In computing, booting (booting up) is a bootstrapping process that starts operating systems when the user turns on a computer system. A boot sequence is the initial set of operations that the computer performs when power is switched on. The bootloader typically loads the main operating system for the computer.

History

The computer word boot is short for 'bootstrap' (short for 'bootstrap load'). The term bootstrap began as a metaphor derived from pull straps sewn onto the backs of leather boots with which a person could pull on their boots without outside help. In computers in the 1950s, pressing a bootstrap button caused a hardwired program to read a bootstrap program from a punched card and then execute the loaded boot program which loaded a larger system of programs from punched cards into memory, without further help from the human operator. In a computing context, that word has been used since at least 1958.

The GE 645 (c. 1965) had a 'BOOT' button – it could be that the contraction started as a way to label the button with fewer letters than the full word.

The Multics operating system (c. 1967) had a boot command. Multics documents also refer to 'boot tapes', but it is hard to determine exactly when that term was first used.

In the Unix operating system, the earliest reference for 'boot' is probably in The Unix Programmer's Manual, first edition 1971.11.03.

The bootstrap concept was used in the IBM 701 computer (1952-1956) which had a "load button" which initiated reading of the first 36-bit word from a punched card in a card reader, or from a magnetic tape unit, or drum unit (predecessor of the harddisk drive). The left 18-bit half-word was then executed as an instruction which read additional words into memory.

Boot loader

A computer's central processor can only execute program code found in Read-Only Memory (ROM) and Random Access Memory (RAM). Modern operating systems and application program code and data are stored on nonvolatile data storage devices, such as hard disc drives, CD, DVD, USB flash drive, and floppy disk. When a computer is first powered on, it does not have an operating system in ROM or RAM. The computer must initially execute a small program stored in ROM along with the bare minimum of data needed to access the nonvolatile devices from which the operating system programs and data are loaded into RAM.

The small program that starts this sequence of loading into RAM, is known as a bootstrap loader, bootstrap or boot loader. This small boot loader program's only job is to load other data and programs which are then executed from RAM. Often, multiple-stage boot loaders are used, during which several programs of increasing complexity sequentially load one after the other in a process of chain loading.

Early computers (such as the PDP-1 through PDP-8 and early models of the PDP-11) had a row of toggle switches on the front panel to allow the operator to manually enter the binary boot instructions into memory before transferring control to the CPU. The boot loader would then read the second-stage boot loader (called Binary Loader of paper tape with checksum) or the operating system in from an outside storage medium such as paper tape, punched card, or a disk drive.