This article explains the process through which the Linux kernel loads. In a more technical sense, it explains how the kernel gets into memory and what it actually does until the first user process begins its execution.

Boot Processes

At a very high level, the Linux boot processes pass through the five steps described below. Some texts can describe it in four or six steps; however, the basic mechanism remains the same. The boot process starts at the moment either when you start a shutdown system or restart an already running system.

Irrespective of the computer or OS standard ("IBM-compatible"), computer systems use either the classic BIOS/MBR [1] boot process or the modern UEFI/GPT boot process. UEFI firmware and conventional BIOS both manage system boot, but the boot method and settings differ for each.

BIOS-Based Booting

The classic BIOS/MBR boot process (as shown in Figure 1) involves five steps: the BIOS POST, loading the MBR, the bootloader phase, the kernel phase, and the startup process.

Figure 1: The traditional BIOS booting sequence.

Step 1: The BIOS POST

The BIOS is located on a chip on the motherboard of a computer in read-only or flash memory and performs the Power-On Self-Test (POST) operation. This is not special to Linux itself. It is related to the hardware initialization mechanism and is similar among all operating systems. The POST is a series of tests to ensure that the computer's hardware is functioning properly. Basically, it does hardware verification in terms of availability and integrity, as well as checking things such as the BIOS itself, CPU registers, RAM, and everything else the computer requires. During the POST operation, the system displays system messages and installs the required video drivers if needed [2].