Link to this headingBooting

Steps:

  1. POST (Power On Self Test)
  2. BIOS Firmware
    • Initialize CPU, Memory, etc.
    • Check for hardware errors
    • Search for boot loader
  3. Boot loader
    • In the MBR (Master Boot Record)/Boot Sector. The first 512 bytes of the device. The last bytes of this sector must end in 55AA.
    • Transfer from 16 bit to real mode

Link to this headingBoot loader

References:

Custom Boot loader:

[BITS 16] ; Start in 16 bit mode [ORG 0x7c00] ; Start Code at this address CODE_OFFSET equ 0x8 DATA_OFFSET equ 0x10 start: cli ; Clear Interrupts mov ax, 0x00 mov ds, ax mov es, ax mov ss, ax mov sp, 0x7c00 ; Start the stack at 0x7c00 going up to 0x0000 sti ; Enable Interrupts jmp load_PM mov si, message ; Move message address into the SI register print: lodsb ; Deference address at SI and load into the AL register. Then increment the SI register cmp al, 0 ; Check if reached the end of the string je done mov ah, 0x0E ; Set the "display char" argument int 0x10 ; Send the common/video Interrupt command jmp print ; Continue the print loop done: cli ; Clear Interrupts hlt ; Stop the CPU load_PM: cli ; Clear Interrupts lgdt[gdt_descriptor] ; Load the GDT from the descriptor mov eax, cr0 ; Clear the CR0 register or al, 1 ; Set the Protected Mode enable mov cr0, eax jmp CODE_OFFSET:pmode_main message: db "Hello World", 0 ; GDT Implementation https://wiki.osdev.org/Global_Descriptor_Table gdt_start: db 0x00000000 ; NULL entry db 0x00000000 ; GDT Entry 1 ; Code Segment Descriptor https://wiki.osdev.org/Global_Descriptor_Table#Segment_Descriptor dw 0xFFFF ; Limit dw 0x0000 ; Base db 0x00 ; Base db 10011010b ; Access Byte: Present Bit, Code Segment Bit, Executable Bit, Read Bit db 11001111b ; Flags and Limit Byte: Granularity Flag, 4KB Blocks, 32bit segment db 0x00 ; Base ; Data Segment Descriptor https://wiki.osdev.org/Global_Descriptor_Table#Segment_Descriptor dw 0xFFFF ; Limit dw 0x0000 ; Base db 0x00 ; Base db 10010010b ; Access Byte: Present Bit, Code Segment Bit, Read Bit db 11001111b ; Flags and Limit Byte: Granularity Flag, 4KB Blocks, 32bit segment db 0x00 ; Base gdt_end: gdt_descriptor: dw gdt_end - gdt_start - 1; Size of GDT dd gdt_start ; Start of GDT Address [BITS 32] pmode_main: mov ax, DATA_OFFSET ; Setup 32 bit registers mov ds, ax mov es, ax mov fs, ax mov gs, ax mov ebp, 0x9C00 ; Setup stack mov esp, ebp in al, 0x92 ; or al, 2 ; out 0x92, al ; times 510 - ($ -$$) db 0 ; Pad the data so that we can have the full 512 block db 0x55 ; Ending data db 0xAA

Build & Run:

>>> nasm -f bin boot.asm -o boot.bin >>> hexyl boot.bin ┌────────┬─────────────────────────┬─────────────────────────┬────────┬────────┐ │00000000│ fa b8 00 00 8e d8 8e c0 ┊ 8e d0 bc 00 7c fb be 1e │××⋄⋄××××┊×××⋄|×ו│ │00000010│ 7c ac 3c 00 74 06 b4 0e ┊ cd 10 eb f5 fa f4 48 65 │|×<⋄t•×•┊ו××××He│ │00000020│ 6c 6c 6f 20 57 6f 72 6c ┊ 64 00 00 00 00 00 00 00 │llo Worl┊d⋄⋄⋄⋄⋄⋄⋄│ │00000030│ 00 00 00 00 00 00 00 00 ┊ 00 00 00 00 00 00 00 00 │⋄⋄⋄⋄⋄⋄⋄⋄┊⋄⋄⋄⋄⋄⋄⋄⋄│ │* │ ┊ │ ┊ │ │000001f0│ 00 00 00 00 00 00 00 00 ┊ 00 00 00 00 00 00 55 aa │⋄⋄⋄⋄⋄⋄⋄⋄┊⋄⋄⋄⋄⋄⋄U×│ └────────┴─────────────────────────┴─────────────────────────┴────────┴────────┘ >>> qemu-system-x86_64 -hda ./boot.bin

Link to this headingLinux

Booting modern Intel CPUs
https://binarydebt.wordpress.com/2018/10/06/how-does-an-x86-processor-boot/
ArchLinux Boot process
Linux Insides Booting
https://raw.githubusercontent.com/Stefan20162016/linux-insides-code/master/bootloader.asm minimal Linux bootloader
https://github.com/shikaan/osle A tiny and mighty boot sector OS

  1. Real Mode
  2. Start executing commands from 8086 reset vector

Linux Bootloaders:

  • BIOS
  • MBR
  • GRUB
  • Kernel
  • INIT/Systemd

Link to this headingMac Booting

Booting on the Apple T2
Mac Secure Boot
AsahiLinux Boot Process