Step 2: Loading the MBR

After the first boot-up checks have been completed, the BIOS will find the primary disk among the available disks and scan the first 512 bytes. Normally, the boot device is found in the Master Boot Record (MBR). The MBR (Figure 2) is a 512-byte area that contains 446 bytes of bootloader code, a 64-byte partition table, and the final two bytes for the boot signature. Initially, the BIOS is not aware of filesystems. As a result, the MBR is stored in a specific location on the disk, which is the first sector of the boot disk. Note that there is an MBR partition on every hard disk.

Figure 2: The MBR Block layout.

Step 3: The Bootloader Phase

After loading the MBR, the system executes the 446 bytes of the primary bootloader. This code is stored at the start of the MBR. The boot sector of the boot disk is located at this location. The bootloader code or the bootstrap code is usually called the Stage 1 bootloader. This code is too small to do any major function like calling a kernel. However, it can simplify things by locating and loading another set of blocks from the partition that contains all the code needed for loading the kernel. This code is actually the Stage 2 bootloader.

The most common Stage 2 bootloader for Linux systems is GNU GRand Unified Bootloader (GRUB). However, there are a number of other options that are not dependent on a certain OS. The main purpose of the Stage 2 bootloader program is to locate the Linux kernel code (a disk image), which is usually inside the /boot filesystem [3], decompress it, read it into memory as per the GRUB configuration file, and transfer control of the system over to the kernel to move on the boot process.

Step 4: The Kernel Phase

Once the kernel starts running, it initializes the system's hardware to determine what devices are present. Also, it initializes the system's device drivers, which allow it to communicate with the hardware. Further, it sets up the system's memory management, and then it mounts the root filesystem, which contains the root directory of the system. Finally, it starts the init process from /sbin/init, which is the systemd process. Systemd is a replacement for the old SysVinit process and is the first userspace process to run on the system with the PID of 1.