Unraveling Linux Boot: From BIOS Power‑On to Kernel Init

Linux System Boot Process

When a computer powers on, the CPU starts in real mode at address 0xFFFF0, which points to the BIOS stored in a ROM chip. The BIOS performs the Power‑On Self‑Test (POST), initializes hardware, and builds an interrupt vector table in the first 1 KB of RAM.

Boot Sequence and MBR

The BIOS follows the boot order configured in CMOS, reads the first sector of the selected device (the Master Boot Record, 512 bytes). The MBR contains a 446‑byte primary boot loader, a 64‑byte partition table for up to four primary partitions, and the 0x55AA signature.

The primary boot loader scans the partition table, finds the active partition, loads its secondary boot loader (e.g., GRUB or LILO) into memory, and transfers control.

Secondary Boot Loader (GRUB)

GRUB can read ext2/ext3 filesystems, presents a menu for selecting an operating system, and loads the chosen Linux kernel image (zImage or bzImage) into memory using BIOS interrupt 0x13 services.

Kernel Decompression and Transition to Protected Mode

The compressed kernel image is decompressed (via decompress_kernel), placed in high memory, and the kernel’s start_kernel() routine initializes hardware, memory management, and the init process.

Init Process and Runlevels

The first user‑space program is /sbin/init, which reads /etc/inittab to set the runlevel (0‑6) and executes the appropriate rc scripts, loads kernel modules, and finally starts /bin/login for user authentication.

# This script will be executed *after* all the other init scripts.
# You can put your own initialization stuff in here if you don’t
# want to do the full Sys V style init stuff.