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From Power On to Main(): How an OS Boots

SystemsOS

When you press the power button, magic happens. But it’s not magic - it’s legacy engineering layered on newer standards.

Before your beautiful React app loads, your CPU has to wake up in a mode that resembles a 40-year-old Intel 8086 chip.

1. The Firmware Phase (BIOS or UEFI)

The PSU stabilizes power. The CPU wakes up and jumps to a reset vector. Firmware code runs on a flash chip.

BIOS path (legacy):

  1. Runs POST (Power-On Self-Test).
  2. Scans for a bootable device.
  3. Reads the first 512 bytes (Sector 0) into memory at 0x7C00 and jumps there.

UEFI path (modern):

  1. Loads a boot manager from the EFI System Partition (ESP).
  2. Boots a .efi executable (like GRUB or systemd-boot).
  3. Passes control with a richer API and memory map.

2. The Bootloader

Legacy BIOS gives you only 512 bytes of stage-1 bootloader space. That’s why modern bootloaders are multi-stage:

  1. Stage 1: Tiny loader in the MBR.
  2. Stage 2: Understands filesystems and loads the kernel image.

Bootloaders also pass critical data to the kernel: memory maps, CPU features, and boot arguments.

3. CPU Mode Switch

The CPU starts in Real Mode (16-bit). It can address only 1MB of RAM. A real OS needs Protected Mode (32-bit) or Long Mode (64-bit).

; Switch to Protected Mode (simplified)
cli                   ; Disable interrupts
lgdt [gdt_descriptor] ; Load Global Descriptor Table
mov eax, cr0
or eax, 1             ; Set the PE (Protection Enable) bit
mov cr0, eax
jmp 08h:clear_pipe    ; Far jump to flush CPU pipeline

In 64-bit mode, you also enable paging and load a 64-bit GDT.

4. Kernel Entry

The bootloader jumps to the kernel entry point (often a C function like kmain()), but the kernel still needs to set up basics:

  1. Stack: Allocate a safe stack for C code.
  2. IDT/GDT: Interrupt and descriptor tables.
  3. Paging: Virtual memory and page tables.
  4. Device drivers: Keyboard, screen, disk, timers.

There is no printf. There is no malloc. You have to build them yourself.

5. Userspace Init

Once the kernel is stable, it launches the first user process:

  • Linux: init or systemd
  • BSD: init

That process starts services, networking, and finally your login manager or shell.

That is the joy of systems programming: an entire world built from nothing but a few bytes of boot code.

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