What is need to boot a computer?

Applying power to a computer's circuitry, normally by depressing the "On" switch, gives the microprocessor the voltage it, and its associated clock circuits, need to function. After any internal self-initialization the processor might do, it loads a fixed address onto its address bus, determined by the manufacturer, and starts reading instructions from memory at that address, incrementing its instruction counter as it goes. In small microcontroller circuits, that may be the end of the story; it just keeps reading and executing instructions from ROM (Read-Only Memory) until the program ends (if ever), or some unexpected interrupt or exception occurs which halts it.

In the case of modern desktop computers, however, typically the BiOS (Basic Input/Output System) is the ROM which is addressed at initialization, and this can consist of many megabytes of initialization and test code until some means of long-term storage (disk, USB flash device, CD- or DVD-ROM) is loaded. This will typically have several stages of loaders, where the first 512-byte sector has code which knows where to fetch a secondary loader; eventually a loader is capable of reading the filesystem of the disk well enough to locate the operating system itself.

The operating system then typically has its own test and initialization routines to go through, and while some OSes such as Windows may by default hide all this behind a "splash screen", others like some Linux distributions will display screen after screen of quickly-scrolling information letting you know exactly what's going on. Once it has mapped all the computer's circuitry: memory (RAM), disks, sound cards, and other hardware to its internal representations of these devices, it typically displays a login prompt. The computer is now said to be "booted": lifted up by its own bootstraps.

In the case of a "turnkey" or "kiOSk" system, the login may be bypassed, and the system might not be considered "booted" until it starts executing the process for which it was designed. This would be the case at info-kiOSks at airports, or point-of-sale (POS) machines (modern cash registers) in stores.