Section 4: What is UEFI?

The Big Picture: A Modern Replacement

By the 2000s the old BIOS was holding PCs back: 16-bit code, a 2 TB disk limit, text-only menus. The industry built a replacement from scratch. It is called UEFI. It does the same job as the BIOS (check hardware, set up devices, start the OS), but it is built like a small operating system of its own, written in modern code, with a mouse-friendly screen, network support and security features.

4.1 Definition (Unified Extensible Firmware Interface)

What it is: UEFI stands for Unified Extensible Firmware Interface.

Important: UEFI is a specification (a written standard, a rulebook). Companies such as AMI, Insyde and Phoenix write firmware that follows it. It is managed by the UEFI Forum, a group of many companies.

Where it came from:

What UEFI looks like to you: Often a nicer graphical setup screen, usable with a mouse, often with a language menu and "EZ Mode" and "Advanced Mode". The setup screen is called "UEFI Setup" or "UEFI Firmware Settings", but most people still call it "BIOS".

4.2 Advantages Over BIOS

What it is: UEFI fixes the problems from Section 1.3.

A key idea: GPT versus MBR. UEFI works best with the newer disk layout, GPT (GUID Partition Table). Here is a picture of a GPT disk:

Figure 5.5 — A GPT disk from start to end. At the very start: LBA 0 holds a Protective MBR, LBA 1 the Primary GPT Header, then the partition entries 1 to 128 (up to LBA 33). Partitions begin at LBA 34. At the very end of the disk sit a second copy of the entries and a Secondary GPT Header. Diagram: Kbolino, CC BY-SA 2.5, via Wikimedia Commons.

(LBA means Logical Block Address: just the number of a sector, counting from 0.)

Why the "Protective MBR"? At LBA 0, GPT disks keep a fake MBR that says "this whole disk is used". Old tools that only know MBR see a full disk and will not damage the real GPT data.

Check your disk in Windows (PowerShell):

Get-Disk | Select-Object Number, FriendlyName, PartitionStyle

PartitionStyle shows GPT or MBR.

4.3 32-bit vs 64-bit

What it is: The "bitness" of UEFI means which kind of code the firmware runs. Most PCs today use 64-bit UEFI with a 64-bit CPU.

Why this matters:

• UEFI starts the CPU in real mode at reset, but quickly moves it into protected mode and then 64-bit mode. The firmware and its drivers run in 64-bit, with lots of memory and fast code.
• The OS loader is a 64-bit program (on Windows bootmgfw.efi). The firmware's bitness must match it: a 64-bit UEFI loads a 64-bit loader.
• Some small tablets and cheap PCs have a 32-bit UEFI on a 64-bit CPU. This is called mixed mode. Normal 64-bit Windows installers may not boot on them without special help.

File name convention. UEFI looks for a fallback loader named for the architecture:

Check on Windows: In msinfo32, System Type shows "x64-based PC", and BIOS Mode shows UEFI.

4.4 Modular Architecture

What it is: UEFI is built from many separate modules (small pieces of code), not one big program. The firmware is a pile of these pieces, and the boot process loads them in a set order.

Why modular?

• Different makers can write different pieces (the CPU maker writes the CPU part, the board maker the board part, the firmware company the rest).
• New hardware only needs a new driver, not a new firmware.
• Bugs can be fixed in one module.

The big layers:

┌─────────────────────────────────────────────────┐
│ Operating system (Windows, Linux)               │
├─────────────────────────────────────────────────┤
│ UEFI boot loader / applications (.efi files)    │
├─────────────────────────────────────────────────┤
│ UEFI services: Boot Services + Runtime Services │
├─────────────────────────────────────────────────┤
│ UEFI drivers and protocols                      │
├─────────────────────────────────────────────────┤
│ Platform firmware (chipset, CPU, board code)    │
├─────────────────────────────────────────────────┤
│ Hardware                                        │
└─────────────────────────────────────────────────┘

The boot phases (preview). UEFI follows the PI (Platform Initialization) design, a companion standard. The firmware moves through several phases:

You will see these in detail in Section 5.

Free code. A well-known open-source UEFI codebase is EDK II (from the TianoCore project). Many commercial firmwares are built from it.

Windows tip: Look at the firmware's own version info:

Get-CimInstance Win32_BIOS | Format-List Manufacturer, SMBIOSBIOSVersion, ReleaseDate

Summary