✅ What is UEFI?

Unified Extensible Firmware Interface (UEFI) is a specification that defines the software interface between an operating system and platform firmware.
It replaces the legacy BIOS and provides a more flexible and powerful environment for system initialization and OS booting.

✅ Boot Services and Runtime Services

UEFI firmware goes beyond simply initializing hardware—it provides various system services that can be used both before and after the operating system boots. These services are categorized into two main types:

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1️⃣ Boot Services

These are services that are available before the operating system is loaded.
They are primarily used by UEFI applications, such as bootloaders or .efi programs.

Once the OS kernel is loaded, the firmware exits boot services, making them no longer available to the OS.

📌 Key Features:

• Memory allocation and deallocation (AllocatePages, FreePages)

• Event timers and delays

• Protocol discovery and registration

• Driver loading and management

• Device initialization by UEFI drivers

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2️⃣ Runtime Services

These services remain available even after the OS has booted.
They allow the operating system to continue interacting with the firmware for things like timekeeping and system variables.

📌 Key Features:

• Getting and setting system time/date (GetTime, SetTime)

• Reading and writing UEFI variables (NVRAM) (GetVariable, SetVariable)

• Performing firmware-based system reset (ResetSystem)

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🔄 Comparison: Boot Services vs. Runtime Services

✅ Technologies Introduced in UEFI

1️⃣ Fast Boot

✔ Summary

Fast Boot is a feature designed to significantly reduce boot time by simplifying the system initialization process and minimizing the Power-On Self Test (POST) duration.

🧩 Detailed Explanation

In traditional BIOS-based systems, the boot process typically follows this sequence:
Power ON → Load BIOS → Initialize all hardware → Load OS
This process is often slow, especially during hardware initialization.

In contrast, UEFI with Fast Boot can display the motherboard manufacturer’s logo and begin loading the operating system almost immediately, resulting in dramatically faster boot times.

Fast Boot works by skipping the initialization of "trusted devices"—devices that do not require reinitialization during every boot.

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🔍 How does Fast Boot determine “trusted hardware”?

UEFI firmware uses the following criteria to decide which devices can be safely skipped:

Many motherboards also offer Fast Boot level options:

• Minimal: Skip most devices

• Thorough: Reinitialize all devices

• Auto: Determine based on system state

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💡 Real-World Examples

• Windows "Fast Startup" combines UEFI’s Fast Boot concept with hybrid sleep to accelerate boot time.

• On some motherboards, when Fast Boot is enabled, pressing Del or F2 to enter the BIOS/UEFI setup may not work. In this case, you can use Windows' Advanced Startup Options to reboot into firmware settings.

• In Linux or dual-boot environments, Fast Boot may cause device detection issues, so disabling Fast Boot is often recommended.

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✍️ One-line Summary

Fast Boot is a UEFI technology that dramatically improves boot speed by skipping initialization of devices that were stable and unchanged in the previous boot.

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2️⃣ Secure Boot

✔ Summary

Secure Boot is a security feature that only allows digitally signed code to run during the boot process, helping prevent the execution of low-level malware such as rootkits and bootkits.

🧩 Detailed Explanation

• Every boot component (e.g., bootloader, kernel) must have a valid digital signature.

• The firmware will only execute files with verified signatures.

• The list of trusted public keys is stored in UEFI NVRAM, typically organized into structures like DB (allow list) and DBX (deny list).

• If a bootloader is changed or tampered with, and the signature doesn’t match, the system will block the boot process.

💡 Real-World Examples

• Only signed bootloaders such as Microsoft’s bootmgfw.efi or a Secure Boot-compatible GRUB loader are allowed to execute.

• When installing a Linux distribution, it's important to check whether it is Secure Boot-compatible.

• Secure Boot helps protect the kernel and drivers from unauthorized modifications and root-level malware attacks.

3️⃣ Bootloader Management at the Firmware Level

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✔ Summary

In UEFI systems, the firmware remembers and manages bootloaders (programs that start the operating system).
When you install an OS, its bootloader is automatically registered, and the firmware lets you choose which OS to boot first.

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🧩 Explained Simply

✅ In the BIOS era:

• The system only remembered which disk to boot from, not which OS.

• It blindly ran the bootloader from the MBR (Master Boot Record) of the selected disk.

• If you wanted multiple operating systems, you had to manually install and configure something like GRUB.

✅ In the UEFI era:

• Bootloaders exist as separate .efi files in the EFI System Partition (ESP).

• UEFI recognizes each bootloader and associates it with a specific OS.

• Each one is stored and managed under names like Boot0000, Boot0001, etc.

• You can also set the boot order (BootOrder) to determine which OS loads first.

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💡 Real-World Example

Let’s say you install both Windows and Ubuntu on the same system:

• Ubuntu automatically registers its bootloader as grubx64.efi.

• Windows already has bootmgfw.efi registered.

• UEFI stores both entries with names and order in NVRAM.

• When you power on your computer, UEFI decides “Which OS should I boot?” based on the configured boot order.

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💡 Tip

• On Linux, you can use efibootmgr to view or change boot entries.

• On Windows, you can use bcdedit for similar tasks.

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4️⃣ Standard Interface for OS-Level Access to Firmware Settings

✔ Summary

UEFI provides a standard interface that allows the operating system to access and modify firmware settings—such as boot order, Secure Boot state, and UEFI variables—from within the OS.
Thanks to this, users and system software can manage bootloaders, change settings, or control security features without rebooting into the firmware setup.

🧩 Explained Simply

✅ In the BIOS era:

• You had to press Del or F2 during boot to enter the BIOS setup screen.

• It was impossible to access or modify BIOS settings from within the OS.

✅ In the UEFI era:

• The OS can read and change UEFI settings directly, thanks to:

  • UEFI Runtime Services

  • EFI Variables exposed by the firmware

From within the OS, you can:

• Change the boot order

• Enable or disable Secure Boot

• Specify which bootloader to run next (BootNext)

• Reboot directly into the UEFI firmware setup screen

💡 Real-World Examples

🪟 On Windows:

shutdown /r /fw /t 0

→ Reboots directly into the UEFI settings screen

bcdedit /set {current} path \EFI\ubuntu\grubx64.efi

→ Changes the current bootloader path

🧾 Summary Table

* Secure Boot changes may require elevated permissions or BIOS password