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How a Computer Operates
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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