Boot Process

• x86: i386 typically uses a BIOS, x64 typically uses UEFI
• ARM is not standardized

BIOS

• Binary Input and Output System
  • Stored in ROM on the chip
• Legacy circa 2017

Steps

1. Execute BIOS Init (drivers, etc.)
2. Run Power-On Self Test (POST)
   • Checks what peripherals are connected and working
3. Get boot device via NVRAM (no standard)
   • NVRAM (non-volatile RAM) stores the settings for the BIOS
4. Load boot sector into volatile RAM
5. Execute Boot Block program from the boot sector (one block)
6. Bootstrap OS with Bootloader routine
   • Bootloader

Storage

UEFI

• Unified Extensible Firmware Interface
• Main difference: boot settings are stored in a standardized way
  • Allows for editing settings when running the system (and then rebooting)

Steps

1. Execute architecture firmware
2. Execute EFI manager (init & drivers)
3. Get boot device via NVRAM (standardized)
4. On disks, get boot loader from EFI partition

Storage

• Protective Master Boot Record: Tells a BIOS machine that the disk is not invalid so it would not try to reformat it if it is accidentally plugged into a BIOS machine
• Secondary header copies the secondary partition entries back into the primary in the case that it is for the first gets corrupted