A Layman's Guide to Linux: The Boot Loader

The Boot Loader is the part of the computer that gets executed first. When you only have one Operating system on your computer, it simply loads the Kernel but when you have multiple Operating Systems or multiple versions of the Linux Kernel then, it allows you to choose which one you wish to start.

This Blog is Part 2 of my series The Layman's Guide to Linux. In this part, I'll discuss what happens when you turn on your Computer.

When we turn on the Computer, it undergoes a process called POST (Power On Self Test). During this process, the Basic Input-Output System or the BIOS initialize the Hardware components and tests the main memory. Simultaneously in a quick flash, the branding from the Manufacturer of the system is displayed.

This is followed by a direction to press a certain combination of keys to gain access to the Boot configuration or to run the setup.

BIOS software is stored on a ROM chip on the motherboard. After this remaining Boot Process is controlled by Operating System. The successor to the BIOS is Unified Extensible Firmware Interface also known as EUPHY. Regardless of which specification of the two your system is using, when referring to the Initial Boot sequence, the term BIOS is commonly used.

The setup software that the computer uses is controlled by CMOS. A complementary metal-oxide-semiconductor i.e., CMOS is the small amount of memory on the computer motherboard that stores the BIOS setting.

Information on Date, Time and the most important peripherals are loaded from CMOS values (First 64 bytes of memory located in CMOS chip). This CMOS chip is kept alive by the CMOS battery even when the computer is turned off.

The MBR or the Master Boot Record is the information which gives the computer instructions to either present the Menu or jump ahead to load the Operating System depending on how things are configured.

On a Linux Machine, the Master Boot Record processes its instructions and sends you to the Boot Loader screen.

After POST, the system control passes from BIOS to the Boot Loader. The boot loader is generally stored on one of the Hard Disks of the computer either in the boot sector or the EFI (Extensible Firmware Interface) partition.

When you initially install Linux, the software will ask you where to install the Boot Loader. You can also perform these actions manually when you divide disk partitions and then specify which partition should the Boot Flag.

Linux Boot Loader is responsible for loading the Kernel image, initial RAM disk or Filesystem (contains critical files and device drivers) into the memory.

Today the most widely used Boot Loader on the PC is the Microsoft Windows Boot manager. If you ever decide to install windows after you install Linux on your system, then you will not be able to boot Linux again until you use a rescue disk and install an alternative Boot Loader.

Fortunately, there are some good alternatives. They will automatically detect your Operating System and present you with the menu to choose the Operating system you want to run at the moment.

The most widely used on the majority of Linux Distributions is from the GNU Project. It is the Grand Unified Boot Loader or the GRUB. GRUB loads the kernel or the filesystem into the memory from the /boot directory in your root filesystem. In addition to performing this function, special commands can be passed before loading the kernel so that specialized hardware may be able to operate. Command Line parameters to set backlight or screen resolution are a few examples.

This was all about the Boot Loader, how it functions, and the series of steps your system undergoes right from turning on to the computer to start working on it.

If you are new to Linux and want to learn more about it then check out my entire series of Blogs: A Lyman's Guide to Linux. This series is like a crash course that will introduce you to the Fundamentals of the Linux Operating system.

If you want to explore further and learn more about the Boot Process then watch these videos:

Let's Connect:

Twitter: https://twitter.com/_aviral07

LinkedIn: https://www.linkedin.com/in/aviral07/

Showwcase: https://www.showwcase.com/aviralsharma07