The role of BIOS and UEFI
When you press the physical ON button on your computer, what exactly happens? Even before Windows appears, a series of crucial steps are executed thanks to what is called the system firmware — historically the BIOS, today often replaced or supplemented by the UEFI.
Role of BIOS / UEFI
The BIOS (Basic Input/Output System) and the UEFI (Unified Extensible Firmware Interface) are firmwares pre-installed on the motherboard. Their role is:
- To perform the initial hardware tests at startup (POST — Power-On Self Test).
- To locate a boot device (e.g., hard drive, SSD) containing an operating system.
- To configure basic hardware components (RAM, CPU, input/output devices).
- To provide an interface between the hardware and the operating system.
- To allow access to certain system options (boot order, password, virtualization options, etc.).
What happens at startup
Here is a simplified sequence of what happens from the moment power arrives:
-
Pressing the power button: Power is supplied to the motherboard.
-
Activation of the firmware chip: A small non-volatile memory, often a ROM (EEPROM) soldered to the motherboard, contains the BIOS or the UEFI.
-
POST (Power-On Self Test): The firmware checks the RAM, processor, keyboard, graphics cards, etc.
-
Initialization of buses and devices: The CPU relies on the firmware to know how to communicate with devices (PCIe, USB, etc.).
-
Detection of the boot device: The firmware reads the first sector of the boot disk (MBR for BIOS, or GPT table + .efi file for UEFI).
-
Loading the bootloader: The BIOS launches a program like GRUB or Windows Boot Manager, which in turn launches the operating system.
-
Handover to the operating system: Once the operating system kernel is launched, the BIOS/UEFI remains in the background (UEFI can remain accessible via its runtime services).
Where are BIOS and UEFI stored?
They are stored in non-volatile memory on the motherboard, often in the form of an EEPROM chip.
Differences between BIOS and UEFI
The table below summarizes the main differences between BIOS and UEFI:
| Aspect | BIOS | UEFI |
|---|---|---|
| User interface | Text only | Graphical interface possible |
| Maximum disk size | ~2 TB (MBR limit) | > 9 ZB with GPT |
| Supported partition types | MBR | GPT |
| Secure Boot | ❌ No | ✅ Yes |
| Possible extension (drivers) | ❌ No | ✅ Yes, via EFI applications |
| Boot speed | Slower | Faster |
| Presence on recent systems | Rare | Standard since ~2015 |
Where does BIOS / UEFI fit in the system architecture?
In a modern PC, the BIOS/UEFI acts as the first link between:
- The CPU, which has no "program" in memory initially.
- The chipset, which manages communication between the CPU and devices.
- Storage, input/output, and communication devices.
Accessing BIOS/UEFI on Windows
To access the BIOS/UEFI, you generally need to press a specific key (like F2, DEL, ESC) right after turning on the computer. This varies depending on the motherboard manufacturer.