Boot-strapping

In most cases, the operating system is not the first code to run on the computer at startup (boot) time. The initial code executing on the computer is usually loaded from firmware, which is stored in read only memory (ROM). This is sometimes called the BIOS or boot ROM.

The firmware loads and executes code located on a removable disk or hard drive, and contained within the first sector of the drive, referred to as the boot sector. The code stored on the boot sector is called the boot loader, and is responsible for loading the operating system's kernel from disk and starting it running.

Some simple boot loaders are designed to locate one specific operating system and load it, although many modern ones have the capacity to allow the user to choose from a number of operating systems.