Tag Archives: Master Boot Record

What happens when you turn a computer on?

Working on a computer is so easy nowadays that we find even children handling them expertly. However, several things start to happen when we turn on the power to a computer, before it can present the nice user-friendly graphical user interface (GUI) screen that we call the desktop. In a UNIX-like operating system, the computer goes through a process of booting, BIOS, Master Boot Record, Bootstrap Loading, grub, init, before reaching the operating level.


As soon as you switch on the computer, the motherboard initializes its own firmware to get the CPU running. Some registers, such as the Instruction Pointer of the CPU, have permanent values that point to a fixed memory location in a read only memory (ROM) containing the basic input output system (BIOS) program. The CPU begins executing the BIOS from the ROM.


The BIOS program has several important functions, which begin with the power on self-test (POST) to ensure all the components present in the system are functioning properly. POST indicates any malfunction in the form of audible beeps. You have to refer to the Beep Codes of the motherboard to decipher them. If the computer passes the test for the video card, it displays the manufacturer’s logo on its screen.

After checking, BIOS initializes the various hardware devices. This allows them to operate without conflicts. Most BIOSs follow the ACPI create tables for initializing the devices in the computer.

In the next stage, the BIOS looks for an Operating System to load. The search sequence follows an order predefined by the manufacturer in the BIOS settings. However, the user can change this Boot Order to alter the actual search. In general, the search order starts with the hard disk, CD-ROMs, and thumb drives. If the BIOS does not find a suitable operating system, it displays an error. Otherwise, it reads the master boot record (MBR) to know where the operating system is located.

Master Boot Record

In most cases, the operating system resides in the hard disk. The first sector of the hard disk is the master boot record (MBR), and its structure is independent of the operating system. It consists of a special program, the bootstrap loader, and a partition table. The partition table is actually a list of all the partitions in the hard disk and their file system types. The bootstrap loader contains the code to start loading the operating system. Complex operating systems such as Linux use the grand unified boot loader (GRUB), which allows selecting of one of the several operating systems present on the hard disk. Booting an operating system using GRUB is a two-stage process.


Stage one of the GRUB is a tiny program and its only task is to call stage two, which contains the main code for loading the Linux Kernel and the file system into the RAM. The Kernel is the core component of the operating system, remains in the RAM throughout the session, and controls all aspects of the system through its drivers and modules. The last step of the kernel boot sequence is the init, which determines the initial run-level of the system. Unless otherwise instructed, it brings the computer to the graphical user interface (GUI) for the user to interact.