After an operating system is generated, it must be available for use by the hardware. But how does the hardware know where the kernel is, or how to load that kernel? The procedure of starting a computer by loading the kernel is known as Booting the system. Hence it needs a special program, stored in ROM to do this job known as the Bootstrap loader. Example: BIOS (boot input-output system). A modern PC BIOS (Basic Input/Output System) supports booting from various devices. Typically, the BIOS will allow the user to configure a boot order. If the boot order is set to:

Then the BIOS will try to boot from the CD drive first, and if that fails then it will try to boot from the hard disk drive, and if that fails then it will try to boot from the network, and if that fails then it won’t boot at all. Booting is a startup sequence that starts the operating system of a computer when it is turned on. A boot sequence is the initial set of operations that the computer performs when it is switched on. Every computer has a boot sequence. Bootstrap loader locates the kernel, loads it into main memory, and starts its execution. In some systems, a simple bootstrap loader fetches a more complex boot program from disk, which in turn loads the kernel. 

Process of Booting

Dual Booting: When two operating systems are installed on the computer system then it is called dual booting. In fact, multiple operating systems can be installed on such a system. But how does the system know which operating system is to boot? A boot loader that understands multiple file systems and multiple operating systems can occupy the boot space. Once loaded, it can boot one of the operating systems available on the disk. The disk can have multiple partitions, each containing a different type of operating system. When a computer system turns on, a boot manager program displays a menu, allowing the user to choose the operating system to use.