OS Booting

• Boot Loaders
• Full process of Booting OS
• See Disk_Partitioning

System Firmware Standards

BIOS

• Basic Input/Output System
• Legacy firmware standard
• Lacks native support for modern hardware stacks (NVMe drives, USB 3+, or complex mouse/graphics interfaces)
• Performs Power-On Self-Test (POST) to verify basic hardware (CPU, RAM, CMOS)
• Automatically reads the first sector (LBA 0) of the designated boot drive into RAM address 0x7C00
• Jump execution to the MBR code

UEFI

• Unified Extensible Firmware Interface
• Initially created by Intel as EFI but now under UEFI Forum
  • https://uefi.org/
  • https://en.wikipedia.org/wiki/UEFI
  • It replaces the legacy BIOS
  • Boots faster than BIOS
• Defines
  • GPT scheme
  • Secure Boot
  • CSM Boot
  • EFI System Partition
  • Boot Loaders
  • UEFI Shell
  • NVRAM Variables
  • GUI and native mouse navigation support
  • Better POST support
  • support extensible drivers like PXE (Network Boot) or storage controllers (NVMe/RAID/AHCI)
• Uses Secure Boot by default and only loads signed boot loaders
• UEFI Firmware has Built in UEFI Shell
  • Also available in ESP at \shellx64.efi
• Stores boot priorities and state telemetry in non-volatile hardware storage (NVRAM)
  • accessible via /sys/firmware/efi/efivars on Linux
• NVRAM Boot Variables
  • BootOrder
    • A hexadecimal boot order sequence
  • BootXXXX
    • Human-readable device tag
    • Direct hardcoded structural entry path to the boot file on the ESP
    • Example: Boot0001, Boot0002

POST

• Power-On Self Test
• Confirms presence of essential hardware components
  • Central Processing Unit (CPU)
  • System Memory (RAM)
  • Graphics Card/Video Module
• Execute Diagnostic System Integrity Checks
• Prevent Cascade Hardware Damage
• Initialize Motherboard Chipsets