To boot (as a verb; also "to boot up") a computer is to
load an operating system into the computer's main
memory or random access memory (RAM). Once the operating system is loaded
(and, for example, on a PC, you see the initial Windows or Mac desktop screen),
it's ready for users to run applications. Sometimes you'll see an
instruction to "reboot" the operating system. This simply means to
reload the operating system (the most familiar way to do this on PCs is
pressing the Ctrl, Alt, and Delete keys at the same time).
On larger computers (including mainframes), the equivalent term for
"boot" is "initial program load" (IPL) and for "reboot" is
"re-IPL." Boot is also used as a noun for the act of booting, as in
"a system boot." The term apparently derives from bootstrap which is a small strap or loop
at the back of a leather boot that enables you to pull the entire boot on.
There is also an expression, "pulling yourself up by your own
bootstraps," meaning to leverage yourself to success from a small
beginning. The booting of an operating system works by loading a very small
program into the computer and then giving that program control so that it in
turn loads the entire operating system.
Booting or loading an operating system is different than
installing it, which is generally an initial one-time activity. (Those who buy
a computer with an operating system already installed don't have to worry about
that.) When you install the operating system, you may be asked to identify
certain options or configuration choices. At the end of installation, your
operating system is on your hard disk ready to be booted (loaded) into random
access memory, the computer storage that is closer to the microprocessor and
faster to work with than the hard disk. Typically, when an operating system is
installed, it is set up so that when you turn the computer on, the system is
automatically booted as well. If you run out of storage (memory) or the
operating system or an application program encounters an error, you may get an
error message or your screen may "freeze" (you can't do anything). In
these events, you may have to reboot the operating system.
How Booting Works
Note: This procedure may differ slightly for Mac, UNIX, OS/2, or other
operating systems.
When you turn on your computer, chances are that the operating
system has been set up to boot (load into RAM) automatically in this sequence:
1.
As soon as the computer is turned on,
the basic input-output system (BIOS) on your system's read-only memory (ROM) chip is "woken up" and takes
charge. BIOS is already loaded because it's built-in to the ROM chip and,
unlike random access memory (RAM), ROM contents don't get erased when the
computer is turned off.
2.
BIOS first does a power-on self test
(POST) to make sure all the computer's
components are operational. Then the BIOS's boot program looks for the special
boot programs that will actually load the operating system onto the hard disk.
3.
First, it looks on drive A (unless
you've set it up some other way or there is no diskette drive) at a specific
place where operating system boot files are located. If there is a diskette in
drive A but it's not a system disk, BIOS will send you a message that drive A
doesn't contain a system disk. If there is no diskette in drive A (which is the
most common case), BIOS looks for the system files at a specific place on your
hard drive.
4.
Having identified the drive where
boot files are located, BIOS next looks at the firstsector (a 512-byte area) and copies
information from it into specific locations in RAM. This information is known
as the boot record or Master Boot Record.
5.
It then loads the boot record into a
specific place (hexadecimal address 7C00) in RAM.
6.
The boot record contains a program
that BIOS now branches to, giving the boot record control of the computer.
7.
The boot record loads the initial
system file (for example, for DOS systems, IO.SYS) into RAM from the diskette
or hard disk.
8.
The initial file (for example,
IO.SYS, which includes a program called SYSINIT) then loads the rest of the
operating system into RAM. (At this point, the boot record is no longer needed
and can be overlaid by other data.)
9.
The initial file (for example,
SYSINIT) loads a system file (for example, MSDOS.SYS) that knows how to work
with the BIOS.
10. One of the first operating system files that is loaded is a system
configuration file (for DOS, it's called CONFIG.SYS). Information in the
configuration file tells the loading program which specific operating system
files need to be loaded (for example, specific device driver.
11. Another special file that is loaded is one that tells which
specific applications or commands the user wants to have included or performed
as part of the boot process. In DOS, this file is named AUTOEXEC.BAT. In
Windows, it's called WIN.INI.
12. After all operating system files have been loaded, the operating
system is given control of the computer and performs requested initial commands
and then waits for the first interactive user input.
Reboot (warm boot, cold boot)
To reboot is to restart a computer and reload the
operating system. The most common reasons to reboot are because the
installation of new software or hardware requires it, or because applications
are not responding for some reason. On computers running Windows, you can
usually reboot by selecting "turn off computer" from the start menu
and then clicking "restart" in the window that pops up. Another way
(and one that works sometimes when the first way doesn't) is through the Ctrl-Alt-Delete keystroke combination, which was
developed as an easy way to reboot a computer that would nevertheless be an
unlikely accidental keystroke combination.
Rebooting a computer through the menu option or the keystroke
combination is sometimes referred to as a warm
boot, perhaps because it is more gentle than the alternative cold boot(simply pressing the
computer's power button once to turn it off and then again to turn it back on).
On larger computers (including mainframes), the
equivalent term for "boot" is "initial program load" (IPL) and for
"reboot" is "re-IPL."
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