What Is a Boot Loader?

A boot loader is a very small piece of software code that runs during the boot sequence of a computer or other device. It copies critical files and components of an operating system from a storage device, like a hard drive, into the computer’s main memory. The program will then give the operating system control and quit until the next boot. Boot loaders vary in complexity and are also used in devices like smartphones, tablets, and video game consoles.

The boot process in most personal computers (PCs) is a complex event with multiple stages. The central processing unit (CPU) inside most PCs is not capable of running programs directly from a storage device, like a hard disk, so the first piece of software to run when the computer is turned on is loaded from a special microchip on the motherboard, the computer’s central circuit board. This microchip may contain one of several different specialized programs, including the basic input output system (BIOS) or the unified extensible firmware interface (UEFI), that perform hardware testing and initialization functions before searching hard disks and removable storage devices for a boot loader. Code from the boot loader is then transferred into system memory, which takes control of the computer and moves the boot sequence onto the next step.

Once in control, the boot loader proceeds to load the essential drivers and critical system files needed to run an operating system such as Microsoft Windows®, Linux, or Mac OS X®. Boot loaders are often platform specific, since drivers and system files vary widely between different systems. They may also run in stages, with each stage of the program loading a more complex and powerful piece of software. On computers that have more than one operating system installed, a program called a boot manager lets users select a system to boot and hands over control to the appropriate boot loader. After an operating system is loaded, the boot loader relinquishes its control of the system and exits.

Smartphones, tablets, video game consoles, consumer electronics products, and many other types of devices also use boot loaders. If a device goes through a period of booting or loading when first powered on, there is a good chance it is using a boot loader. Some technology enthusiasts have been known to hack or replace the boot loaders of certain devices in order to run custom software or enable more advanced features.