From Power On to Main(): How an OS Boots
When you press the power button, magic happens. But it’s not magic—it’s legacy engineering from the 1980s.
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 BIOS/UEFI Phase
The PSU stabilizes power. The CPU wakes up. It looks at a specific memory address (0xFFFFFFF0 on modern x86) for the Reset Vector. This jumps to the BIOS (or UEFI) code on the motherboard’s flash chip.
The BIOS runs POST (Power-On Self-Test). RAM okay? Keyboard connected? Then, it looks for a bootable device (HDD, SSD, USB).
It reads the first 512 bytes (Sector 0) of the drive. This is the MBR (Master Boot Record). If the last 2 bytes are 0x55AA, it loads that code into memory at 0x7C00 and jumps there.
2. The Bootloader (Legacy Mode)
You now have 512 bytes of code space. That’s microscopic. Modern bootloaders (like GRUB) are split into stages.
- Stage 1: Fits in the MBR. Its only job is to load Stage 2.
- Stage 2: Understands file systems (FAT32, EXT4) and loads the Kernel image.
3. Entering Protected Mode
Your CPU is still in Real Mode (16-bit). It can only address 1MB of RAM. To run a real OS, we need Protected Mode (32-bit) or Long Mode (64-bit).
; Switch to Protected Mode
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
4. The Kernel
Finally, the bootloader passes control to the kernel’s entry point (often a designated C function like kmain()).
From here, you are the god of the machine. You have to write drivers for the screen, the keyboard, the disk. There is no printf. There is no malloc. You have to build them yourself.
That is the joy of Systems Programming.