changes do not break anything. If you change the boot order or disable a hardware feature, you can always change it back. If you are worried, write down the original settings before you change anything, or use the "Load Defaults" option to reset everything to factory settings. Only a few settings—like flashing the BIOS itself—can cause serious problems.

Can I update BIOS to fix problems or improve performance?

Yes. Manufacturers release BIOS updates to fix bugs, add features, or improve compatibility with new hardware. You can check your computer manufacturer's support website to see if an update is available. BIOS updates are usually safe, but they should be done carefully—do not turn off your computer during the update, and make sure your power supply is stable.

Why does my computer take so long to start if BIOS runs first?

BIOS itself is fast—it usually finishes in a few seconds. The delay you see is mostly Windows loading from the hard drive, which takes longer. UEFI computers start faster than BIOS computers because UEFI is more efficient, but the difference is usually only a few seconds.

Is BIOS the same as the CMOS?

No. BIOS is the software program that runs at startup. CMOS is the chip that stores your BIOS settings. They work together: BIOS reads the settings from CMOS and uses them to configure your hardware.

Can I access BIOS from inside Windows?

On newer computers with UEFI, yes. Windows 10 and 11 have a "Restart to UEFI Firmware Settings" option in the Advanced Startup menu. On older computers with traditional BIOS, you must restart and press the BIOS key during startup. Restarting to BIOS from inside Windows is faster and safer than the old method.