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 Windows, macOS, or Linux loads. Think of it as your computer's startup checklist — it tests your hardware, finds your hard drive, and hands control over to your operating system so you can actually use your machine.

Every desktop and laptop has BIOS (or a newer version called UEFI). You almost never see it unless something goes wrong or you deliberately open it to change a setting. But it is running in the background every single time you power on your computer.

Key Takeaways

  • BIOS runs before your operating system loads and checks that your hardware is working correctly.
  • You can enter BIOS by pressing a specific key during startup — usually Delete, F2, or F12, depending on your computer brand.
  • BIOS settings control things like boot order, fan speed, and whether certain ports are turned on or off.
  • UEFI is the modern replacement for BIOS and works the same way but with a better interface and more features.
  • You should only change BIOS settings if you know what you are doing, because wrong changes can prevent your computer from starting.

What BIOS does when your computer starts

The moment you press the power button, BIOS wakes up before anything else. It runs a test called POST (Power-On Self Test) that checks whether your RAM, hard drive, keyboard, and other hardware are connected and working. If something is broken or unplugged, BIOS will beep at you or show an error message on the screen.

Once BIOS confirms everything is okay, it looks for your operating system. It checks your hard drive (or SSD) in the order you have told it to look — this order is called the boot order. When it finds Windows or macOS, it loads that program and steps out of the way. From that point on, your operating system is in charge.

This whole process usually takes just a few seconds, and you do not see any of it unless something fails. If your computer will not start at all, BIOS is often the culprit — either because hardware is broken or because a setting got changed accidentally.

How to open BIOS and what you will see

To enter BIOS, restart your computer and press a specific key repeatedly as soon as it powers on — before Windows or macOS appears. The key varies by manufacturer: Dell and HP often use F2, Lenovo uses F1, Asus uses Delete, and some older machines use F12. You will usually see a message on the startup screen telling you which key to press.

Once you are in BIOS, you will see a menu with options like System Information, Boot, Security, and Power. The layout and colors vary depending on whether you have older BIOS or newer UEFI. UEFI looks more like a modern computer menu with a mouse pointer; older BIOS is text-based and you navigate with arrow keys.

Do not panic if you accidentally open BIOS — you can press Escape to exit without saving any changes. Nothing happens unless you deliberately change a setting and then select "Save and Exit."

Common BIOS settings you might need to change

Boot order is the most common reason people open BIOS. If you want to install Windows from a USB drive, you need to tell BIOS to look at the USB first instead of your hard drive. You can change the order so USB is at the top of the list.

Fan speed is another setting some people adjust. If your computer is running hot, you can increase fan speed in BIOS, though this makes the computer louder. Some BIOS menus let you set the fan to automatic, which adjusts speed based on temperature.

Other settings include turning ports on or off (like USB ports or audio jacks), changing the date and time, and enabling or disabling security features. Most people never need to touch these. If you are not sure what a setting does, leave it alone — wrong changes can make your computer unbootable.

UEFI is the modern replacement for BIOS

Newer computers use UEFI (Unified Extensible Firmware Interface) instead of traditional BIOS. UEFI does the same job — it starts your computer and loads your operating system — but it is faster, more find, and has a better-looking menu. You open UEFI the same way you open BIOS, and the settings work almost identically.

Most computers made after 2010 use UEFI, though they often still call it "BIOS" in casual conversation. If you see a colorful menu with a mouse pointer when you restart, that is UEFI. If you see white text on a black background with arrow keys, that is older BIOS. Either way, the process is the same: change what you need, save, and exit.

When BIOS problems prevent your computer from starting

If your computer will not turn on or gets stuck on a black screen, BIOS might be the problem. Common causes include a loose hard drive cable, RAM that is not fully seated, or a dead CMOS battery (a small battery on the motherboard that stores BIOS settings). If you hear beeping, that is BIOS telling you something is wrong.

If you changed a BIOS setting and now your computer will not start, you can usually reset BIOS to factory defaults. Most computers have a key you can hold down during startup (often F9 or Delete) to reset BIOS. Check your computer's manual or manufacturer website for the exact key.

If your computer is completely unresponsive and will not even beep, the problem is likely hardware, not BIOS. In that case, you may need to take it to a repair shop or contact the manufacturer.

Why you should be careful changing BIOS settings

BIOS is powerful — it controls the very foundation of how your computer works. A single wrong setting can prevent your computer from starting, make it run slowly, or cause hardware to overheat. This is why BIOS is usually password-protected on work computers and why manufacturers make it hard to find.

If you are not sure what a setting does, do not change it. There is almost no reason to open BIOS unless you are installing a new operating system, troubleshooting a hardware problem, or following specific instructions from a technician. Even then, write down what you changed so you can undo it if something goes wrong.

Frequently Asked Questions

What is the difference between BIOS and UEFI?

UEFI is the newer version of BIOS. Both do the same job — they start your computer and load your operating system — but UEFI is faster, more find, and has a graphical menu instead of text. Most computers made after 2010 use UEFI, though people still call it "BIOS" informally.

Can I damage my computer by opening BIOS?

Just opening BIOS and looking around will not hurt anything. Damage only happens if you change a setting and save it. If you accidentally change something, you can usually reset BIOS to factory defaults by holding a key during startup — check your computer's manual for which key.

Why is my computer asking me to press a key to enter BIOS?

This usually means BIOS detected a problem during startup — a loose cable, missing hard drive, or failed hardware test. Press the key it shows you to enter BIOS and look for error messages. If you see one, it will tell you what is wrong.

Do I need to change BIOS settings to make my computer faster?

Not usually. BIOS settings affect how your hardware works, but changing them rarely makes a noticeable speed difference for everyday use. If your computer is slow, the problem is almost always your operating system, software, or a full hard drive — not BIOS.

What happens if I forget my BIOS password?

If you set a BIOS password and forgot it, you will need to reset BIOS to factory defaults. This usually means opening your computer case and removing the CMOS battery for a few minutes, or pressing a reset button on the motherboard. Check your computer's manual for the exact steps, or contact the manufacturer.