# BIOS and UEFI

> The first program a PC runs: it checks the hardware and starts the operating system.

- Level: Intermediate
- Topics: Hardware
- URL: https://tidbit.legios.com.ar/bios-and-uefi/
- In Spanish: [BIOS y UEFI](https://tidbit.legios.com.ar/es/bios-and-uefi/)

When you press the power button, the processor knows nothing about your disk, your keyboard or Windows. It starts by running a small program stored in a chip on the motherboard. On older PCs that program was the **BIOS** (Basic Input/Output System); on almost every computer made since the early 2010s it is **UEFI** (Unified Extensible Firmware Interface), although many people still call it "the BIOS" out of habit.

Its job is short and essential. It checks that the CPU, the RAM and the other parts respond, sets them up, finds a disk with an operating system and hands control to the **boot loader**, the small program that starts the system proper. It also keeps settings like the boot order, the clock and whether virtualization is turned on. You reach its setup screen by pressing a key such as `F2` or `Del` right after power-on.

UEFI replaced the BIOS because the old design had hard limits: it could not boot from disks larger than roughly 2 TB and offered little protection. UEFI handles big disks, starts faster, has a setup screen you can use with a mouse, and adds **Secure Boot**, which only lets boot loaders signed with trusted keys start. That makes it much harder for malware to hide underneath the operating system. Because it is firmware, manufacturers occasionally release updates for it, usually to fix bugs or security holes.

## An analogy

The stage manager who arrives before everyone else, turns on the lights, checks the microphones and then hands the stage to the main act. The audience barely notices, but without it the show never starts.

## Keep in mind

- It runs from a chip on the motherboard every time the computer turns on.
- UEFI is the modern replacement for the BIOS; many people still call both "BIOS".
- Secure Boot only lets signed, trusted boot loaders start.
- Updates fix bugs and security holes, but a failed one can leave the machine unable to start.

## Go deeper

- [BIOS on Wikipedia](https://en.wikipedia.org/wiki/BIOS)
- [UEFI on Wikipedia](https://en.wikipedia.org/wiki/UEFI)
- [Booting on Wikipedia](https://en.wikipedia.org/wiki/Booting)

## Related terms

- [CPU](https://tidbit.legios.com.ar/cpu/): The chip that runs your programs, one instruction after another, very fast.
- [Chip](https://tidbit.legios.com.ar/semiconductor-chip/): A sliver of silicon carrying billions of tiny switches that compute and remember.
- [Motherboard](https://tidbit.legios.com.ar/motherboard/): The main circuit board that every other part of a computer plugs into.
- [RAM](https://tidbit.legios.com.ar/ram/): The computer's short-term memory: fast, small and wiped when you turn it off.
- [Operating system](https://tidbit.legios.com.ar/operating-system/): The base software that runs a device and lets every app share its hardware.
- [Hypervisor](https://tidbit.legios.com.ar/hypervisor/): The software layer that splits one physical computer into many virtual machines.
- [Malware and ransomware](https://tidbit.legios.com.ar/malware-and-ransomware/): Software built to harm you; ransomware locks your files and charges to unlock them.
- [Firmware](https://tidbit.legios.com.ar/firmware/): Software built into a device to run its hardware, from routers to washing machines.