The PC boot process

Booting is the sequence a computer follows to bring itself from a powered-off state to a running operating system. When a PC is switched on, firmware stored on the motherboard takes control first. Traditionally this was the BIOS (Basic Input/Output System), which performs a power-on self-test to check that essential hardware is working, initialises devices, and then looks for a startup program on a boot device. The term “booting” comes from the idea of a machine pulling itself up by its bootstraps from almost nothing.

Having selected a boot device according to a configured order, the firmware loads a small piece of code from it into memory and hands over control. That code, in turn, locates and loads the operating system, which initialises drivers for the hardware and presents the user environment. A boot disk is any medium prepared with this startup code and enough of a system to run; historically these were floppy disks, later CDs, and today most often USB keys or the internal drive.

A bootable USB key is a flash drive written with the necessary boot code and files so the firmware will start from it. Such keys are widely used to install operating systems, to run diagnostic or recovery tools when the main system will not start, and to carry portable environments. Creating one requires writing the appropriate boot information and copying the system files, after which the device must be selected in the firmware's boot order. Understanding this chain — firmware, boot code, then operating system — helps in diagnosing startup failures.

Frequently asked questions

What does the BIOS do at startup?

The BIOS runs a power-on self-test to check the hardware, initialises devices, and then loads startup code from the boot device, which goes on to load the operating system.

What is a boot disk?

A boot disk is any medium, such as a USB key, CD, or floppy, prepared with startup code and system files so the computer can start from it.

Why use a bootable USB key?

Bootable USB keys are used to install operating systems and to run diagnostic or recovery tools when the main system fails to start, because the computer can boot directly from them.