the complete chain from power ON → kernel loaded

os.md

The complete chain from power ON → kernel loaded, with every step mentioned, but kept simple and short.

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Power-on → Kernel load (x86, BIOS, Linux 0.01 style)

1. Power ON

   • Electricity reaches motherboard, CPU resets

2. CPU reset state

   • CPU enters real mode (16-bit)
   • Registers set to fixed values
   • Instruction pointer = 0xFFFF0

3. CPU fetches first instruction

   • Address 0xFFFF0 maps to BIOS ROM
   • CPU reads and executes BIOS code

4. BIOS starts running (from ROM)

   • Tests CPU
   • Initializes chipset
   • Initializes RAM
   • Initializes keyboard, disk, video

5. POST (Power-On Self Test)

   • Checks memory, basic hardware
   • Shows BIOS screen / beep if error

6. BIOS selects boot device

   • Uses boot order (floppy / HDD)
   • Finds a bootable disk

7. BIOS reads boot sector

   • Reads first 512 bytes (sector 0)
   • Loads it into RAM at 0x7C00

8. BIOS jumps to 0x7C00

   • Control passes to bootloader

9. Bootloader starts (boot.s)

   • Still 16-bit real mode
   • Uses BIOS interrupts

10. Bootloader relocates itself

    • Moves from 0x7C00 → 0x90000

11. Bootloader loads kernel

    • Reads kernel from disk
    • Loads it at 0x10000

12. Bootloader prepares CPU

    • Disables interrupts
    • Enables A20 line
    • Sets up GDT & IDT

13. Switch to protected mode

    • Sets PE bit
    • CPU becomes 32-bit

14. Jump to kernel entry

    • Far jump to address 0x00000000
    • Execution enters head.s

15. Kernel startup code (head.s)

    • Sets segments
    • Sets stack
    • Sets paging
    • Calls main() in C

16. Kernel is running

    • Hardware control taken from BIOS
    • OS is now in charge

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One-line summary

Power → CPU reset → BIOS (ROM) → Boot sector (RAM)
→ Bootloader → Protected mode → Kernel