The Linux Boot Process: From Power Button to Kernel

Summary

The Linux Boot Process: From Power Button to Kernel Saturday. October 25, 2025 - 15 mins Part 1 — From power button to the kernel’s first breath You press the power button. A second later a wall of text scrolls by, or a logo fades in, and eventually Linux appears. What happens in between is not magic. It is a careful handshake between tiny programs and a very literal CPU. This part follows that handshake until the very first line of C code inside the Linux kernel runs. The very first instruction When power stabilizes, the CPU resets itself to a tiny, old‑fashioned mode called real mode. Real mode dates back to the original 8086 chip. The rules are simple on purpose. Memory addresses are built from two values the CPU keeps in special fast storage called registers. You combine a segment and an offset like this: physical_address = (segment << 4) + offset If you see numbers like 0xFFFFFFF0, that is hexadecimal. Hex is base 16. We write 0x in front to make that clear. 0x10 is 16 in everyday counting. 0x100000 is 1 megabyte. Hex lines up nicely with how hardware stores bits, which is why you see it everywhere in low‑level code. Right after reset the CPU jumps to a special address called the reset vector at 0xFFFFFFF0. Think of it as a permanent bookmark that says “start here.” There is room for almost nothing at that address, so manufacturers put a short jump there that passes control to the firmware on your motherboard. Tiny explainer: register A register is a tiny slot inside the CPU. It holds a number the CPU is using right now. Names like CS and IP are register names. CS means “code segment,” which marks the current neighborhood for instructions. IP means “instruction pointer,” which marks which instruction comes next. BIOS and UEFI The firmware is a small starter program baked into your board. BIOS stands for Basic Input Output System. It is the older style. BIOS does a quick health check called POST, looks at the boot order, and tries each device. If it finds a disk...