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Booting
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http://en.wikipedia.org/wiki/Booting
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In computing, booting (also known as booting up) is a process that begins when a user turns on a
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computer system and prepares the computer to perform its normal operations. On modern computers, this
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typically involves loading and starting an operating system. The boot sequence is the initial set of operations
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that the computer performs when power is switched on. A boot loader is a computer program that typically
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loads the main operating system or runtime environment for the computer after completion of a power-on self-
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test.
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Since the IBM 701 in the 1950s, booting has been used to load programs and operating systems into the
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main memory of stored-program computers, including mainframe computers, minicomputers, microcomputers,
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personal computers, and consumer electronics devices. Some simpler embedded systems do not require a boot
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sequence to begin functioning and may run programs stored in read-only memory (ROM) when turned on.
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There are numerous examples of single and multi-stage boot sequences that begin with the execution of boot
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program(s) stored in boot ROMs. During the booting process, the binary code of an operating system or
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runtime environment may be loaded from nonvolatile secondary storage (such as a hard disk drive) into volatile,
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or random-access memory (RAM) and then executed. The booting concept is also known as "Initial Program
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Load" for IBM mainframe computers.
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The computer word boot is short for bootstrap (itself short for bootstrap load). The term bootstrap
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derives from the idiom to pull oneself up by one's bootstraps. The term refers to the paradox that a computer
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cannot run without first loading software but must be running before any software can be loaded.[2]
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Upon starting, a personal computer's x86 CPU runs the instruction located at the memory location CS:IP
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F000:FFF0 of the BIOS, which wraps to linear address 0xFFFF0, the BIOS entry inside the ROM. This memory
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location is close to the end of the 1 MB of system memory accessible in real mode. It typically contains a jump
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instruction that transfers execution to the location of the BIOS start-up program. This program runs a power-on
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self-test (POST) to check and initialize required devices. The BIOS goes through a pre-configured list of non-
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volatile storage devices ("boot device sequence") until it finds one that is bootable. A bootable device is defined
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as one that can be read from, and the last two bytes of the first sector contain the word 0xAA55 (also known as
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the boot signature).The boot process can be considered complete when the computer is ready to interact with
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the user, or the operating system is capable of running system programs or application programs. Typical
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modern personal computers boot in about one minute, of which about 15 seconds are taken by a power-on self-
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test (POST) and a preliminary boot loader, and the rest by loading the operating system and other software.
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Time spent after the operating system loading can be considerably shortened to as little as 3 seconds[12] by
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bringing the system up with all cores at once, as with coreboot.[13] Large servers may take several minutes to
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boot and start all their services.