What does BIOS stand for?
BIOS stands for Basic Input/Output System. Legacy firmware that initializes hardware and starts the operating system. Modern systems use UEFI, which adds Secure Boot.
The BIOS is the firmware on a PC's motherboard that runs first when the computer is switched on. It tests the basic hardware in the power-on self-test, initializes memory, storage and other devices, and then loads the boot loader of the operating system from the configured boot device. Its settings, such as the boot order and the clock, are kept in non-volatile memory. On current PCs the original BIOS design has been replaced by UEFI, although the setup screen is still commonly called the BIOS. Because it runs before any operating system, firmware that has been tampered with is very hard to detect, which is why updates must come from the manufacturer.
BIOS in practice: an example
A PC shows no bootable device after a second drive is added. The technician opens the firmware setup and finds that the new, empty drive is first in the boot order. Putting the system drive back in first place fixes the start-up. Nothing was wrong with Windows.
What BIOS is confused with
BIOS vs UEFI: what is the difference?
The legacy BIOS is the older firmware design, which boots from MBR disks. UEFI is the current design, with support for GPT disks and Secure Boot. People often say BIOS for both. What UEFI stands for
BIOS vs CMOS: what is the difference?
The BIOS is the firmware program. CMOS is the traditional name for the small memory, kept alive by a coin battery on older boards, that stores its settings and the clock.
Which exams cover BIOS?
BIOS is on the official CompTIA acronym list for two of the four exams covered here.
- Security+ SY0-801: on the acronym list, not tied to one objective.
- A+ 220-1201 and 220-1202: 220-1201 3.0 (Hardware); 220-1202 2.0 (Security).
Each number is a CompTIA exam objective and the name in brackets is its domain. The exam puts the term in a scenario; it does not ask you to recite the expansion.
Check yourself: two practice questions
Written by CertSprint for this page. They are not taken from any exam.
1. A PC powers on but tries to start from the network before the internal drive. Where is this order changed?
- In the firmware (BIOS or UEFI) setup
- In the printer settings
- In the browser
- In the screen saver settings
Show the answer
A. In the firmware (BIOS or UEFI) setup. The boot order is a firmware setting, applied before any operating system is loaded.
2. A desktop PC loses the date and time every time it is unplugged. Which part is the most likely cause?
- The power supply fan
- The CMOS battery
- The network cable
- The graphics card
Show the answer
B. The CMOS battery. The coin battery keeps the clock running and, on older boards, the firmware settings while the PC has no power. A flat battery resets them.
Sources: NIST SP 800-147, BIOS Protection Guidelines; Microsoft Learn: Boot to UEFI mode or legacy BIOS mode; NIST glossary: Basic Input/Output System. The acronym and its expansion come from CompTIA's official acronym lists, linked from each exam glossary above. The explanation, example and questions are CertSprint's own, checked against these sources on 11 November 2026.