Understanding Linux Startup: BIOS/UEFI

BIOS = Basic Input/Output System

• Type: Firmware (software permanently stored in hardware)

• Location: ROM chip soldered onto the motherboard

• Age: Introduced in 1975, used for decades

• Purpose: First program that runs when you power on your computer

How BIOS Works:

1. Power On → BIOS chip activates immediately

2. POST (Power-On Self-Test) → Checks if hardware components are working

   • Tests CPU, RAM, keyboard, drives, etc.

   • You hear beeps if something fails

3. Hardware Initialization → Activates and configures detected hardware

4. Boot Device Selection → Looks for bootable devices in order (HDD, USB, CD, Network)

5. MBR Loading → Reads the Master Boot Record (first 512 bytes of disk)

6. Hand-off → Transfers control to the bootloader

BIOS Limitations:

❌ Uses 16-bit processor mode (slow and outdated) ❌ Supports only MBR partition scheme (max 4 primary partitions) ❌ Can't handle drives larger than 2TB ❌ Limited user interface (usually text-based) ❌ No network/remote management capabilities ❌ No secure boot features (vulnerable to bootkits/rootkits)

UEFI = Unified Extensible Firmware Interface

• Type: Modern firmware replacement for BIOS

• Introduced: Mid-2000s, became standard around 2010

• Developed by: Intel (originally called EFI), now maintained by UEFI Forum

How UEFI Works:

1. Power On → UEFI firmware initializes

2. POST → Hardware verification (faster than BIOS)

3. UEFI Boot Manager → Reads boot configuration from NVRAM

4. EFI System Partition (ESP) → Looks for bootloader in GPT partition

5. Bootloader Execution → Runs .efi files directly

6. OS Loading → Launches operating system

UEFI Advantages:

✅ Uses 32-bit or 64-bit mode (much faster) ✅ Supports GPT partition scheme (128+ partitions) ✅ Handles drives larger than 2TB (up to 9.4 ZB!) ✅ Graphical user interface with mouse support ✅ Network capabilities (remote diagnostics) ✅ Secure Boot → Prevents unauthorized OS loading ✅ Faster boot times ✅ Better hardware compatibility

BIOS:

• Firmware location: ROM chip on motherboard

• Boot sector: First 512 bytes of disk (MBR)

• Bootloader location: /dev/sda (first sector)

UEFI:

• Firmware location: Flash memory on motherboard

• EFI System Partition (ESP): /boot/efi or /efi

• Bootloader location: /boot/efi/EFI/[distro]/ (e.g., /boot/efi/EFI/ubuntu/grubx64.efi)

• Partition type: FAT32 (usually 100-550 MB)

• Boot Order: Which device to boot from first (HDD, USB, Network)

• Secure Boot: Enable/disable signature verification (UEFI only)

• CSM (Compatibility Support Module): Enables BIOS mode on UEFI systems

• Fast Boot: Skip some POST checks for faster startup

• Virtualization (VT-x/AMD-V): Enable CPU virtualization features

• TPM: Trusted Platform Module for encryption

# Check if your system uses UEFI or BIOS

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

# View EFI variables (UEFI systems)

ls /sys/firmware/efi/efivars/

# Check partition scheme

sudo fdisk -l /dev/sda | grep "Disklabel type"

# Output: "gpt" = UEFI/GPT, "dos" = BIOS/MBR

• Press during boot: F2, F10, F12, Del, or Esc (varies by manufacturer)

• From Linux (UEFI only):

systemctl reboot --firmware-setup

Basic Input/Output System

Unified Extensible Firmware Interface

On a chip on the motherboard (ROM for BIOS, flash memory for UEFI)

Power-On Self-Test - a diagnostic testing sequence that checks if hardware is functioning properly during system startup, performed by BIOS/UEFI

MBR (Master Boot Record)

GPT (GUID Partition Table)

2 TB (Terabytes)

4 primary partitions (or 3 primary + 1 extended)

128 or more partitions

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

A FAT32 partition (usually 100-550 MB) that stores UEFI bootloaders and is typically mounted at /boot/efi

A UEFI feature that verifies digital signatures of bootloaders and OS kernels to prevent unauthorized or malicious software from loading during boot

Performs POST (Power-On Self-Test) to verify hardware functionality

/boot/efi or /efi

FAT32 (vfat)

UEFI operates in 32/64-bit mode (vs BIOS's 16-bit mode) and can initialize hardware in parallel, resulting in faster boot times

systemctl reboot --firmware-setup

Compatibility Support Module - allows UEFI systems to boot in legacy BIOS mode

Detect and initialize hardware, perform POST, and locate/launch the bootloader

sudo fdisk -l /dev/sda | grep "Disklabel type"(Output shows "gpt" for GPT or "dos" for MBR)

.efi (e.g., grubx64.efi, shimx64.efi)

BIOS/UEFI → Bootloader → Kernel → Init System (Systemd)

In NVRAM (Non-Volatile RAM) on the motherboard, accessible in Linux at /sys/firmware/efi/efivars/

512 bytes (first sector of the disk)

True - UEFI supports PXE (Preboot Execution Environment) for network booting