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BIOS and UEFI

Main source of this document: LPIC-1 Exam 101 Learning Material: 101.2

UEFI and BIOS short overview​

What is it?​

BIOS (Basic Input/Output System) and UEFI (Unified Extensible Firmware Interface) are pre-installed firmware typically integrated into the motherboard. UEFI is the successor to BIOS and is what we will encounter in most modern computers. As such, we will mostly focus on UEFI.

note

The term "BIOS" is often used to refer to this firmware that stands between hardware and OS, regardless of it being UEFI or the actual BIOS.

We can usually access BIOS/UEFI by pressing the delete button on the keyboard during the power-on process.

Broadly, what is it for?​

Loading and passing the control OS kernel​

One of the main responsibilities of BIOS/UEFI is to load and launch the bootloader and pass control to it (which in turn loads the OS kernel and passes control to it).

Tweaking low-level hardware settings​

We can tweak low-level hardware settings via BIOS/UEFI. For example, you usually enable memory overclocking from the BIOS/UEFI when building gaming PCs.

UEFI: Deeper Dive​

UEFI can detect and run EFI Applications. One example of an EFI application is a bootloader such as GRUB. EFI Applications are stored in a partition called ESP (EFI System Partition).

The settings of UEFI are stored in a non-volatile memory called NVRAM. This setting persists even when the power is turned off. NVRAM stores UEFI settings such as boot order, hardware configuration, etc. When you change a configuration, UEFI updates the content of NVRAM.

We can think of UEFI as a platform, NVRAM stores the settings for the platform, and EFI Applications as the applications that can run on the UEFI platform.

Pre-bootloader boot steps of UEFI​

On machines with UEFI, the pre-operating system boot steps are:

  1. The POST (power-on self-test) is executed to identify hardware failures
  2. UEFI activates basic components to load the system, such as video output, keyboard, and storage
  3. UEFI firmware reads configurations stored at NVRAM and executes the pre-defined EFI application stored in ESP's filesystem. Usually, this is a bootloader such as GRUB.

Extra things to note​

  • The EFP is compatible with FAT12, FAT16, and FAT32 filesystem and ISO-9660 for optical media.
  • Secure boot is a part of the UEFI standard, which only allows the execution of EFI applications authorized by the hardware manufacturer.
  • GUID Partition Table is also a part of the UEFI standard.

BIOS: Difference to UEFI​

To boot with BIOS, we need something called MBR (Master boot record). MBR is a data structure located on the first 512 bytes of a disk. It contains the first stage of the bootloader.

Instead of detecting external partitions for the bootloader, BIOS looks for MBR in the first storage device (in the order defined in the BIOS config utility). It then loads the first stage of the bootloader, which in turn calls the second stage. The second stage of the bootloader is responsible for presenting boot options and loading the OS kernel.

Last updated: July 7, 2023