 The magazine of the Melbourne PC User Group Computer Anatomy What Is Inside That Beige Box? James Cox |
Have you ever sat back and wondered what is happening inside that box
sitting on your desk? How does pushing a key on the keyboard result in a character appearing on your monitor?
This article will help if you wish to know a bit more about some of the components inside and some of the key
devices that you plug into your computer.
For the purpose of this discussion, I will assume that we are talking about a desktop PC that has been built
in the last three years. Don't worry if your PC is older than three years (so is mine!), or that you have a
laptop, as the functionality built into your computer is still the same. It is just that the location of some
parts will be different. There is a popular trend for many of the functions that were performed in the past
by plug-in cards, to be incorporated into the design of the motherboard, and from there to the CPU.
Power Supply
| When you plug your power cable into your PC, you are actually
plugging into a socket in the power supply unit that has been fitted inside your case. This component is
responsible for converting the 240 volt AC mains power to low voltage DC power needed by PC components. The
power supply generates +3.3V, +5V, +12V, -5V and -12V of which the latter two are not much used anymore. The
+3.3V and +5V circuits supply power to all the electronic circuitry in the system (motherboard, adapter cards
and disk drive logic boards), and through the port sockets on the motherboard that poke out through the rear
of the case, to the peripherals that don't have their own power supply (e.g. keyboard, mouse).
Power supplies are often overlooked, but are essential to the proper operation of your system which requires a good, steady supply of DC power at the appropriate voltages. These voltages must be constant, right up to the maximum current your system will draw under load. |
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Poor quality power units, or units not rated
for the amount of power your system requires, can lead to unstable operation and computer problems that are
hard to identify. Take this into consideration when you start adding drives and cards to your system, as they
will increase its total power requirement. How much more can your power supply take?
CPU
The CPU (Central Processing Unit) is the brain of the system. It executes all the program code from the
operating system and the applications you run, and does most of the calculating and processing of data. It
sends commands to direct the actions of all the other components in the PC and it manages the distribution of
data to/from those various components.
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The CPU is the
fastest component in your system. It works hard only sometimes, maybe when you play a game involving 3D
graphics, or perhaps compile a very large program or run a very large spread-sheet; but most of the time it
is just waiting for you to press the next key, or for the next bit of data to arrive from the Internet,
through your modem.
Main Memory or RAM (Random Access Memory)
Memory is temporary storage where the processor can access program code and data. It is temporary because
any information stored there is lost when the system loses power or is rebooted.
Physically, memory in a computer usually consists of 1- 4 small boards containing special chips. These
boards, characterised by the rows of "pins" along the bottom edge, are plugged into appropriate slots on the
motherboard. Each memory chip consists of an array of a huge number of small memory circuits. The circuits
can be set individually by the CPU during a write operation which stores bits of data, or read by the CPU as
the data is being retrieved.
A low or high (on or off) voltage state in those small circuits represents a "0" or "1", which are the only
two possible states of a binary digit (a "bit"). The high and low voltage state of each circuit is constantly
refreshed.
While you work on your computer, all your work is stored here until it's saved to the hard disk or other
storage device. This is why it is important NOT to have any interruption to your power
supply.
Motherboard
As its name implies, everything inside your PC (apart from the disk drives and power supply unit) is
part of the motherboard, or sits in a socket on the motherboard, or is connected to it by cables (e.g disk
drives and power supply). So it plays an important structural role.
Boiling it down to its essentials, the motherboard comprises the chipset, Super I/O chip, BIOS ROM,
communications pathways called buses and a whole lot of sockets to plug things into.
The chipset, usually a pair of chips, determines a lot of the features of your system. For example it
determines what type of CPU and memory you can use. The chipset provides controllers for the buses and
transfers data from one bus to another. These controllers are referred to as the North Bridge and
South Bridge, unless the manufacturer uses some proprietary name for marketing purposes. The North Bridge
connects to the faster buses and the South Bridge to the slower buses. (If you are keen to learn about that,
the box below contains a more detailed description.)
Other functions can also be integrated into the motherboard and as I wrote at the beginning there is a trend
for motherboards to be designed in this fashion. The most common are integrated video and audio adapters.
This can save most of the cost of an adapter card, but the performance is usually at the lower end of the
spectrum.
Video Adapter Card
The main components on this card are some video memory, a video processor and a digital-to-analog converter.
The computer's CPU and video processor work together to generate a digital representation of an image and
store it in the video memory. More advanced video processors will speed up the rendering of 3D images in
graphics applications and games by taking on some of the processing that the CPU would otherwise have to do.
The digital-to-analog converter continuously reads through memory containing the image and generates the
analog electrical signal that your monitor needs, on order to display the output from your
computer.
Hard Disk Drive
The function of the hard disk drive is to provide convenient persistent storage for data and programs. By
persistent I mean that the information will survive when the power to the computer is turned off. Hard disks
provide convenient storage for large amounts of data as it is always accessible. You don't have to find and
insert the next floppy disk, or data storage tape.
Be aware that hard drives do not provide permanent storage. Lots of things can go wrong and your data can be
lost. You simply must look at it this way... If you don't backup your data, obviously you
don't really want it! It can and ultimately will be lost without warning.
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Your hard disk drive gets its name from the rigid, or hard disks it contains. These
are made of aluminium or glass coated with material which can be magnetised. The disks are mounted on a
spindle which is spun by a motor. Drive heads are small transducers that can produce a magnetic field
to magnetise the disk surface when a voltage is applied to them, or generate a voltage as they pass through a
changing magnetic field.
Binary data is written to the disk by magnetising small areas with fields of two different
directions. The head will produce a voltage pulse as it passes over the junction between areas of different
field direction and in this way data can be read. There are two heads for each disk: one for each side. Each
head is connected to an actuator arm that can swing out over or below the disk so that the head can hover
over any point between the centre and the edge of the disk while it is spinning. All the arms are ganged
together and move as one, so that only one head is writing or reading at any time.
All the above is enclosed within a sealed unit. The remaining element of a hard drive disk unit is the disk
controller. This is a circuit board with all the electronics required to control the motor and the actuator
arm assembly. It also produces the voltage waveform applied to the heads to write data and decodes the
waveform generated by the head when reading data, as well as handling communications with the
motherboard.
Hard disk drives are slower than memory but faster than any other type of drive. After the Internet, waiting
for my hard drive is the delay I notice most on my system. The red LED on the front of your case indicates
the drive is in use.
Floppy Disk Drive
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The floppy disk drive works on the same principals as a hard
disk drive, however it cannot record at such high data densities as the hard disk drive. This is because of
the nature of the disk material, and because it is not rigid, nor fixed to the drive motor. |
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CD-ROM Drive
CD-ROMs encode data by having small areas either strongly reflective to light or non-reflective to light. To
read this data, a CD-ROM drive employs a laser diode to emit an infrared light beam which is reflected onto a
track on the CD-ROM by a mirror that is positioned by a motor. The light reflected by the disc is gathered
and directed by a system of lenses to a photo detector that converts the reflected light pulses into an
electrical signal, which is then decoded by the drive electronics and sent to the motherboard.
These devices are slower than hard disks, but much faster than floppy disks.
Sound Card
A sound card converts digital data into an analog signal that can be used as input to speakers
or headphones. It also does the reverse, taking a signal from a microphone or other audio equipment and
converting it into digital data. A sound card usually also has a game port connection which enables you to
use a joystick.
Keyboard
The keyboard is the primary device by which someone can input commands or data into a computer system.
Pressing down on a key closes a switch below the key. All the key switches are connected in an array, so that
a processor inside the keyboard can identify which key has been pressed when a switch is closed. The
processor then sends a signal along the cable to the motherboard containing a code for the key that has been
pressed. Once you lift your finger off the key, the key switch opens and the keyboard processor sends another
signal with a code to indicate the release of the key.
Mouse
| You will have seen the mechanical part of a mouse if you have ever removed the ring shaped plate from its underside to clean out the dust and dirt it collects. A ball sits on the plate and rests against two rollers that are arranged at a 90 degree angle to each other. Connected to the rollers are wheels with spokes protruding from them. As your hand moves the mouse, the ball rolls across the surface the mouse is resting on and rotates the rollers and their attached wheels. The spokes on these wheels interrupt infrared light from an LED. Small optical sensors detect the interrupted pattern of light and thus the movement of the wheels. Each roller/wheel combination picks up movement along one of two perpendicular axes. The simple circuitry inside the mouse transmits this movement information and the status of the buttons, which are simple contact switches, to the motherboard. |
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| North Bridge The North Bridge connects to the faster buses, which are:
Linked to the North Bridge by a proprietary bus which we don't need to consider, the South Bridge connects to the following slower buses, devices and ports:
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About the Author
James Cox has been fascinated with computers since first encountering a Tandy TRS-80 at school. He studied
electrical engineering and worked in the oil industry for 13 years, first encountering a PC in 1986. Recently
he has been looking after his young daughter at home and is a member of the Melb PC iHelp
team.
Reprinted from the March 2002 issue of PC Update, the
magazine of Melbourne PC User Group, Australia
 
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