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The average person who uses a
computer on a regular basis
doesn't think about what happens
inside a computer once the power
is turned on. As long as their
version of MS Windows pops up
within a few seconds, most
people are quite content to
continue on with what they want
to do on their computer. A
computer goes through many
processes from the moment the
power is turned on before its
operating system (ex. Windows,
Linux) is fully loaded and takes
over.
The operating system is stored
on the hard disk of a computer.
It is stored on the hard disk
because this type of storage is
much less expensive and an
operating system requires a
large amount of storage space.
So, in order to make computers
more economical, they are
designed to use a combination of
ROM, DRAM, and hard disks. An
explanation of each follows.
Once the power switch is turned
on, the "boot-up" process
begins. To "boot-up" a computer
simply means to start it.
Electricity then flows through
all of the chips and their
circuits. The instructions for
what the computer is supposed to
do next are found in the Read
Only Memory, Basic Input/Output
System (ROM BIOS). ROM is memory
that can only be read from and
has information that is
permanently burned into it. It
is nonvolatile and will not be
lost or disappear once the power
is turned off.
ROM BIOS or just BIOS, is
designed to begin giving
commands as soon as it receives
power. The BIOS contains an
entire set of instructions, in
effect a computer program
written into the chip that
manages the boot-up process.
Without the BIOS, the computer
would not know what to do next.
The first task that BIOS
completes is to make sure that
all of the hardware components
are working properly (for
example: disk drives, external
buses, the mouse, the printer).
This is called a power-on
self-test (POST). After the POST
is complete, the BIOS activates
other chips on different cards
installed in the computer (SCSI
and graphics cards) and provides
a set of low-level routines that
the operating system uses to
interface to different hardware
devices such as the keyboard,
mouse, printer, etc.
Once the POST is complete, the
BIOS hands the next stage in the
boot-up process over to the
central processing unit (CPU).
The CPU is a one chip processor
or microprocessor that has two
distinct capabilities:
1. The CPU carries out all of
the mathematical and logical
operations including basic math
and comparisons of two or more
numbers.
2. The CPU has the ability to
intelligently manage the flow of
instructions and data going into
and out of its circuits.
The last instruction that the
ROM sends to the CPU is to go to
a specific location or address
to find its next instruction. An
address is a string of numbers
that gives directions to where
something can be found, much
like an address on an envelope.
Computers use addresses to keep
track of information much the
same way as the post office uses
them to find residences and
businesses. The bigger the
number in an address the more
locations it can refer to. Most
current computers use a 32-bit
address space for memory, which
means that there can be over
four billion separate locations
to hold information.
The instruction that the ROM
BIOS wants the CPU to carry out
is sent through a chip on a bus
(a set of wires) to the address
specified. The data bus is able
to carry information into and
out of the chip within the CPU.
The information is not available
within the CPU so it has to look
elsewhere. The CPU then sends
the address on another bus
called an address bus. When the
CPU does this, it is called a
fetch. The address bus is
"fetching" information from
elsewhere within the computer.
The address bus is only able to
carry instructions out of the
CPU.
The address bus fetches
information from the computer's
memory. Memory is a type of
silicon chip that can hold
instructions or data. This type
of memory can be read from or
written to by the CPU, but this
type of memory or Dynamic Random
Access Memory (DRAM) is
volatile. Once the power is
turned off, the DRAM looses its
memory or information. Since the
DRAM is basically a blank slate,
the CPU has within, a set of
sequential instructions as to
where to look for the required
information.
Before the address bus can get
to memory, it has to pass
through a set of chips called a
chipset. The chipset refers to a
group of chips that provide an
intelligent interface for the
core components of a computer -
CPU, memory, graphics, I/O
system, described as core logic
or glue logic. If the
information that the chipset
requires is not in memory, the
chipset then sends or redirects
it to the Input/Output (I/O)
bus. The I/O bus connects the
chipset to other places where
the information is stored, such
as the hard disk. The hard disk
allows the CPU to read from it
and to write to it. The hard
disk is non-volatile so it
retains its data or information
once the power is turned off. A
hard disk is much slower at
retrieving data from than memory
but memory is much more
expensive.
Once the hard disk receives the
address (via the I/O bus and
chipset), it retrieves the
information and sends it back
through the chipset and then
puts it on the address bus back
into the CPU. The chipset
functions as a bridge for the
two buses; the I/O bus and the
address bus.
The CPU uses a four step
sequence: fetch, decode,
execute, and store. Since the
CPU does not retain its memory,
it has to obtain its information
or fetch the information from
elsewhere within the computer.
To help with the speed of the
process of fetching, the CPU has
a pre-fetch area to make the
information available more
quickly.
Once the information has been
fetched, it has to be decoded.
Part of the decoding process of
the CPU is to decide which
circuits are appropriate to use
for executing the instructions.
Once that decision has been
made, the CPU begins to execute
the instructions. The part of
the CPU where the actual
execution of instructions takes
place is called the Arithmetic
Logical Unit (ALU). The ALU
includes groups of transistors,
known as logic gates, which are
organized to carry out basic
mathematical and logical
operations. Logic gates are
grouped into electrical circuits
that execute the CPU's
instructions such as "add" two
numbers or "compare" two
numbers.
The final step of the CPU is to
store the information. This
final step takes place after the
ALU completes its calculations.
The results of the calculations
are stored on a chip that has an
area called a register.
Registers can be accessed more
quickly than any other kind of
memory but are only for
temporary holding (storage) of
information.
The CPU also has a clock within
it to keep the timing of all of
the flow of information and
processes of the computer. This
clock is vital to the
synchronization of all of the
processes of the computer. This
CPU clock controls all of the
operations on its chip. The
processes of the CPU can also be
interrupted by an external
interrupt controller chip which
is part of the chipset. The
chipset contains a small
database of interrupt vector
(numerical table). When an
interrupt signal comes onto the
chip, the CPU saves what it is
doing and goes to the interrupt
vector to find the address of
the instruction that the
interrupt is telling it to
execute instead. Once it is
finished with the interrupt, it
goes back to what it was doing.
The CPU finds what it was doing
in a register called a stack. If
interrupts were not possible,
the CPU would have to complete
one task before it could start
another causing the speed to be
greatly reduced.
Now that the CPU has found the
operating system, loaded it into
memory, the operating system
takes over and the computer is
now ready to be used by its
owner. The user can now check
email, play a game, or do
whatever they wanted to do when
they started the computer.
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