Glossary · Hardware

BIOS and UEFI

BIOS and UEFI are firmware that initializes hardware at power-on and starts the operating system; UEFI is the modern replacement for BIOS.

Updated October 6, 2026

BIOS and UEFI are the firmware layers stored on the motherboard that run first when a computer starts. They test and initialize hardware, then load the bootloader. UEFI is the modern successor to legacy BIOS.

Why it matters

UEFI supports large disks via GPT, faster boot, a graphical setup menu, and security features such as Secure Boot. Legacy BIOS uses MBR partitions and a 2 TB boot disk limit.

You enter the setup screen with a key such as Del, F2 or Esc during startup to change boot order, enable virtualization, or adjust memory settings.

Example

$ [ -d /sys/firmware/efi ] && echo UEFI || echo Legacy BIOS
UEFI

This Linux check reveals which mode the current system booted in.

Common mistakes

  • Installing an OS in legacy mode on a disk meant for UEFI, causing boot failure.
  • Interrupting a firmware update, which can brick the board.
  • Leaving virtualization disabled and then wondering why virtual machines fail.

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