What the BIOS is
The BIOS, which stands for Basic Input/Output System, is a type of firmware - software built into a chip on a computer's motherboard. It is the very first code that runs when you turn on a computer, before the operating system loads. The BIOS is responsible for waking up and checking the hardware, then handing control over to the operating system. It acts as the essential bridge between the physical machine and the software that runs on it.
What the BIOS does at startup
When you press the power button, the BIOS springs into action. It performs a self-check of the computer's essential hardware to make sure components like memory and storage are present and working. It initializes these components so they are ready to use. Then it locates the operating system on a storage device and starts loading it, transferring control so the OS can take over. This startup sequence is the BIOS's most visible and important job.
Firmware, not ordinary software
Unlike the programs you install and run, the BIOS is firmware: software stored permanently on a hardware chip so it is always available the instant the computer powers on, even before any operating system exists on the machine. This permanence is essential, because something has to run first to get everything else going. The BIOS lives close to the hardware and provides the low-level foundation that the rest of the system is built upon.
BIOS settings
The BIOS also provides a settings screen you can usually enter by pressing a key right as the computer starts. Here you can adjust low-level options, such as the order in which the computer looks for an operating system to boot, and basic hardware configuration. These settings are stored so they persist between restarts. For most people the defaults are fine, but the BIOS setup is useful for troubleshooting and configuring how the machine starts.
From BIOS to UEFI
The traditional BIOS has largely been succeeded in modern computers by a newer kind of firmware called UEFI, which serves the same fundamental purpose but with more capabilities, a friendlier interface, and support for modern hardware. People often still say 'BIOS' when referring to this startup firmware in general. Whether called BIOS or UEFI, its role is the same: the first software to run, preparing the hardware and launching the operating system.
Why it matters
The BIOS is the unsung foundation that brings a computer to life every time you power it on. Understanding what the BIOS is - firmware that initializes the hardware and starts the operating system - clarifies what actually happens in those first moments after you press the power button. For anyone curious about how computers start up, or who needs to troubleshoot boot problems, the BIOS is a foundational concept.
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See also: What is an operating system? Explained, What is a kernel? Explained simply.
Sources
Published date reflects the original event date (2023-10-03). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.
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