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In computing, boot is a process that starts operating systems when the user turns on his PC.
Most computer systems can only execute code found in the memory (ROM or RAM); modern operating systems are mostly stored on hard disk drives. Just after a computer has been turned on, it doesn't have an operating system in memory. The computer's hardware alone cannot perform complicated actions of the operating system, such as loading a program from disk on its own; so a seemingly irresolvable paradox is created: to load the operating system into memory, one appears to need to have an operating system already installed.
The solution to the paradox involves using a special small program, called a bootstrap loader or boot loader. This program doesn't have the full functionality of an operating system, but is tailor-made specifically so that it is capable of loading enough other software for the operating system to start. Often, multiple-stage boot loaders are used, in which several small programs summon each other, until the last of them loads the operating system.
Early programmable computers had toggle switches on the front panel to allow the operator to place the bootloader into the program store before starting the CPU. This would then read the operating system in from an outside storage medium such as paper tape.
Pseudo-assembly code for the bootloader might be as simple as the following eight instructions:
0: set the P register to 8 1: check paper tape reader ready 2: if not ready, jump to 1 3: read a byte from paper tape reader to accumulator 4: if end of tape, jump to 8 5: store accumulator to address in P register 6: increment the P register 7: jump to 1
In modern computers the bootstrapping process begins with the CPU executing software contained in ROM at a predefined address (the CPU is programmed to execute this software after reset without outside help). This software contains rudimentary functionality to search for devices eligible to participate in booting, and load a small program from a special section of the most promising device. The small program is most often not itself an operating system, but only a second-stage boot loader, such as LILO. It will then be able to load the operating system proper, and finally transfer execution to it. The system will initialize itself, and may load device drivers and other programs that are needed for the normal operation of the OS.
The boot process is considered complete when the computer is ready to answer queries from the outside. Typical modern PCs boot in about a minute (of which about 15 seconds are taken by the preliminary boot loaders, and the rest - by the one loading the operating system), while large servers may take several minutes to boot and to start all services; to ensure high availability, they bring up some services before others.
Most embedded systems must boot almost instantly -- for instance, waiting a minute for the television to come up is not acceptable. Therefore they have their whole operating system in ROM or flash memory, so it can be executed directly.
see boot sequence, which overlaps somewhat.