‫مكونات الحاسب وتجميعه‬
‫الجزء النظري‬
‫ معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
‫ هـ‬1429-1428 ‫الفصل األول العام الدراسي‬
:‫المراجع‬
http://www.howstuffworks.com/motherboard.htm
http://computer.howstuffworks.com/microprocessor.htm
http://webster.cs.ucr.edu/AoA/Windows/HTML/SystemOrganization.html
http://www.howstuffworks.com/ram.htm
http://en.wikipedia.org/wiki/DDR_SDRAM
http://en.wikipedia.org/wiki/DDR2_SDRAM
http://en.wikipedia.org/wiki/DDR3_SDRAM
http://www.pcstats.com/articleview.cfm?articleID=2175
http://computer.howstuffworks.com/cache.htm
http://computer.howstuffworks.com/virtual-memory.htm
http://computer.howstuffworks.com/hard-disk.htm
http://www.pcguide.com/ref/hdd/file/structPartitions-c.html
http://www.pcguide.com/ref/hdd/perf/perf/ext/fileType-c.html
http://www.pcguide.com/ref/hdd/op/jump.htm
http://electronics.howstuffworks.com/cd.htm
http://computer.howstuffworks.com/cd-burner.htm
http://electronics.howstuffworks.com/hd-dvd.htm
http://electronics.howstuffworks.com/dvd.htm
http://mimech.com/printers/
http://home.howstuffworks.com/inkjet-printer.htm
http://www.howstuffworks.com/laser-printer.htm
http://en.wikipedia.org/wiki/Daisy_wheel_printer
http://en.wikipedia.org/wiki/Thermal_printer
http://computer.howstuffworks.com/power-supply.htm
http://en.wikipedia.org/wiki/Dye-sublimation_printer
http://computer.howstuffworks.com/mouse.htm
http://computer.howstuffworks.com/keyboard.htm
http://computer.howstuffworks.com/rom.htm
http://www.howstuffworks.com/bios.htm
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Motherboards ‫اللوحة األم‬
If you've ever taken the case off of a computer, you've seen the
one piece of equipment that ties everything together -- the
motherboard. A motherboard allows all the parts of your computer
to receive power and communicate with one another.
Motherboards have come a long way in the last twenty years. The
first motherboards held very few actual components. The first IBM
PC motherboard had only a processor and card slots. Users
plugged components like floppy drive controllers and memory into
the slots.
Today, motherboards typically boast a wide variety of built-in
features, and they directly affect a computer's capabilities and
potential for upgrades. In this article, we'll look at the general
components of a motherboard. Then, we'll closely examine five
points that dramatically affect what a computer can do.
Form Factor
A motherboard by itself is useless, but a computer has to have one
to operate. The motherboard's main job is to hold the computer's
microprocessor chip and let everything else connect to it.
Everything that runs the computer or enhances its performance is
either part of the motherboard or plugs into it via a slot or port.
A modern motherboard
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The shape and layout of a motherboard is called the form factor.
The form factor affects where individual components go and the
shape of the computer's case. There are several specific form
factors that most PC motherboards use so that they can all fit in
standard cases. For a comparison of form factors, past and
present, check out Motherboards.org.
The form factor is just one of the many standards that apply to
motherboards. Some of the other standards include:
The socket for the microprocessor determines what kind of Central
Processing Unit (CPU) the motherboard uses.
The chipset is part of the motherboard's logic system and is
usually made of two parts -- the northbridge and the southbridge.
These two "bridges" connect the CPU to other parts of the
computer.
The Basic Input/Output System (BIOS) chip controls the most
basic functions of the computer and performs a self-test every time
you turn it on. Some systems feature dual BIOS, which provides a
backup in case one fails or in case of error during updating.
The real time clock chip is a battery-operated chip that maintains
basic settings and the system time.
The slots and ports found on a motherboard include:
Peripheral Component Interconnect (PCI)- connections for video,
sound and video capture cards, as well as network cards
Accelerated Graphics Port (AGP) - dedicated port for video cards.
Integrated Drive Electronics (IDE) - interfaces for the hard drives
Universal Serial Bus or FireWire - external peripherals
Memory slots
Some motherboards also incorporate newer technological
advances:
Redundant Array of Independent Discs (RAID) controllers allow
the computer to recognize multiple drives as one drive.
PCI Express is a newer protocol that acts more like a network than
a bus. It can eliminate the need for other ports, including the AGP
port.
Rather than relying on plug-in cards, some motherboards have onboard sound, networking, video or other peripheral support.
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A Socket 754 motherboard
Many people think of the CPU as one of the most important parts of a
computer. We'll look at how it affects the rest of the computer in the next
section.
Sockets and CPUs
The CPU is the first thing that comes to mind when many people
think about a computer's speed and performance. The faster the
processor, the faster the computer can think. In the early days of
PC computers, all processors had the same set of pins that would
connect the CPU to the motherboard, called the Pin Grid Array
(PGA). These pins fit into a socket layout called Socket 7. This
meant that any processor would fit into any motherboard.
-4-
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A Socket 939 motherboard
Today, however, CPU manufacturers Intel and AMD use a variety
of PGAs, none of which fit into Socket 7. As microprocessors
advance, they need more and more pins, both to handle new
features and to provide more and more power to the chip.
Current socket arrangements are often named for the number of
pins in the PGA. Commonly used sockets are:
Socket 478 - for older Pentium and Celeron processors
Socket 754 - for AMD Sempron and some AMD Athlon processors
Socket 939 - for newer and faster AMD Athlon processors
Socket AM2 - for the newest AMD Athlon processors
Socket A - for older AMD Athlon processors
A Socket LGA755 motherboard
The newest Intel CPU does not have a PGA. It has an LGA, also
known as Socket T. LGA stands for Land Grid Array. An LGA is
different from a PGA in that the pins are actually part of the socket,
not the CPU.
Anyone who already has a specific CPU in mind should select a
motherboard based on that CPU. For example, if you want to use
one of the new multi-core chips made by Intel or AMD, you will
need to select a motherboard with the correct socket for those
chips. CPUs simply will not fit into sockets that don't match their
PGA.
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The CPU communicates with other elements of the motherboard
through a chipset. We'll look at the chipset in more detail next.
Chipsets
The chipset is the "glue" that connects the microprocessor to the
rest of the motherboard and therefore to the rest of the computer.
On a PC, it consists of two basic parts -- the northbridge and the
southbridge. All of the various components of the computer
communicate with the CPU through the chipset.
The northbridge and southbridge
The northbridge connects directly to the processor via the front
side bus (FSB). A memory controller is located on the northbridge,
which gives the CPU fast access to the memory. The northbridge
also connects to the AGP or PCI Express bus and to the memory
itself.
The southbridge is slower than the northbridge, and information
from the CPU has to go through the northbridge before reaching
the southbridge. Other busses connect the southbridge to the PCI
bus, the USB ports and the IDE or SATA hard disk connections.
Chipset selection and CPU selection go hand in hand, because
manufacturers optimize chipsets to work with specific CPUs. The
chipset is an integrated part of the motherboard, so it cannot be
removed or upgraded. This means that not only must the
motherboard's socket fit the CPU, the motherboard's chipset must
work optimally with the CPU.
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Next, we'll look at busses, which, like the chipset, carry information
from place to place.
Bus Speed
A bus is simply a circuit that connects one part of the motherboard
to another. The more data a bus can handle at one time, the faster
it allows information to travel. The speed of the bus, measured in
megahertz (MHz), refers to how much data can move across the
bus simultaneously.
Busses connect different parts of the motherboard
to one another
Bus speed usually refers to the speed of the front side bus (FSB),
which connects the CPU to the northbridge. FSB speeds can
range from 66 MHz to over 800 MHz. Since the CPU reaches the
memory controller though the northbridge, FSB speed can
dramatically affect a computer's performance.
Here are some of the other busses found on a motherboard:

The back side bus connects the CPU with the level 2 (L2)
cache, also known as secondary or external cache. The
processor determines the speed of the back side bus.
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The memory bus connects the northbridge to the memory.

The IDE or ATA bus connects the southbridge to the disk
drives.

The AGP bus connects the video card to the memory and
the CPU. The speed of the AGP bus is usually 66 MHz.

The PCI bus connects PCI slots to the southbridge. On
most systems, the speed of the PCI bus is 33 MHz. Also
compatible with PCI is PCI Express, which is much faster
than PCI but is still compatible with current software and
operating systems. PCI Express is likely to replace both
PCI and AGP busses.
The faster a computer's bus speed, the faster it will operate -- to a
point. A fast bus speed cannot make up for a slow processor or
chipset.

Now let's look at memory and how it affects the motherboard's
speed.
-8-
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CPU: Central Processing Unit ‫المعالج‬
The computer you are using to read this page uses a
microprocessor to do its work. The microprocessor is the heart of
any normal computer, whether it is a desktop machine, a server or
a laptop. The microprocessor you are using might be a Pentium, a
K6, a PowerPC, a Sparc or any of the many other brands and
types of microprocessors, but they all do approximately the same
thing in approximately the same way.
Intel 4004 chip
A microprocessor -- also known as a CPU or central processing
unit -- is a complete computation engine that is fabricated on a
single chip. The first microprocessor was the Intel 4004, introduced
in 1971. The 4004 was not very powerful -- all it could do was add
and subtract, and it could only do that 4 bits at a time. But it was
amazing that everything was on one chip. Prior to the 4004,
engineers built computers either from collections of chips or from
discrete components (transistors wired one at a time). The 4004
powered one of the first portable electronic calculators.
If you have ever wondered what the microprocessor in your
computer is doing, or if you have ever wondered about the
differences between types of microprocessors, then read on. In
this article, you will learn how fairly simple digital logic techniques
allow a computer to do its job, whether its playing a game or spell
checking a document!
-9-
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The Intel 8080 was the first microprocessor in a home computer.
See more microprocessor pictures.
Microprocessor Progression: Intel
The first microprocessor to make it into a home computer was the
Intel 8080, a complete 8-bit computer on one chip, introduced in
1974. The first microprocessor to make a real splash in the market
was the Intel 8088, introduced in 1979 and incorporated into the
IBM PC (which first appeared around 1982). If you are familiar with
the PC market and its history, you know that the PC market moved
from the 8088 to the 80286 to the 80386 to the 80486 to the
Pentium to the Pentium II to the Pentium III to the Pentium 4. All of
these microprocessors are made by Intel and all of them are
improvements on the basic design of the 8088. The Pentium 4 can
execute any piece of code that ran on the original 8088, but it does
it about 5,000 times faster!
The following table helps you to understand the differences
between the different processors that Intel has introduced over the
years.
Name
Date Transistors Microns
Clock
speed
Data
width
MIPS
8080
1974
6,000
6
2 MHz
8 bits
0.64
8088
1979
29,000
3
5 MHz
16 bits
8-bit bus
0.33
80286
1982
134,000
1.5
6 MHz
16 bits
1
80386
1985
275,000
1.5
16 MHz
32 bits
5
80486
1989
1,200,000
1
25 MHz
32 bits
20
Pentium
1993
3,100,000
0.8
60 MHz
32 bits
64-bit bus
100
Pentium II
1997
7,500,000
0.35
233 MHz
32 bits
~300
64-bit bus
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Pentium III
1999
9,500,000
0.25
450 MHz
32 bits
~510
64-bit bus
Pentium 4
2000 42,000,000
0.18
1.5 GHz
32 bits
~1,700
64-bit bus
Pentium 4
"Prescott"
2004 125,000,000
0.09
3.6 GHz
32 bits
~7,000
64-bit bus
Compiled from The Intel Microprocessor Quick Reference Guide and TSCP
Benchmark Scores
Information about this table:
What's a Chip?
A chip is also called an integrated circuit. Generally it is a small,
thin piece of silicon onto which the transistors making up the
microprocessor have been etched. A chip might be as large as an
inch on a side and can contain tens of millions of transistors.
Simpler processors might consist of a few thousand transistors
etched onto a chip just a few millimeters square.






The date is the year that the processor was first introduced. Many
processors are re-introduced at higher clock speeds for many years
after the original release date.
Transistors is the number of transistors on the chip. You can see
that the number of transistors on a single chip has risen steadily over
the years.
Microns is the width, in microns, of the smallest wire on the chip. For
comparison, a human hair is 100 microns thick. As the feature size on
the chip goes down, the number of transistors rises.
Clock speed is the maximum rate that the chip can be clocked at.
Clock speed will make more sense in the next section.
Data Width is the width of the ALU. An 8-bit ALU can
add/subtract/multiply/etc. two 8-bit numbers, while a 32-bit ALU can
manipulate 32-bit numbers. An 8-bit ALU would have to execute four
instructions to add two 32-bit numbers, while a 32-bit ALU can do it in
one instruction. In many cases, the external data bus is the same
width as the ALU, but not always. The 8088 had a 16-bit ALU and an
8-bit bus, while the modern Pentiums fetch data 64 bits at a time for
their 32-bit ALUs.
MIPS stands for "millions of instructions per second" and is a rough
measure of the performance of a CPU. Modern CPUs can do so many
different things that MIPS ratings lose a lot of their meaning, but you
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can get a general sense of the relative power of the CPUs from this
column.
From this table you can see that, in general, there is a relationship
between clock speed and MIPS. The maximum clock speed is a
function of the manufacturing process and delays within the chip.
There is also a relationship between the number of transistors and
MIPS. For example, the 8088 clocked at 5 MHz but only executed
at 0.33 MIPS (about one instruction per 15 clock cycles). Modern
processors can often execute at a rate of two instructions per clock
cycle. That improvement is directly related to the number of
transistors on the chip and will make more sense in the next
section.
Photo courtesy Intel Corporation
Intel Pentium 4 processor
Core 2
Sometimes abbreviated as C2, the Intel Core 2 is a family of
processors first introduced with the release of the Intel Core 2 Duo
processor on July 27, 2006. The Core 2 processors include the
Core 2 Duo (C2D), Core 2 Extreme (C2E), Core 2 Quad (C2Q),
and Core 2 Solo.
The Core 2 Extreme was first released on July 29, 2006 and has
4MB of shared L2 cache like the Intel Core 2 Duo but has a higher
clock speed and much more capabilities to be overclocked.
Dual processor
Computer that has two separate processors that work together.
Dual processors are commonly used for intensive processing
demands and improves the computer's overall processing
efficiency. It is important to note that not all operating systems and
software programs support dual processors. Often this is true with
older operating systems such as Microsoft Windows 98; however,
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newer operating systems such as Microsoft Windows XP do
support dual processors.
GHz
1. Short for gigahertz, GHz is a unit of measurement for alternating
current (AC) or electromagnetic (EM) wave frequencies equal to
1,000,000,000 Hz.
2. When referring to a computer processor or CPU, GHz is a clock
frequency, also known as a clock rate or clock speed, representing a
cycle of time. An oscillator circuit supplies a small amount of electricity
to a crystal each second that is measured in MHz or GHz, where "Hz"
is the abbreviation of Hertz, "M" representing Mega, or one million, and
"G" representing Giga, or one thousand million. In addition to GHz and
MHz, there is KHz, or 1,000 Hz.
Heat sink
An electronic device that incorporates either a fan and/or a peltier
device that allows a hot device such as a processor to keep cool.
There are two types of heatsinks: active and passive. Active
heatsinks utilize power and are usually a fan type or some other
peltier cooling device. If you are looking to purchase an active
heatsink, it is recommended that you purchase fans with ballbearing motors which generally last much longer than sleeve
bearings. Passive heatsinks are 100% reliable, as they have no
mechanical components. Passive heatsinks are made of an
aluminum-finned radiator that dissipates heat through convection.
For Passive heatsinks to work to their full capacity, it is
recommended that there is a steady air flow moving across the
fins. To the right is an example image of a heat sink that is both
active and passive.
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Additional information and help with computer heat sinks and fans
can be found by clicking here.
Itanium
Identification name for a brand of Intel processors utilizing the 64bit RISC architecture and based on Explicitly Parallel Instruction
Computing design philosophy, which states that the compiler
should decide which instructions be executed together.
- 14 -
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The System Bus ‫ناقل النظام‬
The system bus connects the various components of a VNA
machine. The 80x86 family has three major busses: the address
bus, the data bus, and the control bus. A bus is a collection of
wires on which electrical signals pass between components in the
system. These busses vary from processor to processor. However,
each bus carries comparable information on all processors; e.g.,
the data bus may have a different implementation on the 80386
than on the 8088, but both carry data between the processor, I/O,
and memory.
A typical 80x86 system component uses standard TTL logic
levels1. This means each wire on a bus uses a standard voltage
level to represent zero and one2. We will always specify zero and
one rather than the electrical levels because these levels vary on
different processors (especially laptops).
1) The Data Bus
The 80x86 processors use the data bus to shuffle data between
the various components in a computer system. The size of this bus
varies widely in the 80x86 family. Indeed, this bus defines the
"size" of the processor.
Every modern x86 CPU from the Pentium on up employs a 64-bit
wide data bus. Some of the earlier processors used 8-bit, 16-bit, or
32-bit data busses, but such machines are sufficiently obsolete
that we do not need to consider them here..
You'll often hear a processor called an eight, 16, 32, or 64 bit
processor. While there is a mild controversy concerning the size of
a processor, most people now agree that the minimum of either
the number of data lines on the processor or the size of the largest
general purpose integer register determines the processor size.
The modern x86 CPUs all have 64-bit busses, but only provide 32bit general purpose integer registers, so most people classify these
devices as 32-bit processors.
Although the 80x86 family members with eight, 16, 32, and 64 bit
data busses can process data up to the width of the bus, they can
also access smaller memory units of eight, 16, or 32 bits.
Therefore, anything you can do with a small data bus can be done
with a larger data bus as well; the larger data bus, however, may
access memory faster and can access larger chunks of data in one
memory operation. You'll read about the exact nature of these
- 15 -
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memory accesses a little later (see "The Memory Subsystem" on
page 140).
2) The Address Bus
The data bus on an 80x86 family processor transfers information
between a particular memory location or I/O device and the CPU.
The only question is, "Which memory location or I/O device? " The
address bus answers that question. To differentiate memory
locations and I/O devices, the system designer assigns a unique
memory address to each memory element and I/O device. When
the software wants to access some particular memory location or
I/O device, it places the corresponding address on the address
bus. Circuitry associated with the memory or I/O device recognizes
this address and instructs the memory or I/O device to read the
data from or place data on to the data bus. In either case, all other
memory locations ignore the request. Only the device whose
address matches the value on the address bus responds.
With a single address line, a processor could create exactly two
unique addresses: zero and one. With n address lines, the
processor can provide 2n unique addresses (since there are 2n
unique values in an n-bit binary number). Therefore, the number of
bits on the address bus will determine the maximum number of
addressable memory and I/O locations. Early x86 processors, for
example, provided only 20 bit address busses. Therefore, they
could only access up to 1,048,576 (or 220) memory locations.
Larger address busses can access more memory.
Table 12: 80x86 Family Address Bus Sizes
Processor
Address Bus
Size
Max Addressable
Memory
In English!
8088, 8086, 80186,
20
80188
1,048,576
One Megabyte
80286, 80386sx
24
16,777,216
Sixteen
Megabytes
80386dx
32
4,294,976,296
Four Gigabytes
80486, Pentium
32
4,294,976,296
Four Gigabytes
Pentium Pro, II, III,
IV
36
68,719,476,736
64 Gigabytes
Future 80x86 processors (e.g., the AMD "Hammer") will probably
support 40, 48, and 64-bit address busses. The time is coming
- 16 -
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when most programmers will consider four gigabytes of storage to
be too small, much like they consider one megabyte insufficient
today. (There was a time when one megabyte was considered far
more than anyone would ever need!).
3) The Control Bus
The control bus is an eclectic collection of signals that control how
the processor communicates with the rest of the system. Consider
for a moment the data bus. The CPU sends data to memory and
receives data from memory on the data bus. This prompts the
question, "Is it sending or receiving?" There are two lines on the
control bus, read and write, which specify the direction of data
flow. Other signals include system clocks, interrupt lines, status
lines, and so on. The exact make up of the control bus varies
among processors in the 80x86 family. However, some control
lines are common to all processors and are worth a brief mention.
The read and write control lines control the direction of data on the
data bus. When both contain a logic one, the CPU and memoryI/O are not communicating with one another. If the read line is low
(logic zero), the CPU is reading data from memory (that is, the
system is transferring data from memory to the CPU). If the write
line is low, the system transfers data from the CPU to memory.
The byte enable lines are another set of important control lines.
These control lines allow 16, 32, and 64 bit processors to deal with
smaller chunks of data. Additional details appear in the next
section.
The 80x86 family, unlike many other processors, provides two
distinct address spaces: one for memory and one for I/O. While the
memory address busses on various 80x86 processors vary in size,
the I/O address bus on all 80x86 CPUs is 16 bits wide. This allows
the processor to address up to 65,536 different I/O locations. As it
turns out, most devices (like the keyboard, printer, disk drives, etc.)
require more than one I/O location. Nonetheless, 65,536 I/O
locations are more than sufficient for most applications. The
original IBM PC design only allowed the use of 1,024 of these.
Although the 80x86 family supports two address spaces, it does
not have two address busses (for I/O and memory). Instead, the
system shares the address bus for both I/O and memory
addresses. Additional control lines decide whether the address is
intended for memory or I/O. When such signals are active, the I/O
devices use the address on the L.O. 16 bits of the address bus.
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When inactive, the I/O devices ignore the signals on the address
bus (the memory subsystem takes over at that point).
1Actually, newer members of the family tend to use lower voltage
signals, but these remain compatible with TTL signals.
2TTL logic represents the value zero with a voltage in the range
0.0-0.8v. It represents a one with a voltage in the range 2.4-5v. If
the signal on a bus line is between 0.8v and 2.4v, it's value is
indeterminate. Such a condition should only exist when a bus line
is changing from one state to the other.
- 18 -
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RAM: Random access memory‫ذاكرة الوصول العشوائية‬
Random access memory (RAM) is the best known form of
computer memory. RAM is considered "random access" because
you can access any memory cell directly if you know the row and
column that intersect at that cell.
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The opposite of RAM is serial access memory (SAM). SAM stores
data as a series of memory cells that can only be accessed
sequentially (like a cassette tape). If the data is not in the current
location, each memory cell is checked until the needed data is
found. SAM works very well for memory buffers, where the data is
normally stored in the order in which it will be used (a good
example is the texture buffer memory on a video card). RAM data,
on the other hand, can be accessed in any order.
In this article, you'll learn all about what RAM is, what kind you
should buy and how to install it.
Dynamic RAM
Similar to a microprocessor, a memory chip is an integrated circuit
(IC) made of millions of transistors and capacitors. In the most
common form of computer memory, dynamic random access
memory (DRAM), a transistor and a capacitor are paired to create
a memory cell, which represents a single bit of data. The capacitor
holds the bit of information -- a 0 or a 1 (see How Bits and Bytes
Work for information on bits). The transistor acts as a switch that
lets the control circuitry on the memory chip read the capacitor or
change its state.
A capacitor is like a small bucket that is able to store electrons. To
store a 1 in the memory cell, the bucket is filled with electrons. To
store a 0, it is emptied. The problem with the capacitor's bucket is
that it has a leak. In a matter of a few milliseconds a full bucket
becomes empty. Therefore, for dynamic memory to work, either
the CPU or the memory controller has to come along and recharge
all of the capacitors holding a 1 before they discharge. To do this,
the memory controller reads the memory and then writes it right
back. This refresh operation happens automatically thousands of
times per second.
- 19 -
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The capacitor in a dynamic RAM memory cell is like a leaky
bucket.
It needs to be refreshed periodically or it will discharge to 0.
This refresh operation is where dynamic RAM gets its name.
Dynamic RAM has to be dynamically refreshed all of the time or it
forgets what it is holding. The downside of all of this refreshing is
that it takes time and slows down the memory.
Memory cells are etched onto a silicon wafer in an array of
columns (bitlines) and rows (wordlines). The intersection of a
bitline and wordline constitutes the address of the memory cell.
- 20 -
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Memory is made up of bits arranged in a two-dimensional grid.
In this figure, red cells represent 1s and white cells represent 0s.
In the animation, a column is selected and then rows are charged
to write data into the specific column.
DRAM works by sending a charge through the appropriate column
(CAS) to activate the transistor at each bit in the column. When
writing, the row lines contain the state the capacitor should take
on. When reading, the sense-amplifier determines the level of
charge in the capacitor. If it is more than 50 percent, it reads it as a
1; otherwise it reads it as a 0. The counter tracks the refresh
sequence based on which rows have been accessed in what
order. The length of time necessary to do all this is so short that it
is expressed in nanoseconds (billionths of a second). A memory
chip rating of 70ns means that it takes 70 nanoseconds to
completely read and recharge each cell.
Memory cells alone would be worthless without some way to get
information in and out of them. So the memory cells have a whole
support infrastructure of other specialized circuits. These circuits
perform functions such as:




Identifying each row and column (row address select and column
address select)
Keeping track of the refresh sequence (counter)
Reading and restoring the signal from a cell (sense amplifier)
Telling a cell whether it should take a charge or not (write enable)
Other functions of the memory controller include a series of tasks
that include identifying the type, speed and amount of memory and
checking for errors.
Static RAM works differently from DRAM. We'll look at how in the
next section.
Static RAM:
Static RAM uses a completely different technology. In static RAM,
a form of flip-flop holds each bit of memory (see How Boolean
Logic Works for details on flip-flops). A flip-flop for a memory cell
takes four or six transistors along with some wiring, but never has
to be refreshed. This makes static RAM significantly faster than
dynamic RAM. However, because it has more parts, a static
memory cell takes up a lot more space on a chip than a dynamic
memory cell. Therefore, you get less memory per chip, and that
makes static RAM a lot more expensive.
Static RAM is fast and expensive, and dynamic RAM is less
expensive and slower. So static RAM is used to create the CPU's
- 21 -
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speed-sensitive cache, while dynamic RAM forms the larger
system RAM space.
Memory chips in desktop computers originally used a pin
configuration called dual inline package (DIP). This pin
configuration could be soldered into holes on the computer's
motherboard or plugged into a socket that was soldered on the
motherboard. This method worked fine when computers typically
operated on a couple of megabytes or less of RAM, but as the
need for memory grew, the number of chips needing space on the
motherboard increased.
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The solution was to place the memory chips, along with all of the
support components, on a separate printed circuit board (PCB)
that could then be plugged into a special connector (memory bank)
on the motherboard. Most of these chips use a small outline J-lead
(SOJ) pin configuration, but quite a few manufacturers use the thin
small outline package (TSOP) configuration as well. The key
difference between these newer pin types and the original DIP
configuration is that SOJ and TSOP chips are surface-mounted to
the PCB. In other words, the pins are soldered directly to the
surface of the board, not inserted in holes or sockets.
Memory chips are normally only available as part of a card called a
module. You've probably seen memory listed as 8x32 or 4x16.
These numbers represent the number of the chips multiplied by
the capacity of each individual chip, which is measured in
megabits (Mb), or one million bits. Take the result and divide it by
eight to get the number of megabytes on that module. For
example, 4x32 means that the module has four 32-megabit chips.
Multiply 4 by 32 and you get 128 megabits. Since we know that a
byte has 8 bits, we need to divide our result of 128 by 8. Our result
is 16 megabytes!
In the next section we'll look at some other common types of RAM.
- 22 -
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Types of RAM
The following are some common types of RAM:







SRAM: Static random access memory uses multiple
transistors, typically four to six, for each memory cell but
doesn't have a capacitor in each cell. It is used primarily for
cache.
DRAM: Dynamic random access memory has memory
cells with a paired transistor and capacitor requiring constant
refreshing.
FPM DRAM: Fast page mode dynamic random access
memory was the original form of DRAM. It waits through the
entire process of locating a bit of data by column and row and
then reading the bit before it starts on the next bit. Maximum
transfer rate to L2 cache is approximately 176 MBps.
EDO DRAM: Extended data-out dynamic random access
memory does not wait for all of the processing of the first bit
before continuing to the next one. As soon as the address of
the first bit is located, EDO DRAM begins looking for the next
bit. It is about five percent faster than FPM. Maximum
transfer rate to L2 cache is approximately 264 MBps.
SDRAM: Synchronous dynamic random access memory
takes advantage of the burst mode concept to greatly
improve performance. It does this by staying on the row
containing the requested bit and moving rapidly through the
columns, reading each bit as it goes. The idea is that most of
the time the data needed by the CPU will be in sequence.
SDRAM is about five percent faster than EDO RAM and is
the most common form in desktops today. Maximum transfer
rate to L2 cache is approximately 528 MBps.
DDR SDRAM: Double data rate synchronous dynamic
RAM is just like SDRAM except that is has higher bandwidth,
meaning greater speed. Maximum transfer rate to L2 cache
is approximately 1,064 MBps (for DDR SDRAM 133 MHZ).
RDRAM: Rambus dynamic random access memory is a
radical departure from the previous DRAM architecture.
Designed by Rambus, RDRAM uses a Rambus in-line
memory module (RIMM), which is similar in size and pin
configuration to a standard DIMM. What makes RDRAM so
different is its use of a special high-speed data bus called the
Rambus channel. RDRAM memory chips work in parallel to
achieve a data rate of 800 MHz, or 1,600 MBps. Since they
operate at such high speeds, they generate much more heat
than other types of chips. To help dissipate the excess heat
Rambus chips are fitted with a heat spreader, which looks
like a long thin wafer. Just like there are smaller versions of
DIMMs, there are also SO-RIMMs, designed for notebook
computers.
- 23 -
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



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Credit Card Memory: Credit card memory is a proprietary
self-contained DRAM memory module that plugs into a
special slot for use in notebook computers.
PCMCIA Memory Card: Another self-contained DRAM
module for notebooks, cards of this type are not proprietary
and should work with any notebook computer whose system
bus matches the memory card's configuration.
CMOS RAM: CMOS RAM is a term for the small amount of
memory used by your computer and some other devices to
remember things like hard disk settings -- see Why does my
computer need a battery? for details. This memory uses a
small battery to provide it with the power it needs to maintain
the memory contents.
VRAM: VideoRAM, also known as multiport dynamic
random access memory (MPDRAM), is a type of RAM used
specifically for video adapters or 3-D accelerators. The
"multiport" part comes from the fact that VRAM normally has
two independent access ports instead of one, allowing the
CPU and graphics processor to access the RAM
simultaneously. VRAM is located on the graphics card and
comes in a variety of formats, many of which are proprietary.
The amount of VRAM is a determining factor in the resolution
and color depth of the display. VRAM is also used to hold
graphics-specific information such as 3-D geometry data and
texture maps. True multiport VRAM tends to be expensive,
so today, many graphics cards use SGRAM (synchronous
graphics RAM) instead. Performance is nearly the same, but
SGRAM is cheaper.
For a comprehensive examination of RAM types, check out the
Kingston Technology Ultimate Memory Guide.
The type of board and connector used for RAM in desktop
computers has evolved over the past few years. The first types
were proprietary, meaning that different computer manufacturers
developed memory boards that would only work with their specific
systems. Then came SIMM, which stands for single in-line memory
module. This memory board used a 30-pin connector and was
about 3.5 x .75 inches in size (about 9 x 2 cm). In most computers,
you had to install SIMMs in pairs of equal capacity and speed. This
is because the width of the bus is more than a single SIMM. For
example, you would install two 8-megabyte (MB) SIMMs to get 16
megabytes total RAM. Each SIMM could send 8 bits of data at one
time, while the system bus could handle 16 bits at a time. Later
SIMM boards, slightly larger at 4.25 x 1 inch (about 11 x 2.5 cm),
used a 72-pin connector for increased bandwidth and allowed for
up to 256 MB of RAM.
- 24 -
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From the top: SIMM, DIMM and SODIMM memory modules
As processors grew in speed and bandwidth capability, the
industry adopted a new standard in dual in-line memory module
(DIMM). With a whopping 168-pin or 184-pin connector and a size
of 5.4 x 1 inch (about 14 x 2.5 cm), DIMMs range in capacity from
8 MB to 1 GB per module and can be installed singly instead of in
pairs. Most PC memory modules and the modules for the Mac G5
systems operate at 2.5 volts, while older Mac G4 systems typically
use 3.3 volts. Another standard, Rambus in-line memory module
(RIMM), is comparable in size and pin configuration to DIMM but
uses a special memory bus to greatly increase speed.
Many brands of notebook computers use proprietary memory
modules, but several manufacturers use RAM based on the small
outline dual in-line memory module (SODIMM) configuration.
SODIMM cards are small, about 2 x 1 inch (5 x 2.5 cm), and have
144 or 200 pins. Capacity ranges from 16 MB to 1 GB per module.
To conserve space, the Apple iMac desktop computer uses
SODIMMs instead of the traditional DIMMs. Sub-notebook
computers use even smaller DIMMs, known as MicroDIMMs,
which have either 144 pins or 172 pins.
Most memory available today is highly reliable. Most systems
simply have the memory controller check for errors at start-up and
rely on that. Memory chips with built-in error-checking typically use
a method known as parity to check for errors. Parity chips have an
- 25 -
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extra bit for every 8 bits of data. The way parity works is simple.
Let's look at even parity first.
When the 8 bits in a byte receive data, the chip adds up the total
number of 1s. If the total number of 1s is odd, the parity bit is set to
1. If the total is even, the parity bit is set to 0. When the data is
read back out of the bits, the total is added up again and compared
to the parity bit. If the total is odd and the parity bit is 1, then the
data is assumed to be valid and is sent to the CPU. But if the total
is odd and the parity bit is 0, the chip knows that there is an error
somewhere in the 8 bits and dumps the data. Odd parity works the
same way, but the parity bit is set to 1 when the total number of 1s
in the byte are even.
The problem with parity is that it discovers errors but does nothing
to correct them. If a byte of data does not match its parity bit, then
the data are discarded and the system tries again. Computers in
critical positions need a higher level of fault tolerance. High-end
servers often have a form of error-checking known as errorcorrection code (ECC). Like parity, ECC uses additional bits to
monitor the data in each byte. The difference is that ECC uses
several bits for error checking -- how many depends on the width
of the bus -- instead of one. ECC memory uses a special algorithm
not only to detect single bit errors, but actually correct them as
well. ECC memory will also detect instances when more than one
bit of data in a byte fails. Such failures are very rare, and they are
not correctable, even with ECC.
The majority of computers sold today use nonparity memory chips.
These chips do not provide any type of built-in error checking, but
instead rely on the memory controller for error detection.
SDRAM
From Wikipedia, the free encyclopedia
SDRAM means synchronous dynamic random access memory which
is a type of solid state computer memory.
Other dynamic random access memories (DRAM) have an
asynchronous interface which means that it reacts as quickly as
possible to changes in control inputs. SDRAM has a synchronous
interface, meaning that it waits for a clock signal before responding
to its control inputs. It is synchronized with the computer's system
bus, and thus with the processor. The clock is used to drive an
internal finite state machine that pipelines incoming instructions. This
- 26 -
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allows the chip to have a more complex pattern of operation than
DRAM which does not have synchronizing control circuits.
Pipelining means that the chip can accept a new instruction before
it has finished processing the previous one. In a pipelined write,
the write command can be immediately followed by another
instruction without waiting for the data to be written to the memory
array. In a pipelined read, the requested data appears after a fixed
number of clock pulses after the read instruction, cycles during
which additional instructions can be sent. (This delay is called the
latency and is an important parameter to consider when purchasing
SDRAM for a computer.)
Several SDRAM ICs on a PC100 DIMM package.
SDRAM History
Although the concept of synchronous DRAM has been known
since at least the 1970s and was used with early Intel processors,
it was only in 1993 that SDRAM began its path to universal
acceptance in the electronics industry. In 1993, Samsung
introduced its KM48SL2000 synchronous DRAM, and by 2000,
SDRAM had replaced virtually all other types of DRAM in modern
computers, because of its greater speed.
SDRAM latency is not inherently lower than asychronous DRAM.
Indeed,
early
SDRAM
was
somewhat
slower
than
contemporaneous burst EDO DRAM due to the additional logic.
The benefits of SDRAM's internal buffering come from its ability to
interleave operations to multiple banks of memory, thereby
increasing effective bandwidth.
Today, virtually all SDRAM is manufactured in compliance with
standards established by JEDEC, an electronics industry
association that adopts open standards to facilitate interoperability
of electronic components. JEDEC formally adopted its first
SDRAM standard in 1993 and subsequently adopted other
- 27 -
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SDRAM standards, including those for DDR, DDR2 and DDR3
SDRAM.
SDRAM is also available in registered memory varieties, for
systems that need greater scalability.
As of 2007, 168-pin SDRAM DIMMs are not used in new PC
systems, and 184-pin DDR memory has been mostly superseded.
DDR2 SDRAM is the most common type used with new PCs, and
DDR3 motherboards and memory are widely available, but more
expensive than still-popular DDR2 products.
Today, the world's largest manufacturers of SDRAM include:
Samsung Electronics, Micron Technology, Qimonda (formerly
Infineon Technologies) and Hynix.
SDRAM Timing
The fundamental limit on DRAM speed is the read cycle time, the
time between successive read operations to an open row. This
time decreased from 10 ns for 100 MHz SDRAM to 5 ns for DDR400, but has remained relatively unchanged through DDR2-800
and DDR3-1600 generations. However, by operating the interface
circuitry at increasingly higher multiples of the fundamental read
rate, the achievable bandwidth has increased rapidly.
Another limit is the CAS latency, the time between supplying a
column address and receiving the corresponding data. Again, this
has remained relatively constant at 10–15 ns through that last few
generations of DDR SDRAM.
In operation, CAS latency is a specific number of clock cycles
programmed into the SDRAM's mode register and expected by the
DRAM controller. Any value may be programmed, but the SDRAM
will not operate correctly if it is too low. At higher clock rates, the
useful CAS latency in clock cycles naturallly increases. 10–15 ns is
2–3 cycles (CL2–3) of the 200 MHz clock of DDR-400 SDRAM,
CL4-6 for DDR2-800, and CL8-12 for DDR3-1600. Slower clock
cycles will naturally allow lower numbers of CAS latency cycles.
SDRAM modules have their own timing specifications, which may
be slower than those of the chips on the module. When 100 MHz
SDRAM chips first appeared, some manufacturers sold "100 MHz"
modules that could not reliably operate at that speed. In response,
- 28 -
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Intel published the PC100 standard, which outlines requirements
and guidelines for producing a memory module that can operate
reliably at 100 MHz. This standard was widely influential, and the
term "PC100" quickly became a common identifier for 100 MHz
SDRAM modules, and modules are now commonly designated
with "PC"-prefixed numbers (although the actual meaning of the
numbers has changed).
DDR SDRAM
From Wikipedia, the free encyclopedia
DDR SDRAM or double-data-rate synchronous dynamic
random access memory is a class of memory integrated circuit
used in computers. It achieves greater bandwidth than the
preceding single-data-rate SDRAM by transferring data on the
rising and falling edges of the clock signal (double pumped).
Effectively, it doubles the transfer rate without increasing the
frequency of the memory bus.
With data being transferred 64 bits at a time, DDR SDRAM gives a
transfer rate of (memory bus clock rate) × 2 (for dual rate) × 64
(number of bits transferred) / 8 (number of bits/byte). Thus with a
bus frequency of 100 MHz, DDR-SDRAM gives a maximum
transfer rate of 1600 MB/s.
JEDEC has set standards for speeds of DDR SDRAM, divided into
two parts: The first specification is for memory chips and the
second is for memory modules.
As DDR is superseded by the newer DDR2, the older version is
commonly referred to as DDR1.
Chips and modules
Standard
name
DDR-200
Memory
clock
Time
between
signals
100 MHz 10 ns
I/O
Bus
clock
100
MHz
- 29 -
Data
transfers
per second
200 Million
Module
name
Peak
transfer
rate
PC-1600 1.600 GB/s
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DDR-266
133 MHz 7.5 ns
133
MHz
266 Million
PC-2100 2.133 GB/s
DDR-333
166 MHz 6 ns
166
MHz
333 Million
PC-2700 2.667 GB/s
DDR-400
200 MHz 5 ns
200
MHz
400 Million
PC-3200 3.200 GB/s
Note: All above listed are specified by JEDEC as JESD79. All
RAM speeds in-between or above these listed specifications are
not standardized by JEDEC — most often they are simply
manufacturer optimizations using higher-tolerance or overvolted
chips.
The package sizes in which DDR SDRAM is manufactured are
also standardised by JEDEC.
DDR SDRAM memory modules have 184 pins and one notch
There is no architectural difference between DDR SDRAM
designed for different clock frequencies, e.g. PC-1600 (designed to
run at 100 MHz) and PC-2100 (designed to run at 133 MHz). The
number simply designates the speed that the chip is guaranteed to
run at. Hence DDR SDRAM can be run at lower clock speeds than
it was made for (underclocking) or higher clock speeds than it was
made for (overclocking).
DDR SDRAM DIMMs have 184 pins (as opposed to 168 pins on
SDRAM, or, 240 pins on DDR2 SDRAM), and can be differentiated
from SDRAM DIMMs by the number of notches (DDR SDRAM has
one, SDRAM has two). DDR SDRAM operates at a voltage of
2.5 V, compared to 3.3 V for SDRAM. This can significantly reduce
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power usage. Note: some DIMMs have nominal voltage of 2.6 V
[1].
Many new chipsets use these memory types in dual-channel
configurations, which doubles or quadruples the effective
bandwidth.
Chip characteristics


DRAM density. Size of the chip in mebibits. Example: 256 Mibit — 32
MiB chip.
DRAM organization. Written in the form of 64M x 4, where 64M is a
number of storage units (64 million), x4 (pronounced «by 4») —
number of bits per chip, which equals the number of bits per storage
unit. There are x4, x8, and x16 DDR chips. The x4 chips allow the use
of advanced error correction features like Chipkill, memory scrubbing
and Intel SDDC, while the x8 and x16 chips are somewhat more
expensive.
Module characteristics





Size.
# of DRAM Devices. The number of chips is a multiple of 8 for nonECC modules and a multiple of 9 for ECC modules. Chips can occupy
one side (Single Sided) or both sides (Dual Sided) of the module. The
maximum amount of chips per DDR module is 36 (9x4).
# of DRAM rows (ranks). Any given module can have 1, 2 or 4 rows,
but only 1 row of a module can be active at any moment of time. When
a module has 2 or more rows, the memory controller must periodically
switch between them by performing close and open operations.
Timings: CAS Latency (CL), Clock Cycle Time (tCK), Row Cycle Time
(tRC), Refresh Row Cycle Time (tRFC), Row Active Time (tRAS).
Buffering: Registered vs. unbuffered
Module and chip characteristics are inherently linked.
Total module size is a product of one chip size by number of chips.
ECC modules multiply it by 8/9 because they use one bit per every
byte for error correction. A module of any particular size can
therefore be assembled either from 36 small chips, or 18 or 9
bigger ones.
DDR memory bus width per channel is 64 bits (72 for ECC
memory). Total module bit width is a product of bits per chip by
number of chips. It also equals number of ranks (rows) multiplied
by DDR memory bus width. Consequently a module with greater
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amount of chips or using x8 chips instead of x4 will have more
ranks.
Example: Variations of 1 GiB PC2100 Registered DDR SDRAM module
Module size
Number of
chips
Chip size
Chip
organization
Number of
rows (ranks)
1 GiB
36
256 Mibit
64M x 4
2
1 GiB
18
512 Mibit
64M x 8
2
1 GiB
18
512 Mibit
128M x 4
1
This example compares different real-world server memory
modules with a common size of 1 GiB. One should definitely be
careful buying 1 GiB memory module, because all these variations
can be sold under one price position without stating whether they
are x4 or x8, single or dual ranked.
There is a common belief that number of module rows or ranks
equals number of sides. As above data shows, this is not true. One
can find (2-side, 1-rank) or (2-side, 4-rank) modules. One can even
think of 1-side, 2-rank memory module having 16(18) chips on
single side x8 each, but it's unlikely such a module was ever
produced.
DDR2 SDRAM
From Wikipedia, the free encyclopedia
In electronic engineering, DDR2 SDRAM or double-data-rate two
synchronous dynamic random access memory is a random
access memory technology used for high speed storage of the
working data of a computer or other digital electronic device.
It is a part of the SDRAM (synchronous dynamic random access
memory) family of technologies, which is one of many DRAM
- 32 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
(dynamic random access memory) implementations, and is an
evolutionary improvement over its predecessor, DDR SDRAM.
Its primary benefit is the ability to operate the external data bus
twice as fast as DDR SDRAM. This is achieved by improved bus
signaling, and by operating the memory cells at half the clock rate
(one quarter of the data transfer rate), rather than at the clock rate
as in the original DDR. DDR2 memory at the same clock speed as
DDR will provide the same bandwidth but markedly higher latency,
providing worse performance.
Overview
A 512 MiB DDR2 533 module with BGA chips. DDR2 is a 240-pin module
Like all SDRAM implementations, DDR2 stores memory in
memory cells that are activated with the use of a clock signal to
synchronize their operation with an external data bus. Like DDR
before it, DDR2 cells transfer data both on the rising and falling
edge of the clock (a technique called "dual pumping"). The key
difference between DDR and DDR2 is that in DDR2 the bus is
clocked at twice the speed of the memory cells, so four words of
data can be transferred per memory cell cycle. Thus, without
speeding up the memory cells themselves, DDR2 can effectively
operate at twice the bus speed of DDR.
DDR2's bus frequency is boosted by electrical interface
improvements, on-die termination, prefetch buffers and off-chip
drivers. However, latency is greatly increased as a trade-off. The
DDR2 prefetch buffer is 4 bits deep, whereas it is 2 bits deep for
DDR and 8 bits deep for DDR3. While DDR SDRAM has typical
read latencies of between 2 and 3 bus cycles, DDR2 may have
read latencies between 4 and 6 cycles. Thus, DDR2 memory must
be operated at twice the bus speed to achieve the same latency in
nanoseconds.
- 33 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Another cost of the increased speed is the requirement that the
chips are packaged in a more expensive and more difficult to
assemble BGA package as compared to the TSSOP package of
the previous memory generations such as DDR and SDRAM. This
packaging change was necessary to maintain signal integrity at
higher speeds.[citation needed]
Power savings are achieved primarily due to an improved
manufacturing process through die shrinkage, resulting in a drop in
operating voltage (1.8 V compared to DDR's 2.5 V). The lower
memory clock frequency may also enable power reductions in
applications that do not require the highest available speed.
Specification standards
Chips and modules
For use in PCs, DDR2 SDRAM is supplied in DIMMs with 240 pins
and a single locating notch. DIMMs are identified by their peak
transfer capacity (often called bandwidth).
Standard
name
Memory
clock
Cycle
time
I/O
Bus
clock
Data
Module
transfers
name
per second
Peak
transfer rate
DDR2-400
100 MHz 10 ns
200
MHz
400 Million
PC2-3200
3.200 GB/s
DDR2-533
133 MHz 7.5 ns
266
MHz
533 Million
PC2-4200
4.264 GB/s
DDR2-667
166 MHz 6 ns
333
MHz
667 Million
PC2-53001 5.336 GB/s
DDR2-800
200 MHz 5 ns
400
MHz
800 Million
PC2-6400
6.400 GB/s
DDR21066
(planned)
266 MHz
3.75
ns
533
MHz
1066
Million
PC2-8500
(planned)
8.500 GB/s
Note: DDR2-xxx (or DDR-xxx) denotes data transfer rate, and
describes raw DDR chips, whereas PC2-xxxx (or PC-xxxx)
denotes theoretical bandwidth (though it is often rounded up or
down), and is used to describe assembled DIMMs. Bandwidth is
calculated by taking transfers per second and multiplying by eight.
This is because DDR2 memory modules transfer data on a bus
- 34 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
that is 64 data bits wide, and since a byte comprises 8 bits, this
equates to 8 bytes of data per transfer.
1
Some manufacturers label their DDR2-667 sticks as PC2-5400
instead of PC2-5300. At least one manufacturer has reported this
reflects successful testing at a faster-than standard speed.[1]
In addition to bandwidth and capacity variants, modules can
1. Optionally implement ECC, which is an extra data byte lane used
for correcting minor errors and detecting major errors for better
reliability. Modules with ECC are identified by an additional ECC in
their designation. PC2-4200 ECC is a PC2-4200 module with ECC.
2. Be "registered", which improves signal integrity (and hence
potentially clock speed and physical slot capacity) by electrically
buffering the signals at a cost of an extra clock of increased latency.
Those modules are identified by an additional R in their designation,
whereas non-registered (a.k.a. "unbuffered") RAM may be identified
by an additional U in the designation. PC2-4200R is a registered
PC2-4200 module, PC2-4200R ECC is the same module but with
additional ECC.
Note: registered and unbuffered SDRAM generally cannot be mixed on the
same channel.
Debut
DDR2 was introduced in the second quarter of 2003 at two initial
speeds: 200 MHz (referred to as PC2-3200) and 266 MHz (PC24200). Both performed worse than the original DDR specification
due to higher latency, which made total access times longer.
However, the original DDR technology tops out at speeds around
266 MHz (533 MHz effective). Faster DDR chips exist, but JEDEC
has stated that they will not be standardized. These modules are
mostly manufacturer optimizations of highest-yielding chips,
drawing significantly more power than slower-clocked modules,
and usually do not offer much, if any, greater real-world
performance.
DDR2 started to become competitive with the older DDR standard by the end
of 2004, as modules with lower latencies became available. [2]
Backwards compatibility
DDR2 DIMMs are not designed to be backwards compatible with
DDR DIMMs. The notch on DDR2 DIMMs is in a different position
- 35 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
than DDR DIMMs, and the pin density is slightly higher than DDR
DIMMs. DDR2 is a 240-pin module, DDR is a 184-pin module.
Faster DDR2 DIMMs are compatible with slower DDR2 DIMMs;
however, the faster module runs at the slower module's speed.
Using slower DDR2 memory in a system capable of higher speeds
results in the bus running at the speed of the slowest memory in
use.
DDR3 SDRAM:
From Wikipedia, the free encyclopedia
In electronic engineering, DDR3 SDRAM or double-data-rate
three synchronous dynamic random access memory is a
random access memory technology used for high speed storage of
the working data of a computer or other digital electronic devices.
It is a part of the SDRAM family of technologies, which is one of
many DRAM (dynamic random access memory) implementations,
and is an evolutionary improvement over its predecessor, DDR2
SDRAM.
Its primary benefit is the ability to run its I/O bus at four times the
speed of the memory cells it contains, thus enabling faster bus
speeds and higher peak throughputs than earlier technologies.
This is achieved at the cost of higher latency. Also, the DDR3
standard allows for chip capacities of 512 mebibit to 8 gibibit,
effectively enabling memory modules of maximum 16 gibibyte in
size.
Overview
DDR3 memory comes with a promise of a power consumption
reduction of 30% compared to current commercial DDR2 modules
due to DDR3's 1.5 V supply voltage, compared to DDR2's 1.8 V or
DDR's 2.5 V. This supply voltage works well with the 90 nm
fabrication technology used for most DDR3 chips. Some
manufacturers further propose to use "dual-gate" transistors to
reduce leakage of current. McCloskey., Alan. Research: DDR
FAQ. Retrieved on 2007-10-18.
- 36 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
The main benefit of DDR3 comes from the higher bandwidth made
possible by DDR3's 8 bit deep prefetch buffer, whereas DDR2's is
4 bits, and DDR's is 2 bits deep.
Theoretically, these modules could transfer data at the effective
clock rate of 800–1600 MHz (using both edges of a 400–800 MHz
I/O clock), compared to DDR2's current range of effective 400–800
MHz (200–400 MHz clock) or DDR's range of 200–400 MHz (100–
200 MHz). To date, such bandwidth requirements have been
mainly found in the graphics market, where fast transfer of
information between framebuffers is required.
Prototypes were announced in early 2005, and products are
appearing on the market as of mid-[2007], in the form of
motherboards[1] based on Intel's P35 "Bearlake" chipset and
memory DIMMs at speeds up to DDR3-1600.[2]. AMD's roadmap
indicates their own adoption of DDR3 to come in 2008.
DDR3 DIMMs have 240 pins, the same number as DDR2, and are
the same size, but are electrically incompatible and have a
different key notch location.[3]
GDDR3 memory, with a similar name but an entirely dissimilar
technology, has been in use for several years in high-end graphic
cards such as ones from NVIDIA or ATI Technologies, and as
main system memory on the Microsoft Xbox 360. It has sometimes
been incorrectly referred to as "DDR3".
Chips and Modules
Standard
name
Time
Memory
between
clock
signals
I/O
Bus
clock
Data
Module
transfers
name
per second
DDR3800
100
MHz
10 ns
400
MHz
800 Million
PC36400
6.40 GB/s
DDR31066
133
MHz
7.5 ns
533
MHz
1.066
Billion
PC38500
8.53 GB/s
DDR31333
166
MHz
6 ns
667
MHz
1.333
Billion
PC310600
10.67 GB/s[4]
DDR31600
200
MHz
5 ns
800
MHz
1.6 Billion
PC312800
12.80 GB/s
- 37 -
Peak transfer
rate
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Features
DDR3 SDRAM Components:






Introduction of asynchronous RESET pin
Support of system level flight time compensation
On-DIMM Mirror friendly DRAM pin out
Introduction of CWL (CAS Write Latency) per speed bin
On-die IO calibration engine
READ and WRITE calibration
DDR3 Modules:


Fly-by command/address/control bus with On-DIMM termination
High precision calibration resistors
Advantages compared to DDR2




Higher bandwidth performance increase (up to effective 1600 MHz)
Performance increase at low power (longer battery life in laptops)
Enhanced low power features
Improved thermal design (cooler)
Disadvantages compared to DDR2


Commonly higher CAS Latency
Generally costs more than equivalent DDR2 memory
- 38 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Mushkin HP3-10666 2GB DDR3-1333 Memory Kit
Review 3‫مثال على ذاكرة ددر‬
Mushkin is easily recognizable as an enthusiast memory
manufacturer, so it's no surprise the company is quick out of the
gates with fast DDR3, 1333MHz in fact. Mushkin is turning up the
heat and while the DDR3 standard may be in its infancy, from an
overclocker's stand point it's simply a fantastic time.
Given the high price premium on DDR3 RAM, companies are only
releasing enthusiast caliber memory modules. The competition is
so close that what's making it to store shelves is literally the cream
of the crop. Pricey... sure, but fast as you can wish for. When it
comes to performance there are two ways to improve memory,
bump up the speed (which is how most manufacturers have been
doing things) or lower memory timings.
Mushkin has taken the latter approach, which is a nice change
from the usual high speed DDR3 memory we've seen on the
market that has lax timings.
In fact, as of this review Mushkin have the lowest latency DDR3
memory out.
The Mushkin memory that's up for testing today is its 2GB HP310666 (DDR3-1333) memory kit. By default the memory is rated to
run at 1333 MHz with low CAS latency timings of 6-7-6-18, on a
voltage of 1.7-1.8V. Much of the DDR3 memory on the market has
a CAS Latency of 7 or 8, so Mushkin's memory here will perform
faster clock for clock compared to the competition!
The Mushkin HP3-10666 memory is not quite available, but you
can expect prices in the range of $500 for the pair.
- 39 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Mushkin HP3-10666 Memory
RAM Memory Type: DDR3-1333 / PC3-10666
Individual Module Capacity: 10248MB
Native Speed & Latency: 1333 MHz @ CAS 6-7-6-18
Mushkin stick with their custom FrostByte aluminum heatspreaders
to keep the DDR3 memory modules running cool, and safe from
physical damage. The memory heatspreaders are a little taller than
average. They are nice and slim, so there won't be any problems
- 40 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
installing the Mushkin HP3-10666 memory into tight cases, or SFF
machines. PCSTATS does not recommend attempting to remove
heat spreaders from BGA RAM modules, doing so will most likely
result in dead memory! Trust me, we learned the hard way.
Mushkin has an excellent support system, as do virtually all online
memory stores. There are online support forums, online RMA
services, and a toll free 1800 number. If problems arise, you're set
no matter what happens.
Now onto the good stuff.... These 2GB blue babies are rated to run
at DDR3-1333 speeds, with 6-7-6-18 timings and a voltage of 1.71.8V.
So far the DDR3 memory PCSTATS has tested has run at 7-7-720 or 8-8-8-24! As always, with latency timings, lower times are
best. A couple of years ago PCSTATS demonstrated the affects of
memory timings with DDR memory, well the same general rule
applies with DDR2 and DDR3 memory as well.
If you stock your enthusiast grade PC with memory featuring tight
timings, that usually means forgoing high operating memory
speeds for net performance. In other words, users looking at the
Mushkin HP3-10666 should not expect to break technical speed
records. That doesn't mean this modules are going to be slow
though... quite the contrary in fact.
Current DDR3 Chipsets?
If you'd like to equip your computer with DDR3 memory, currently
motherboards based Intel's P35 or X38/X48 Express chipset are
the only option... and then only models specifically built to support
DDR3 DIMMs. The Intel X38 Express chipset is the flagship
choice. If you're an AMD user, you'll have to wait a bit longer;
currently none of its processors utilize DDR3 RAM. Each AMD
processor has its memory controller built right into the core, so
DDR3 can only be adopted when the underlying CPU architecture
calls for it. As it stands, the AMD 'AM3' processor, due late 2008 is
the first AMD CPU road-mapped for DDR3 memory.
PCSTATS will take you for a closer look at the DDR3 memory
standard next, before running through a very exciting overclocking
session!
- 41 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Understanding the Basic of DDR3 Memory
DDR3 memory is not really expected to take hold until mid-2008, when quadcore processors are basically mainstream chips. It's hard to say where AMD
will stand on all of this, its 'K10' microprocessor architecture is not
expected to adopt DDR3 RAM until 2009 according to the last
report PCSTATS saw.
In any case, as with all new memory standards it's important to
state the obvious to minimize confusion.
While DDR2 and DDR3 RAM Dual Inline Memory Modules
(DIMMs) are physically the same size, and contain the same
number of little gold teeth (240), each class of memory is keyed
differently and so neither is interchangeable with the other socket.
DDR3 memory modules will not work in a DDR2 memory socket,
even if the motherboard chipset technically supports the DDR3
standard - as is the case with the Intel P35 Express, and Intel X38
chipset. Conversely, DDR3 memory is not backwards compatible
with DDR2 memory slots.
One obvious difference between DDR-3 memory and its
predecessor is that DDR3 operates with less voltage. DDR-3 RAM
requires 1.5V, while DDR2 demands 1.8V power. Next, unique
memory slots prevent DDR3 memory from being installed in a
DDR2 memory slot, and vice versa. The memory standards
themselves are not interoperable, so neither are the sockets.
- 42 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
These are 1.5V DDR3 memory slots. They look exactly like DDR2
slots, except for the 'key' which is in a slightly different position.
Below is a DDR3 memory module over top of a DDR2 module.
Note the subtle difference where the DDR3 module is 'keyed' with
respect to the DDR2 module. Both memory standards have exactly
240 gold connectors, DDR3 operates at 1.5V, DDR2 at 1.8V.
New memory standards are almost always made incompatible with
old ones. In this case it is because the voltage and data transfer
architectures differ.
Placed edge to edge, it is easy to see the physical difference
between DDR2 and DDR3 modules. Where the module is 'keyed'
is what separates the modules from bing installed in the wrong
slot.
- 43 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
JEDEC standards dictate desktop DDR2 memory speeds to
between 400-800 MHz, although chipset and memory
manufacturers have pushed DDR2 speeds much further. The
JEDEC standard for DDR3 memory started at 800 MHz, running
officially as high as 1600 MHz. While there is a bit of overlap in
terms of speeds, it's not likely to last. At the moment most DDR3
memory is coming in two flavors, 1066 and 1333 MHz. The speed
will increase as the memory standard matures.
- 44 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Cache Memory‫ذاكرة كاش‬
If you have been shopping for a computer, then you have heard
the word "cache." Modern computers have both L1 and L2 caches,
and many now also have L3 cache. You may also have gotten
advice on the topic from well-meaning friends, perhaps something
like "Don't buy that Celeron chip, it doesn't have any cache in it!"
It turns out that caching is an important computer-science process
that appears on every computer in a variety of forms. There are
memory caches, hardware and software disk caches, page caches
and more. Virtual memory is even a form of caching. In this article,
we will explore caching so you can understand why it is so
important.
A Simple Example: Before Cache
Caching is a technology based on the memory subsystem of your
computer. The main purpose of a cache is to accelerate your
computer while keeping the price of the computer low. Caching
allows you to do your computer tasks more rapidly.
To understand the basic idea behind a cache system, let's start
with a super-simple example that uses a librarian to demonstrate
caching concepts. Let's imagine a librarian behind his desk. He is
there to give you the books you ask for. For the sake of simplicity,
let's say you can't get the books yourself -- you have to ask the
librarian for any book you want to read, and he fetches it for you
from a set of stacks in a storeroom (the library of congress in
Washington, D.C., is set up this way). First, let's start with a
librarian without cache.
The first customer arrives. He asks for the book Moby Dick. The
librarian goes into the storeroom, gets the book, returns to the
counter and gives the book to the customer. Later, the client
comes back to return the book. The librarian takes the book and
returns it to the storeroom. He then returns to his counter waiting
for another customer. Let's say the next customer asks for Moby
Dick (you saw it coming...). The librarian then has to return to the
storeroom to get the book he recently handled and give it to the
- 45 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
client. Under this model, the librarian has to make a complete
round trip to fetch every book -- even very popular ones that are
requested frequently. Is there a way to improve the performance of
the librarian?
Yes, there's a way -- we can put a cache on the librarian. In the
next section, we'll look at this same example but this time, the
librarian will use a caching system.
A Simple Example: After Cache
Let's give the librarian a backpack into which he will be able to
store 10 books (in computer terms, the librarian now has a 10book cache). In this backpack, he will put the books the clients
return to him, up to a maximum of 10. Let's use the prior example,
but now with our new-and-improved caching librarian.
The day starts. The backpack of the librarian is empty. Our first
client arrives and asks for Moby Dick. No magic here -- the
librarian has to go to the storeroom to get the book. He gives it to
the client. Later, the client returns and gives the book back to the
librarian. Instead of returning to the storeroom to return the book,
the librarian puts the book in his backpack and stands there (he
checks first to see if the bag is full -- more on that later). Another
client arrives and asks for Moby Dick. Before going to the
storeroom, the librarian checks to see if this title is in his backpack.
He finds it! All he has to do is take the book from the backpack and
give it to the client. There's no journey into the storeroom, so the
client is served more efficiently.
What if the client asked for a title not in the cache (the backpack)?
In this case, the librarian is less efficient with a cache than without
one, because the librarian takes the time to look for the book in his
backpack first. One of the challenges of cache design is to
minimize the impact of cache searches, and modern hardware has
reduced this time delay to practically zero. Even in our simple
librarian example, the latency time (the waiting time) of searching
the cache is so small compared to the time to walk back to the
storeroom that it is irrelevant. The cache is small (10 books), and
the time it takes to notice a miss is only a tiny fraction of the time
that a journey to the storeroom takes.
- 46 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
From this example you can see several important facts about
caching:




Cache technology is the use of a faster but smaller
memory type to accelerate a slower but larger
memory type.
When using a cache, you must check the cache to see
if an item is in there. If it is there, it's called a cache
hit. If not, it is called a cache miss and the computer
must wait for a round trip from the larger, slower
memory area.
A cache has some maximum size that is much smaller
than the larger storage area.
It is possible to have multiple layers of cache. With our
librarian example, the smaller but faster memory type
is the backpack, and the storeroom represents the
larger and slower memory type. This is a one-level
cache. There might be another layer of cache
consisting of a shelf that can hold 100 books behind
the counter. The librarian can check the backpack,
then the shelf and then the storeroom. This would be
a two-level cache.
Computer Caches
A computer is a machine in which we measure time in very small
increments. When the microprocessor accesses the main memory
(RAM), it does it in about 60 nanoseconds (60 billionths of a
second). That's pretty fast, but it is much slower than the typical
microprocessor. Microprocessors can have cycle times as short as
2 nanoseconds, so to a microprocessor 60 nanoseconds seems
like an eternity.
What if we build a special memory bank in the motherboard, small
but very fast (around 30 nanoseconds)? That's already two times
faster than the main memory access. That's called a level 2 cache
or an L2 cache. What if we build an even smaller but faster
memory system directly into the microprocessor's chip? That way,
this memory will be accessed at the speed of the microprocessor
and not the speed of the memory bus. That's an L1 cache, which
on a 233-megahertz (MHz) Pentium is 3.5 times faster than the L2
cache, which is two times faster than the access to main memory.
- 47 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Some microprocessors have two levels of cache built right into the
chip. In this case, the motherboard cache -- the cache that exists
between the microprocessor and main system memory -- becomes
level 3, or L3 cache.
There are a lot of subsystems in a computer; you can put cache
between many of them to improve performance. Here's an
example. We have the microprocessor (the fastest thing in the
computer). Then there's the L1 cache that caches the L2 cache
that caches the main memory which can be used (and is often
used) as a cache for even slower peripherals like hard disks and
CD-ROMs. The hard disks are also used to cache an even slower
medium -- your Internet connection.
Caching Subsystems
Your Internet connection is the slowest link in your computer. So
your browser (Internet Explorer, Netscape, Opera, etc.) uses the
hard disk to store HTML pages, putting them into a special folder
on your disk. The first time you ask for an HTML page, your
browser renders it and a copy of it is also stored on your disk. The
next time you request access to this page, your browser checks if
the date of the file on the Internet is newer than the one cached. If
the date is the same, your browser uses the one on your hard disk
instead of downloading it from Internet. In this case, the smaller
but faster memory system is your hard disk and the larger and
slower one is the Internet.
Cache can also be built directly on peripherals. Modern hard
disks come with fast memory, around 512 kilobytes, hardwired to
the hard disk. The computer doesn't directly use this memory -- the
hard-disk controller does. For the computer, these memory chips
are the disk itself. When the computer asks for data from the hard
disk, the hard-disk controller checks into this memory before
moving the mechanical parts of the hard disk (which is very slow
compared to memory). If it finds the data that the computer asked
for in the cache, it will return the data stored in the cache without
actually accessing data on the disk itself, saving a lot of time.
Here's an experiment you can try. Your computer caches your
floppy drive with main memory, and you can actually see it
happening. Access a large file from your floppy -- for example,
open a 300-kilobyte text file in a text editor. The first time, you will
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see the light on your floppy turning on, and you will wait. The
floppy disk is extremely slow, so it will take 20 seconds to load the
file. Now, close the editor and open the same file again. The
second time (don't wait 30 minutes or do a lot of disk access
between the two tries) you won't see the light turning on, and you
won't wait. The operating system checked into its memory cache
for the floppy disk and found what it was looking for. So instead of
waiting 20 seconds, the data was found in a memory subsystem
much faster than when you first tried it (one access to the floppy
disk takes 120 milliseconds, while one access to the main memory
takes around 60 nanoseconds -- that's a lot faster). You could
have run the same test on your hard disk, but it's more evident on
the floppy drive because it's so slow.
To give you the big picture of it all, here's a list of a normal caching
system:
L1 cache - Memory accesses at full microprocessor
speed (10 nanoseconds, 4 kilobytes to 16 kilobytes in
size)
 L2 cache - Memory access of type SRAM (around 20
to 30 nanoseconds, 128 kilobytes to 512 kilobytes in
size)
 Main memory - Memory access of type RAM (around
60 nanoseconds, 32 megabytes to 128 megabytes in
size)
 Hard disk - Mechanical, slow (around 12 milliseconds,
1 gigabyte to 10 gigabytes in size)
 Internet - Incredibly slow (between 1 second and 3
days, unlimited size)
As you can see, the L1 cache caches the L2 cache, which caches
the main memory, which can be used to cache the disk
subsystems, and so on.

Cache Technology
One common question asked at this point is, "Why not make all of
the computer's memory run at the same speed as the L1 cache, so
no caching would be required?" That would work, but it would be
incredibly expensive. The idea behind caching is to use a small
amount of expensive memory to speed up a large amount of
slower, less-expensive memory.
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In designing a computer, the goal is to allow the microprocessor to
run at its full speed as inexpensively as possible. A 500-MHz chip
goes through 500 million cycles in one second (one cycle every
two nanoseconds). Without L1 and L2 caches, an access to the
main memory takes 60 nanoseconds, or about 30 wasted cycles
accessing memory.
When you think about it, it is kind of incredible that such relatively
tiny amounts of memory can maximize the use of much larger
amounts of memory. Think about a 256-kilobyte L2 cache that
caches 64 megabytes of RAM. In this case, 256,000 bytes
efficiently caches 64,000,000 bytes. Why does that work?
In computer science, we have a theoretical concept called locality
of reference. It means that in a fairly large program, only small
portions are ever used at any one time. As strange as it may
seem, locality of reference works for the huge majority of
programs. Even if the executable is 10 megabytes in size, only a
handful of bytes from that program are in use at any one time, and
their rate of repetition is very high. On the next page, you'll learn
more about locality of reference.
Locality of Reference
Let's take a look at the following pseudo-code to see why locality
of reference works (see How C Programming Works to really get
into it):
Output to screen « Enter a number between 1 and 100 »
Read input from user
Put value from user in variable X
Put value 100 in variable Y
Put value 1 in variable Z
Loop Y number of time
Divide Z by X
If the remainder of the division = 0
then output « Z is a multiple of X »
Add 1 to Z
Return to loop
End
This small program asks the user to enter a number between 1
and 100. It reads the value entered by the user. Then, the program
divides every number between 1 and 100 by the number entered
by the user. It checks if the remainder is 0 (modulo division). If so,
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the program outputs "Z is a multiple of X" (for example, 12 is a
multiple of 6), for every number between 1 and 100. Then the
program ends.
Even if you don't know much about computer programming, it is
easy to understand that in the 11 lines of this program, the loop
part (lines 7 to 9) are executed 100 times. All of the other lines are
executed only once. Lines 7 to 9 will run significantly faster
because of caching.
This program is very small and can easily fit entirely in the smallest
of L1 caches, but let's say this program is huge. The result remains
the same. When you program, a lot of action takes place inside
loops. A word processor spends 95 percent of the time waiting for
your input and displaying it on the screen. This part of the wordprocessor program is in the cache.
This 95%-to-5% ratio (approximately) is what we call the locality of
reference, and it's why a cache works so efficiently. This is also
why such a small cache can efficiently cache such a large memory
system. You can see why it's not worth it to construct a computer
with the fastest memory everywhere. We can deliver 95 percent of
this effectiveness for a fraction of the cost.
For more information on caching and related topics, check out the
links on the next page.
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Virtual Memory ‫الذاكرة الواقعية‬
Virtual memory is a common part of most operating systems on
desktop computers. It has become so common because it provides
a big benefit for users at a very low cost.
In this article, you will learn exactly what virtual memory is, what
your computer uses it for and how to configure it on your own
machine to achieve optimal performance.
Your Browser Does Not Support iFrames
Most computers today have something like 32 or 64 megabytes of
RAM available for the CPU to use (see How RAM Works for
details on RAM). Unfortunately, that amount of RAM is not enough
to run all of the programs that most users expect to run at once.
For example, if you load the operating system, an e-mail program,
a Web browser and word processor into RAM simultaneously, 32
megabytes is not enough to hold it all. If there were no such thing
as virtual memory, then once you filled up the available RAM your
computer would have to say, "Sorry, you can not load any more
applications. Please close another application to load a new one."
With virtual memory, what the computer can do is look at RAM for
areas that have not been used recently and copy them onto the
hard disk. This frees up space in RAM to load the new application.
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Because this copying happens automatically, you don't even
know it is happening, and it makes your computer feel like is has
unlimited RAM space even though it only has 32 megabytes
installed. Because hard disk space is so much cheaper than RAM
chips, it also has a nice economic benefit.
The read/write speed of a hard drive is much slower than RAM,
and the technology of a hard drive is not geared toward accessing
small pieces of data at a time. If your system has to rely too
heavily on virtual memory, you will notice a significant performance
drop. The key is to have enough RAM to handle everything you
tend to work on simultaneously -- then, the only time you "feel" the
slowness of virtual memory is is when there's a slight pause when
you're changing tasks. When that's the case, virtual memory is
perfect.
When it is not the case, the operating system has to constantly
swap information back and forth between RAM and the hard disk.
This is called thrashing, and it can make your computer feel
incredibly slow.
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The area of the hard disk that stores the RAM image is called a
page file. It holds pages of RAM on the hard disk, and the
operating system moves data back and forth between the page file
and RAM. On a Windows machine, page files have a .SWP
extension.
Next, we'll look at how to configure virtual memory on a computer.
Configuring Virtual Memory
Windows 98 is an example of a typical operating system that has
virtual memory. Windows 98 has an intelligent virtual memory
manager that uses a default setting to help Windows allocate hard
drive space for virtual memory as needed. For most
circumstances, this should meet your needs, but you may want to
manually configure virtual memory, especially if you have more
than one physical hard drive or speed-critical applications.
To do this, open the "Control Panel" window and double-click on
the "System" icon. The system dialog window will open. Click on
the "Performance" tab and then click on the "Virtual Memory"
button.
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Click on the option that says, "Let me specify my own virtual
memory settings." This will make the options below that statement
become active. Click on the drop-down list beside "Hard disk:" to
select the hard drive that you wish to configure virtual memory for.
Remember that a good rule of thumb is to equally split virtual
memory between the physical hard disks you have.
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In the "Minimum:" box, enter the smallest amount of hard drive
space you wish to use for virtual memory on the hard disk
specified. The amounts are in megabytes. For the "C:" drive, the
minimum should be 2 megabytes. The "Maximum:" figure can be
anything you like, but one possible upper limit is twice physical
RAM space. Windows default is normally 12 megabytes above the
amount of physical RAM in your computer. To put the new settings
into effect, close the dialog box and restart your computer.
The amount of hard drive space you allocate for virtual memory is
important. If you allocate too little, you will get "Out of Memory"
errors. If you find that you need to keep increasing the size of the
virtual memory, you probably are also finding that your system is
sluggish and accesses the hard drive constantly. In that case, you
should consider buying more RAM to keep the ratio between RAM
and virtual memory about 2:1. Some applications enjoy having lots
of virtual memory space but do not access it very much. In that
case, large paging files work well.
One trick that can improve the performance of virtual memory
(especially when large amounts of virtual memory are needed) is
to make the minimum and maximum sizes of the virtual memory
file identical. This forces the operating system to allocate the
entire paging file when you start the machine. That keeps the
paging file from having to grow while programs are running, which
improves performance. Many video applications recommend this
technique to avoid pauses while reading or writing video
information between hard disk and tape.
Another factor in the performance of virtual memory is the location
of the pagefile. If your system has multiple physical hard drives
(not multiple drive letters, but actual drives), you can spread the
work among them by making smaller pagefiles on each drive. This
simple modification will significantly speed up any system that
makes heavy use of virtual memory.
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‫ القرص الصلب‬Hard Disk
Nearly every desktop computer and server in use today contains
one or more hard-disk drives. Every mainframe and
supercomputer is normally connected to hundreds of them. You
can even find VCR-type devices and camcorders that use hard
disks instead of tape. These billions of hard disks do one thing well
-- they store changing digital information in a relatively permanent
form. They give computers the ability to remember things when the
power goes out.
In this article, we'll take apart a hard disk so that you can see
what's inside, and also discuss how they organize the gigabytes of
information they hold in files!
Hard Disk Basics
Hard disks were invented in the 1950s. They started as large disks
up to 20 inches in diameter holding just a few megabytes. They
were originally called "fixed disks" or "Winchesters" (a code name
used for a popular IBM product). They later became known as
"hard disks" to distinguish them from "floppy disks." Hard disks
have a hard platter that holds the magnetic medium, as opposed
to the flexible plastic film found in tapes and floppies.
At the simplest level, a hard disk is not that different from a
cassette tape. Both hard disks and cassette tapes use the same
magnetic recording techniques described in How Tape Recorders
Work. Hard disks and cassette tapes also share the major benefits
of magnetic storage -- the magnetic medium can be easily erased
and rewritten, and it will "remember" the magnetic flux patterns
stored onto the medium for many years.
In the next section, we'll talk about the main differences between
casette tapes and hard disks.
Capacity and Performance
A typical desktop machine will have a hard disk with a capacity of
between 10 and 40 gigabytes. Data is stored onto the disk in the
form of files. A file is simply a named collection of bytes. The bytes
might be the ASCII codes for the characters of a text file, or they
could be the instructions of a software application for the computer
to execute, or they could be the records of a data base, or they
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could be the pixel colors for a GIF image. No matter what it
contains, however, a file is simply a string of bytes. When a
program running on the computer requests a file, the hard disk
retrieves its bytes and sends them to the CPU one at a time.
There are two ways to measure the performance of a hard disk:
Data rate - The data rate is the number of bytes per
second that the drive can deliver to the CPU. Rates
between 5 and 40 megabytes per second are common.
 Seek time - The seek time is the amount of time between
when the CPU requests a file and when the first byte of
the file is sent to the CPU. Times between 10 and 20
milliseconds are common.
The other important parameter is the capacity of the drive, which
is the number of bytes it can hold.

Inside: Electronics Board
The best way to understand how a hard disk works is to take a
look inside. (Note that OPENING A HARD DISK RUINS IT, so this
is not something to try at home unless you have a defunct drive.)
Here is a typical hard-disk drive:
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It is a sealed aluminum box with controller electronics attached to
one side. The electronics control the read/write mechanism and
the motor that spins the platters. The electronics also assemble
the magnetic domains on the drive into bytes (reading) and turn
bytes into magnetic domains (writing). The electronics are all
contained on a small board that detaches from the rest of the
drive:
Inside: Beneath the Board
Underneath the board are the connections for the motor that spins
the platters, as well as a highly-filtered vent hole that lets internal
and external air pressures equalize:
Removing the cover from the drive reveals an extremely simple but
very precise interior:
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In this picture you can see:


The platters - These typically spin at 3,600 or 7,200 rpm
when the drive is operating. These platters are
manufactured to amazing tolerances and are mirrorsmooth (as you can see in this interesting self-portrait of
the author... no easy way to avoid that!).
The arm - This holds the read/write heads and is
controlled by the mechanism in the upper-left corner. The
arm is able to move the heads from the hub to the edge
of the drive. The arm and its movement mechanism are
extremely light and fast. The arm on a typical hard-disk
drive can move from hub to edge and back up to 50 times
per second -- it is an amazing thing to watch!
Inside: Platters and Heads
In order to increase the amount of information the drive can store,
most hard disks have multiple platters. This drive has three
platters and six read/write heads:
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The mechanism that moves the arms on a hard disk has to be
incredibly fast and precise. It can be constructed using a highspeed linear motor.
Many drives use a "voice coil" approach -- the same technique
used to move the cone of a speaker on your stereo is used to
move the arm.
Storing the Data
Data is stored on the surface of a platter in sectors and tracks.
Tracks are concentric circles, and sectors are pie-shaped wedges
on a track, like this:
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A typical track is shown in yellow; a typical sector is shown in blue.
A sector contains a fixed number of bytes -- for example, 256 or
512. Either at the drive or the operating system level, sectors are
often grouped together into clusters.
The process of low-level formatting a drive establishes the tracks
and sectors on the platter. The starting and ending points of each
sector are written onto the platter. This process prepares the drive
to hold blocks of bytes. High-level formatting then writes the filestorage structures, like the file-allocation table, into the sectors.
This process prepares the drive to hold files.
For more information on hard disks and related topics, check out
the links on the next page.
Hard Disk Connectors and Jumpers
Several different connectors and jumpers are used to configure the
hard disk and connect it to the rest of the system. The number and
types of connectors on the hard disk depend on the data interface
it uses to connect to the system, the manufacturer of the drive, and
any special features that the drive may possess. Instructions for
setting common jumpers are usually printed right on the drive; full
instructions for all jumpers will be in the product's manual, or on
the manufacturer's web site.
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Some of the connectors and jumper pins
3.5", 36 GB, 10,000 RPM SCSI Cheetah drive.
on
a
Power Connector
use a standard, 4-pin male connector plug, that takes one of the
power connectors coming from the power supply. This keyed, 4wire plastic connector provides +5 and +12 voltage to the hard
disk.
A
standard
hard
disk
power
connector.
Note
"4, 3, 2, 1" pin markings on the logic board, and
square shape of solder pad for pin #1.
the
the
Data Interface Connector
Modern hard disk drives use one of two interfaces: IDE (ATA) and
its variants, or SCSI. You can tell immediately by looking at the
back of the hard disk which interface is being used by the drive:
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

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IDE/ATA: A 40-pin rectangular connector. See here for more
information on IDE/ATA cables and connectors.
SCSI: A 50-pin, 68-pin, or 80-pin D-shaped connector (the
same shape used for serial and parallel port connectors). A
50-pin connector means the device is narrow SCSI; 68 pins
means wide SCSI; 80 pins means wide SCSI using single
connector attachment (SCA). See here for more on SCSI
cables and connectors.
A standard hard disk IDE/ATA data interface connector.
If you look closely you can see markings for pins #1, #2, #39 and #40.
A 50-pin SCSI interface connector looks identical except that it
has 25 columns of pins instead of 20 (they look so much alike that
getting the two mixed up is common). A 68-pin SCSI interface
connector is pictured on the parent page of this page.
The connectors on hard disk drives are generally in the form of a
2xN rectangular grid of pins (where N is 20, 25, 34 or 40
depending on the interface). Older ST-506 (also called MFM, RLL)
and ESDI hard disks used two data connectors, one 34 pins and
the other 20 pins. These connectors were often not in the form of
pins but rather card edge connectors, such as those used by ISA
expansion cards. Some SCSI connectors may have different
shapes, especially older ones.
While most current SCSI interface connectors are keyed to prevent
incorrect insertion (because they are D-shaped), this is not always
the case for other interfaces. For this reason, it is important to
make sure that the cable is oriented the correct way before
plugging it in. The cable has a red stripe to indicate wire #1 and
the hard disk uses markers of one form or another to indicate the
matching pin #1.
IDE/ATA Configuration Jumpers
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IDE/ATA hard disks are fairly standard in terms of jumpers. There
are usually only a few and they don't vary greatly from drive to
drive. Here are the jumpers you will normally find:




Drive Select: Since there can be two drives (master and
slave) on the same IDE channel, a jumpers is normally used
to tell each drive if it should function as a master or slave on
the IDE channel. For a single drive on a channel, most
manufacturers instruct that the drive be jumpered as master,
while some manufacturers (notably Western Digital) have a
separate setting for a single drive as opposed to a master on
a channel with a slave. The terms "master" and "slave" are
misleading since the drives really have no operational
relationship. See this section on IDE/ATA jumpering for
more.
Slave Present: Some drives have an additional jumper that
is used to tell a drive configured as master that there is also
a slave drive on the ATA channel. This is only required for
some older drives that don't support standard master/slave
IDE channel signaling.
Cable Select: Some configurations use a special cable to
determine which drive is master and which is slave, and
when this system is used a cable select jumper is normally
enabled.
Size Restriction Jumper: Some larger hard disk drives
don't work properly in older PCs that don't have a BIOS
program modern enough to recognize them. To get around
this, some drives have special jumpers that, when set, will
cause them to appear as a smaller size than they really are
to the BIOS, for compatibility. For example, some 2.5 GB
hard disks have a jumper that will cause them to appear as a
2.1 GB hard disk to a system that won't support anything
over 2.1 GB. These are also sometimes called capacity
limitation jumpers and vary from manufacturer to
manufacturer. See here for more on this.
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Jumper block for an IDE hard disk.The jumpers are labeled "MA"
(master),"SL" (slave) and "CS" (cable select). Other IDE drives will
have slightly different jumper configuration or placement.
Connectors Types
• IDE Connectors
• SATA Connectors
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• SCSI Connectors
File System Type
The file system refers to the structures that are used to organize
data at a high level on the disk; the file system is used by the
operating system to store files, directories and other relevant
information on the disk for later retrieval. As such, the file system is
highly operating-system-dependent. In most cases you don't
generally have a "choice" between different file system types.
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However, in some operating systems you do, and there can be a
performance impact from the choice.
Some file systems store files in packages as small as 512 bytes,
while others store files in larger chunks called allocation units or
clusters. Some are very simple file systems with few features and
little overhead (such as the FAT file system used in DOS and
Windows 9x), and others have many features but comparatively
higher overhead (NTFS used in NT). Windows NT and 2000
typically give you your choice of file system; Windows 2000
supports FAT16, FAT32 and NTFS. See here for more on the
different file systems used in the PC.
Which file system you use can have an effect on overall
performance, but it is relatively small: typically a few percentage
points. It's also difficult to predict exactly what the effect will be for
a given system when selecting from one file system to another.
Since the file system affects so many other usability factors of the
PC, performance is usually not one of the primary factors for
deciding between them. As an example, consider the "FAT vs.
NTFS" decision, which is probably the most common "file system
decision" in the PC world today. These two file systems are so
different in so many ways that most people choose one or the
other for reasons particular to their use, not performance. If you
need the high security and advanced management features of
NTFS, you are probably going to use NTFS even if FAT is a few
percentage points "faster". Similarly, if you need the compatibility
and simplicity of FAT, changing to NTFS for a few ticks on a
benchmark is probably unwise.
One file system choice that is commonly made in part for
performance reasons is "FAT16 vs. FAT32"; this is really a "subfile-system" choice, since FAT16 and FAT32 are really two flavors
of the same file system. The primary performance impact of
changing between these has nothing to do with anything inherently
different between FAT16 or FAT32, but rather the difference in
cluster size that results from the choice. See here for more details
on this.
Partitioning and Volume Position
Partitioning is the process of dividing the hard disk into
subsections, called volumes. It is an important initial step in
preparing a hard disk for use.
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The choice of how the hard disk is partitioned can have a tangible
impact on real-world performance. This is due to several different
but related effects that you should keep in mind when deciding
how to partition your drive:




Cluster Size: The way that the hard disk is partitioned in
most cases determines the cluster size of the partition, which
has a performance impact. See this section for details.
Zone Effects: Modern hard disks use zoned bit recording
to allow more data to be stored on the outer tracks of the
hard disk than the inner ones. This directly impacts the
media transfer rate of the disk when reading one zone of the
disk as opposed to another; see here for details. Hard disks
fill their space starting from the outer tracks and working
inward. This means that if you split a hard disk into three
partitions of equal size, the first partition will have the highest
transfer rate, the second will be lower, and the third lower
still. Therefore, you can put the more important files on the
faster partitions if transfer performance is important to you.
Seek Confinement: Seek times are roughly proportional to
the linear distance across the face of the platter surfaces that
the actuator must move the read/write heads. Using platters
of smaller diameter improves seek time, all else being equal,
and partitioning can have the same net effect. If you split a
drive into multiple partitions, you restrict the read/write heads
to a subsection of the physical disk when seeking, as long as
you stay within the same partition. The tradeoff is that if you
do a lot of moving data between partitions, or accessing
multiple partitions simultaneously, you'll force the heads to
"jump" back and forth between two completely different
areas of the disk, reducing performance. Some who truly
desire performance over all else will buy a hard disk with
double the capacity that they need, partition it in two pieces
and use only the first half! Or use the second half only for
archiving infrequently-used data.
Defragmentation Time: Larger partitions tend to become
full of, well, more data, obviously. A larger partition can take
much longer to defragment than a smaller one. Since
fragmentation reduces performance, some people prefer to
partition their drives to reduce defragmentation time,
enabling them to do it more frequently.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
There are, of course, non-performance issues in partitioning. See
this long description of partitioning issues in the file system
section as well.
Cluster Size
The FAT file system used by DOS and Windows divides all of the
file data on the hard disk into clusters comprised of multiple
sectors. A cluster is normally between 2 kiB and 32 kiB in size, in
powers of two, containing between 4 and 64 sectors of user data.
This is done to make managing the location of data easier.
Clusters and related file system structures are discussed here.
The choice of cluster size has an impact on real-world
performance, though for most people it is not all that significant. In
a nutshell, larger clusters waste more space due to slack but
generally provide for slightly better performance because there will
be less fragmentation and more of the file will be in consecutive
blocks. This occurs because when clusters are larger, fewer of
them are needed than when they are small. A 10,000 byte file
would require three 4 kiB clusters but only one 16 kiB cluster. This
means this file will always be in a contiguous block if stored in a 16
kiB cluster, but could be fragmented if stored in a 4 kiB cluster size
partition. (The slack tradeoff is a waste of 4 kiB more storage in the
case of the 16 kiB clusters.) Small cluster sizes also have a
negative effect on partition because they require larger file
allocation tables, to manage their much larger numbers of clusters.
Traditionally, most people have tried to use cluster sizes as small
as possible in order to reduce slack and make more efficient use of
disk space. This is of course a valid goal, but it has become
increasingly irrelevant today as hard disks approach truly
gargantuan sizes and the price per GB of storage drops to
amazingly low levels. Today, the large file allocation tables
resulting from enormous FAT32 partitions means that balancing
slack reduction with performance effects is also important, unless
you are on a very tight budget. I certainly can't recommend forcing
Windows to use 4 kiB clusters on a 30 GB partition "to reduce
slack" as some people do, because I personally wouldn't want to
take the performance hit of having 30 MiB file allocation tables-and I wouldn't want to have to wait for that puppy to defragment
either!
- 70 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
‫ األقراص الليزرية‬CDs and DVDs
CDs and DVDs are everywhere these days. Whether they are
used to hold music, data or computer software, they have become
the standard medium for distributing large quantities of information
in a reliable package. Compact discs are so easy and cheap to
produce that America Online sends out millions of them every year
to entice new users. And if you have a computer and CD-R drive,
you can create your own CDs, including any information you want.
In this article, we will look at how CDs and CD drives work. We will
also look at the different forms CDs take, as well as what the future
holds for this technology.
Understanding the CD: Material
As discussed in How Analog and Digital Recording Works, a CD
can store up to 74 minutes of music, so the total amount of digital
data that must be stored on a CD is:
44,100 samples/channel/second x 2 bytes/sample x 2
channels x 74 minutes x 60 seconds/minute = 783,216,000
bytes
To fit more than 783 megabytes (MB) onto a disc only 4.8 inches
(12 cm) in diameter requires that the individual bytes be very
small. By examining the physical construction of a CD, you can
begin to understand just how small these bytes are.
A CD is a fairly simple piece of plastic, about four one-hundredths
(4/100) of an inch (1.2 mm) thick. Most of a CD consists of an
injection-molded piece of clear polycarbonate plastic. During
manufacturing, this plastic is impressed with microscopic bumps
arranged as a single, continuous, extremely long spiral track of
data. We'll return to the bumps in a moment. Once the clear piece
of polycarbonate is formed, a thin, reflective aluminum layer is
sputtered onto the disc, covering the bumps. Then a thin acrylic
layer is sprayed over the aluminum to protect it. The label is then
printed onto the acrylic. A cross section of a complete CD (not to
scale) looks like this:
- 71 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Cross-section of a CD
Understanding the CD: The Spiral
A CD has a single spiral track of data,
circling from the inside of the disc to the
outside. The fact that the spiral track
starts at the center means that the CD
can be smaller than 4.8 inches (12 cm) if
desired, and in fact there are now plastic
baseball cards and business cards that
you can put in a CD player. CD business
cards hold about 2 MB of data before the
size and shape of the card cuts off the
spiral.
What the picture on the right does not even begin to impress upon
you is how incredibly small the data track is -- it is approximately
0.5 microns wide, with 1.6 microns separating one track from the
next. (A micron is a millionth of a meter.) And the bumps are even
more miniscule...
Understanding the CD: Bumps
The elongated bumps that make up the track are each 0.5 microns
wide, a minimum of 0.83 microns long and 125 nanometers high.
(A nanometer is a billionth of a meter.) Looking through the
polycarbonate layer at the bumps, they look something like this:
- 72 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
You will often read about "pits" on a CD instead of bumps. They
appear as pits on the aluminum side, but on the side the laser
reads from, they are bumps.
The incredibly small dimensions of the bumps make the spiral
track on a CD extremely long. If you could lift the data track off a
CD and stretch it out into a straight line, it would be 0.5 microns
wide and almost 3.5 miles (5 km) long!
To read something this small you need an incredibly precise discreading mechanism. Let's take a look at that.
CD Player Components
The CD player has the job of finding and reading the data stored
as bumps on the CD. Considering how small the bumps are, the
CD player is an exceptionally precise piece of equipment. The
drive consists of three fundamental components:
 A drive motor spins the disc. This drive motor is precisely
controlled to rotate between 200 and 500 rpm depending
on which track is being read.
 A laser and a lens system focus in on and read the
bumps.
 A tracking mechanism moves the laser assembly so that
the laser's beam can follow the spiral track. The tracking
system has to be able to move the laser at micron
resolutions.
Inside a CD player
- 73 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
What the CD Player Does: Laser Focus
Inside the CD player, there is a good bit of computer technology
involved in forming the data into understandable data blocks and
sending them either to the DAC (in the case of an audio CD) or to
the computer (in the case of a CD-ROM drive).
The fundamental job of the CD player is to focus the laser on the
track of bumps. The laser beam passes through the polycarbonate
layer, reflects off the aluminum layer and hits an opto-electronic
device that detects changes in light. The bumps reflect light
differently than the "lands" (the rest of the aluminum layer), and the
opto-electronic sensor detects that change in reflectivity. The
electronics in the drive interpret the changes in reflectivity in order
to read the bits that make up the bytes.
What the CD Player Does: Tracking
The hardest part is keeping the laser beam centered on the data
track. This centering is the job of the tracking system. The
tracking system, as it plays the CD, has to continually move the
laser outward. As the laser moves outward from the center of the
disc, the bumps move past the laser faster -- this happens
because the linear, or tangential, speed of the bumps is equal to
the radius times the speed at which the disc is revolving (rpm).
Therefore, as the laser moves outward, the spindle motor must
slow the speed of the CD. That way, the bumps travel past the
laser at a constant speed, and the data comes off the disc at a
constant rate.
- 74 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Writing CDs
In response to this demand, electronics manufacturers introduced
an alternative sort of CD that could be encoded in a few easy
steps. CD-recordable discs, or CD-Rs, don't have any bumps or
flat areas at all. Instead, they have a smooth reflective metal
layer, which rests on top of a layer of photosensitive dye.
When the disc is blank, the dye is translucent: Light can shine
through and reflect off the metal surface. But when you heat the
dye layer with concentrated light of a particular frequency and
intensity, the dye turns opaque: It darkens to the point that light
can't pass through.
A CD-R doesn't have the same bumps and lands as a
conventional CD. Instead, the disc has a dye layer underneath
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
a smooth, reflective surface. On a blank CD-R disc, the dye
layer is completely translucent, so all light reflects. The write
laser darkens the spots where the bumps would be in a
conventional CD, forming non-reflecting areas.
By selectively darkening particular points along the CD track, and
leaving other areas of dye translucent, you can create a digital
pattern that a standard CD player can read. The light from the
player's laser beam will only bounce back to the sensor when the
dye is left translucent, in the same way that it will only bounce back
from the flat areas of a conventional CD. So, even though the CDR disc doesn't have any bumps pressed into it at all, it behaves
just like a standard disc.
A CD burner's job, of course, is to "burn" the digital pattern onto a
blank CD. In the next section, we'll look inside a burner to see how
it accomplishes this task.
Burning CDs: Laser Assembly
In the last section, we saw that CD burners darken microscopic
areas of CD-R discs to record a digital pattern of reflective and
non-reflective areas that can be read by a standard CD player.
Since the data must be accurately encoded on such a small scale,
the burning system must be extremely precise. Still, the basic
process at work is quite simple.
The CD burner has a moving laser assembly, just like an ordinary
CD player. But in addition to the standard "read laser," it has a
"write laser." The write laser is more powerful than the read laser,
so it interacts with the disc differently: It alters the surface instead
of just bouncing light off it. Read lasers are not intense enough to
darken the dye material, so simply playing a CD-R in a CD drive
will not destroy any encoded information.
- 76 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
The laser assembly inside a CD burner
On the next page, you'll find out how this write laser operates.
Burning CDs: Write Laser
The write laser moves in exactly the same way as the read laser: It
moves outward while the disc spins. The bottom plastic layer has
grooves pre-pressed into it, to guide the laser along the correct
path. By calibrating the rate of spin with the movement of the laser
assembly, the burner keeps the laser running along the track at a
constant rate of speed. To record the data, the burner simply
turns the laser writer on and off in synch with the pattern of 1s and
0s. The laser darkens the material to encode a 0 and leaves it
translucent to encode a 1.
- 77 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
The machinery in a CD burner looks pretty
much the same as the machinery in any CD
player. There is a mechanism that spins the
disc and another mechanism that slides the
laser assembly.
Most CD burners can create CDs at multiple speeds. At 1x speed,
the CD spins at about the same rate as it does when the player is
reading it. This means it would take you about 60 minutes to
record 60 minutes of music. At 2x speed, it would take you about
half an hour to record 60 minutes, and so on. For faster burning
speeds, you need more advanced laser-control systems and a
faster connection between the computer and the burner. You also
need a blank disc that is designed to record information at this
speed.
The main advantage of CD-R discs is that they work in almost all
CD players and CD-ROMS, which are among the most prevalent
media players today. In addition to this wide compatibility, CD-Rs
are relatively inexpensive.
The main drawback of the format is that you can't reuse the discs.
Once you've burned in the digital pattern, it can't be erased and rewritten. In the mid '90s, electronics manufacturers introduced a
new CD format that addressed this problem. In the next section,
we'll look at these CD-rewritable discs, commonly called CDRWs, to see how they differ from standard CD-R discs.
Erasing CDs
In the last section, we looked at the most prevalent writable CD
technology, CD-R. CD-R discs hold a lot of data, work with most
CD players and are fairly inexpensive. But unlike tapes, floppy
disks and many other data-storage mediums, you cannot re-record
on CD-R disc once you've filled it up.
CD-RW discs have taken the idea of writable CDs a step further,
building in an erase function so you can record over old data you
don't need anymore. These discs are based on phase-change
technology. In CD-RW discs, the phase-change element is a
chemical compound of silver, antimony, tellurium and indium. As
with any physical material, you can change this compound's form
by heating it to certain temperatures. When the compound is
heated above its melting temperature (around 600 degrees
- 78 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Celsius), it becomes a liquid; at its crystallization temperature
(around 200 degrees Celsius), it turns into a solid.
In a CD-RW disc, the reflecting lands and non-reflecting
bumps of a conventional CD are represented by phase shifts
in a special compound. When the compound is in a crystalline
state, it is translucent, so light can shine through to the metal
layer above and reflect back to the laser assembly. When the
compound is melted into an amorphous state, it becomes
opaque, making the area non-reflective.
Phase-change Compounds
In phase-change compounds, these shifts in form can be "locked
into place": They persist even after the material cools down again.
If you heat the compound in CD-RW discs to the melting
temperature and let it cool rapidly, it will remain in a fluid,
amorphous state, even though it is below the crystallization
temperature. In order to crystallize the compound, you have to
keep it at the crystallization temperature for a certain length of time
so that it turns into a solid before it cools down again.
In the compound used in CD-RW discs, the crystalline form is
translucent while the amorphous fluid form will absorb most light.
On a new, blank CD, all of the material in the writable area is in the
crystalline form, so light will shine through this layer to the
reflective metal above and bounce back to the light sensor. To
encode information on the disc, the CD burner uses its write laser,
which is powerful enough to heat the compound to its melting
temperature. These "melted" spots serve the same purpose as the
- 79 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
bumps on a conventional CD and the opaque spots on a CD-R:
They block the "read" laser so it won't reflect off the metal layer.
Each non-reflective area indicates a 0 in the digital code. Every
spot that remains crystalline is still reflective, indicating a 1.
The Erase Laser
As with CD-Rs, the read laser does not have enough power to
change the state of the material in the recording layer -- it's a lot
weaker than the write laser. The erase laser falls somewhere in
between: While it isn't strong enough to melt the material, it does
have the necessary intensity to heat the material to the
crystallization point. By holding the material at this temperature,
the erase laser restores the compound to its crystalline state,
effectively erasing the encoded 0. This clears the disc so new data
can be encoded.
CD-RW discs do not reflect as much light as older CD formats, so
they cannot be read by most older CD players and CD-ROM
drives. Some newer drives and players, including all CD-RW
writers, can adjust the read laser to work with different CD formats.
But since CD-RWs will not work on many CD players, these are
not a good choice for music CDs. For the most part, they are used
as back-up storage devices for computer files.
As we've seen, the reflective and non-reflective patterns on a CD
are incredibly small, and they are burned and read very quickly
with a speeding laser beam. In this system, the chances of a data
error are fairly high. In the next section, we'll look at some of the
ways that CD burners compensate for various encoding problems.
DVDs
It wasn't really that long ago that VHS tapes dominated the home
video market, but now, DVDs have all but wiped them out
completely. Going from tape to disc gave the home theater
experience a major upgrade, and ushered in an era of featurepacked special edition home video.
In this article, you will learn what a DVD consists of, how a DVD
player reads a disc, what to look for when buying a DVD player, a
little DVD history and much more.
- 80 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
The Basics
A DVD is very similar to a CD, but it has a much larger data
capacity. A standard DVD holds about seven times more data
than a CD does. This huge capacity means that a DVD has
enough room to store a full-length, MPEG-2-encoded movie, as
well as a lot of other information.
Here are the typical contents of a DVD movie:
Up to 133 minutes of high-resolution video, in letterbox or
pan-and-scan format, with 720 dots of horizontal
resolution (The video compression ratio is typically 40:1
using MPEG-2 compression.)
 Soundtrack presented in up to eight languages using 5.1
channel Dolby digital surround sound
 Subtitles in up to 32 languages
DVD can also be used to store almost eight hours of CD-quality
music per side.

The format offers many advantages over VHS tapes:





DVD picture quality is better, and many DVDs have Dolby
Digital or DTS sound, which is much closer to the sound
you experience in a movie theater.
Many DVD movies have an on-screen index, where the
creator of the DVD has labeled many of the significant
parts of the movie, sometimes with a picture. With your
remote, if you select the part of the movie you want to
view, the DVD player will take you right to that part, with
no need to rewind or fast-forward.
DVD players are compatible with audio CDs.
Some DVD movies have both the letterbox format, which
fits wide-screen TVs, and the standard TV size format, so
you can choose which way you want to watch the movie.
DVD movies may have several soundtracks on them, and
they may provide subtitles in different languages. Foreign
movies may give you the choice between the version
dubbed into your language, or the original soundtrack
with subtitles in your language.
Storing Data on a DVD
DVDs are of the same diameter and thickness as CDs, and they
are made using some of the same materials and manufacturing
- 81 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
methods. Like a CD, the data on a DVD is encoded in the form of
small pits and bumps in the track of the disc.
A DVD is composed of several layers of plastic, totaling about 1.2
millimeters thick. Each layer is created by injection molding
polycarbonate plastic. This process forms a disc that has
microscopic bumps arranged as a single, continuous and
extremely long spiral track of data. More on the bumps later.
Once the clear pieces of polycarbonate are formed, a thin
reflective layer is sputtered onto the disc, covering the bumps.
Aluminum is used behind the inner layers, but a semi-reflective
gold layer is used for the outer layers, allowing the laser to focus
through the outer and onto the inner layers. After all of the layers
are made, each one is coated with lacquer, squeezed together and
cured under infrared light. For single-sided discs, the label is silkscreened onto the nonreadable side. Double-sided discs are
printed only on the nonreadable area near the hole in the middle.
Cross sections of the various types of completed DVDs (not to
scale) look like this:
DVD formats
- 82 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Each writable layer of a DVD has a spiral
track of data. On single-layer DVDs, the
track always circles from the inside of the
disc to the outside. That the spiral track
starts at the center means that a singlelayer DVD can be smaller than 12
centimeters if desired.
What the image to the right cannot
impress upon you is how incredibly tiny
the data track is -- just 740 nanometers
separate one track from the next (a Data tracks on a DVD
nanometer is a billionth of a meter). And the elongated bumps that
make up the track are each 320 nanometers wide, a minimum of
400 nanometers long and 120 nanometers high. The following
figure illustrates looking through the polycarbonate layer at the
bumps.
DVD pit layout
You will often read about "pits" on a DVD instead of bumps. They
appear as pits on the aluminum side, but on the side that the laser
reads from, they are bumps.
The microscopic dimensions of the bumps make the spiral track on
a DVD extremely long. If you could lift the data track off a single
layer of a DVD, and stretch it out into a straight line, it would be
almost 7.5 miles long! That means that a double-sided, doublelayer DVD would have 30 miles (48 km) of data!
- 83 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
To read bumps this small you need an incredibly precise discreading mechanism.
Data Storage: DVD vs. CD
DVDs can store more data than CDs for a few reasons:
 Higher-density data storage
 Less overhead, more area
 Multi-layer storage
Higher Density Data Storage
Single-sided, single-layer DVDs can store about seven times more
data than CDs. A large part of this increase comes from the pits
and tracks being smaller on DVDs.
Specification
CD
DVD
Track Pitch
1600
nanometers
740
nanometers
Minimum
Pit
830
Length
nanometers
(single-layer
</FONT< td>
DVD)
400
nanometers
Minimum
Pit
Length
830
(double-layer
nanometers
DVD)
440
nanometers
Let's try to get an idea of how much more data can be stored due
to the physically tighter spacing of pits on a DVD. The track pitch
on a DVD is 2.16 times smaller, and the minimum pit length for a
single-layer DVD is 2.08 times smaller than on a CD. By
multiplying these two numbers, we find that there is room for about
4.5 times as many pits on a DVD. So where does the rest of the
increase come from?
Less Overhead, More Area
On a CD, there is a lot of extra information encoded on the disc to
allow for error correction -- this information is really just a repetition
of information that is already on the disc. The error correction
scheme that a CD uses is quite old and inefficient compared to the
method used on DVDs. The DVD format doesn't waste as much
space on error correction, enabling it to store much more real
information. Another way that DVDs achieve higher capacity is by
- 84 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
encoding data onto a slightly larger area of the disc than is done
on a CD.
Multi-Layer Storage
To increase the storage capacity even more, a DVD can have up
to four layers, two on each side. The laser that reads the disc can
actually focus on the second layer through the first layer. Here is a
list of the capacities of different forms of DVDs:
Format
Capacity
Approx. Movie
Time
Single-sided/singlelayer
4.38 GB
2 hours
Single-sided/double7.95 GB
layer
4 hours
Double-sided/single8.75 GB
layer
4.5 hours
Doublesided/double-layer
Over 8 hours
15.9 GB
You may be wondering why the capacity of a DVD doesn't double
when you add a whole second layer to the disc. This is because
when a disc is made with two layers, the pits have to be a little
longer, on both layers, than when a single layer is used. This helps
to avoid interference between the layers, which would cause errors
when the disc is played.
The DVD Player
A DVD player is very similar to a CD player. It has a laser
assembly that shines the laser beam onto the surface of the disc to
read the pattern of bumps (see How CDs Work for details). The
DVD player decodes the MPEG-2 encoded movie, turning it into a
standard composite video signal (see How Television Works for
details). The player also decodes the audio stream and sends it to
a Dolby decoder, where it is amplified and sent to the speakers.
The DVD player has the job of finding and reading the data stored
as bumps on the DVD. Considering how small the bumps are, the
DVD player has to be an exceptionally precise piece of equipment.
The drive consists of three fundamental components:
- 85 -
‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬



‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
A drive motor to spin the disc - The drive motor is
precisely controlled to rotate between 200 and 500 rpm,
depending on which track is being read.
A laser and a lens system to focus in on the bumps and
read them - The light from this laser has a smaller
wavelength (640 nanometers) than the light from the
laser in a CD player (780 nanometers), which allows the
DVD laser to focus on the smaller DVD pits.
A tracking mechanism that can move the laser assembly
so the laser beam can follow the spiral track - The
tracking system has to be able to move the laser at
micron resolutions.
Inside the DVD player, there is a good bit of computer technology
involved in forming the data into understandable data blocks, and
sending them either to the DAC, in the case of audio or video data,
or directly to another component in digital format, in the case of
digital video or data.
The fundamental job of the DVD player is to focus the laser on the
track of bumps. The laser can focus either on the semi-transparent
reflective material behind the closest layer, or, in the case of a
double-layer disc, through this layer and onto the reflective
material behind the inner layer. The laser beam passes through
the polycarbonate layer, bounces off the reflective layer behind it
and hits an opto-electronic device, which detects changes in light.
The bumps reflect light differently than the "lands," the flat areas of
the disc, and the opto-electronic sensor detects that change in
reflectivity. The electronics in the drive interpret the changes in
reflectivity in order to read the bits that make up the bytes.
The hardest part of reading a DVD is keeping the laser beam
centered on the data track. This centering is the job of the
tracking system. As the DVD is played, the tracking system has
to move the laser continually outward. As the laser moves outward
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
from the center of the disc, the bumps move past the laser at an
increasing speed. This happens because the linear, or tangential,
speed of the bumps is equal to the radius times the speed at which
the disc is revolving. So, as the laser moves outward, the spindle
motor must slow the spinning of the DVD so that the bumps travel
past the laser at a constant speed, and the data comes off the disc
at a constant rate.
An interesting thing to note is that if a DVD has a second layer, the
start of that layer's data track can be at the outside of the disc
instead of the inside. This allows the player to transition quickly
from one layer to the next, without a delay in data output, because
it doesn't have to move the laser back to the center of the disc to
read the next layer.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
HD-DVD
If you used to watch movies on videotape, you probably remember
the first time you saw one on DVD. Suddenly, the video and sound
were of much better quality. You could also pause without
distorting the picture, skip from chapter to chapter and zoom in on
the screen. When studios started adding commentary tracks,
"extras" and multiple sound options on each disc, it seemed like
the technology had reached its peak. People couldn't really
imagine a better way to watch a recorded movie than on a DVD.
Then, TVs got a whole lot bigger.
DVDs look best on screens that are smaller than 36 inches, so
they're not always up to the challenge of today's
high-definition (HD) sets. To store and play HD movies, you
need a disc that holds more information, like an HD-DVD. In this
article, we'll explore how HD-DVDs differ from DVDs and what's
happening in the struggle between HD-DVD and Blu-Ray.
The basic idea behind the HD-DVD is really simple -- it looks like a
DVD and acts like a DVD, but it holds more information. A DVD
holds about two hours of standard definition video, but an HD-DVD
can hold about 48 hours.
If you already know how DVDs work, you already know a lot about
HD-DVDs. A DVD stores information as a series of microscopic
pits arranged in a very long spiral. A red laser reads these pits
from the other side, so it sees them as bumps. The bumps reflect
the laser's light to a sensor. Electronics within the DVD player read
the information from the sensor as a digital signal. Check out How
DVDs Work to learn more about how a DVD player does this.
A simplified view of what happens in a DVD player. An HDDVD player is a lot like this, but it can send the signal digitally
rather than converting it to analog.
A HD-DVD player is very similar to a DVD player, but it has a few
notable differences. We'll look at them in the next section.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
HD-DVD Players
An HD-DVD uses the same principles -- it contains a bumpy layer
that reflects light from a laser to a sensor, creating a digital signal.
HD-DVDs are even exactly the same size as DVDs (120
millimeters in diameter and 1.2 millimeters thick). But three
important differences allow them to hold quite a bit more
information than DVDs:
 They use 405 nanometer blue-violet lasers rather than 650
nanometer red lasers.
 The pits are smaller and the tracks are closer together.
 They use more efficient compression to cut down the size
of the files they store.
The color of the laser may seem like a trivial change to make. But
the shorter wavelength of the blue-violet laser is what allows HDDVDs pits to be smaller and arranged closer together. In other
words, it allows the disc to have a much narrower track pitch.
Regular DVDs have a track pitch of 0.74 micrometers, and HDDVDs have a track pitch of 0.40 micrometers. You can imagine
this as the difference between writing with a fine-tipped pen and a
magic marker.
The difference between a red laser and a blue laser is like the
difference between a fine-tipped pen and a magic marker.
The other big difference between DVDs and HD-DVDs involves
how the information on the disc is compressed. Most DVDs use
MPEG-2 compression. HD-DVDs can use MPEG-2, but they
typically use the more efficient MPEG-4, which allows higher video
quality with a smaller file size. HD-DVDs can also use VC-1 (or
Windows Media) compression.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Finally, because of general improvements in the technology, an
HD-DVD player can read information from the disc and deliver it to
the TV about three times as fast as a DVD player can. It can also
send the signal to an HDTV digitally using a High Definition
Multimedia Interface (HDMI), preventing the quality loss that
conversion to analog causes.
One of the first questions people ask about HD-DVD (besides "Is it
better than Blu-ray?") is whether their old DVDs are about to
become obsolete. Let's take a look at what is likely to happen with
players and discs as people upgrade.
Compatibility
If you decided to buy an HD-DVD player the first day they hit the
market, you'd still be able to play your DVDs on it. On the other
hand, if you bought a new movie on HD-DVD, it wouldn't play in
your old DVD player. Since an HD-DVD is exactly the same size
and shape as a regular DVD, it's pretty easy to make new players
that can handle both -- they just need a laser pickup that can read
either format. The Toshiba HD-DVD player that released in April
2006 can read DVDs, HD-DVDs and CDs.
Photo courtesy HowStuffWorks Shopper
Toshiba HD-A1 HD DVD player
Even if HD-DVDs gain widespread use, you should still be able to
buy DVDs -- the majority of homes in the United States don't have
HDTVs, and there's no point in upgrading to HD-DVD without one.
In addition, HD-DVDs can store regular and high-definition content
on the same disc. Twin format discs have two layers -- a DVD
layer on top uses a red laser, and an HD-DVD layer on the bottom
uses a blue laser. The outer layer is transparent to the blue laser.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
The blue laser sees through the outer layer, skipping
straight to the high-definition content.
The other option for including DVD and HD-DVD content on the
same disc is a combination format, which uses a two-sided disc.
A red laser can read the DVD side, and a blue laser can read the
HD-DVD side.
To access the DVD content on a combination HDDVD, simply flip the disc over.
With either option, you can buy one disc that will play in both DVD
and HD-DVD players. If these discs become available in stores,
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
they'll be a good choice if you plan to upgrade to high-definition at
some point in the future. Twin or combination discs will also be
useful for libraries and movie rental stores, since not everyone will
be ready to upgrade their player and right away.
HD-DVDs aren't the only option for high-definition video playback,
though. The other likely candidate for DVD's successor is Blu-ray.
Some people plan to put off upgrading their movies and players
until there's a clear winner in the "format war" between the two.
HD-DVD vs. Blu-Ray
Several companies have developed alternatives to the existing
DVD standard. The two forerunners are HD-DVD and Blu-ray.
Competition between the two has escalated, drawing inevitable
comparisons to the struggle between VHS and Betamax. Here are
the highlights:
 Both formats use blue lasers rather than red.
 Both have the same options for video and audio
compression.
 Blu-ray offers significantly more storage space -- 50 GB on
a dual-layer disc versus HD-DVD's 30 GB.
 The DVD Forum, which creates DVD standards, has
approved HD-DVD and has not approved Blu-ray.
 HD-DVD is less expensive than Blu-ray, partly because
HD-DVDs can be produced on existing equipment, and
Blu-ray discs can't. HD-DVD players are selling for around
$399 (Toshiba HD-A2), and the cheapest Blu-ray player is
selling for around $699 (PlayStation3).
 HD-DVD players hit the market on April 18, 2006, two
months before the first Blu-ray player hit the U.S. market
in June 2006.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
HD-DVD Facts
HD-DVD Capacity:
15 GB single layer, 30 GB dual layer
DVD Capacity:
4.7 GB single layer, 8.4 GB dual layer
Compression:
MPEG-2, MPEG-4, VC-1
High-definition Playback, 15 GB:
4 hours
High-definition playback, 50 GB:
8 hours
Along with other companies, Toshiba, Microsoft and Intel have
sided with HD-DVD. Microsoft plans to release an add-on HD-DVD
drive for its Xbox 360 in November 2006. The Blu-ray Disc
Association, on the other hand, has electronics companies like
Sony (which plans to release the Blu-ray-equipped PS3 in
November) and Pioneer, computer companies like Dell and Apple
and entertainment companies like Disney and Fox on its board of
directors. Most of the motion picture industry seems to support
Blu-ray, in part because the need for new manufacturing
equipment might cut down on piracy.
Photo courtesy Amazon
The Xbox 360 HD-DVD player
Even though Blu-ray seems to have the backing of more of the
industry, the battle isn't over. Some companies, like Hewlett
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Packard, previously supported only Blu-ray but now support both
formats. Critics of Blu-ray point out that it may have more capacity
than any movie could ever use, even with special features. Many
people theorize that HD-DVD will be the winner solely because it is
so much cheaper.
For lots more information about high-definition video and related
technology, check out the links on the next page.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
‫ الطابعات‬Printers
Impact vs. Non-impact
There are several major printer technologies available. These
technologies can be broken down into two main categories with
several types in each:

Impact - These printers have a mechanism that touches
the paper in order to create an image. There are two
main impact technologies:
 Dot matrix printers use a series of small pins
to strike a ribbon coated with ink, causing the
ink to transfer to the paper at the point of
impact.
 Character
printers
are
basically
computerized typewriters. They have a ball
or series of bars with actual characters
(letters and numbers) embossed on the
surface. The appropriate character is struck
against the ink ribbon, transferring the
character's image to the paper. Character
printers are fast and sharp for basic text, but
very limited for other use.
 Non-impact - These printers do not touch the paper
when creating an image. Inkjet printers are part of this
group, which includes:
 Inkjet printers, which are described in this
article, use a series of nozzles to spray drops
of ink directly on the paper.
 Laser printers, covered in-depth in How
Laser Printers Work, use dry ink (toner),
static electricity, and heat to place and bond
the ink onto the paper.

Solid ink printers contain sticks of wax-like
ink that are melted and applied to the paper.
The ink then hardens in place.

Dye-sublimation printers have a long roll of
transparent film that resembles sheets of red-,
blue-, yellow- and gray-colored cellophane
stuck together end to end. Embedded in this
film are solid dyes corresponding to the four
basic colors used in printing: cyan, magenta,
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
yellow and black (CMYK). The print head uses
a heating element that varies in temperature,
depending on the amount of a particular color
that needs to be applied. The dyes vaporize
and permeate the glossy surface of the paper
before they return to solid form. The printer
does a complete pass over the paper for each
of the basic colors, gradually building the
image.

Thermal wax printers are something of a
hybrid of dye-sublimation and solid ink
technologies. They use a ribbon with
alternating CMYK color bands. The ribbon
passes in front of a print head that has a
series of tiny heated pins. The pins cause the
wax to melt and adhere to the paper, where it
hardens in place.

Thermal autochrome printers have the color
in the paper instead of in the printer. There are
three layers (cyan, magenta and yellow) in the
paper, and each layer is activated by the
application of a specific amount of heat. The
print head has a heating element that can vary
in temperature. The print head passes over
the paper three times, providing the
appropriate temperature for each color layer
as needed.
Out of all of these incredible technologies, inkjet printers are by far
the most popular. In fact, the only technology that comes close
today is laser printers.
Inkjet Printers
No matter where you are reading this article, you most likely have
a printer nearby. And there's a very good chance that it is an inkjet
printer. Since their introduction in the latter half of the 1980s,
inkjet printers have grown in popularity and performance while
dropping significantly in price.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
An inexpensive color inkjet printer made by Hewlett
Packard. See more inkjet printer pictures.
An inkjet printer is any printer that places extremely small droplets
of ink onto paper to create an image. If you ever look at a piece of
paper that has come out of an inkjet printer, you know that:



The dots are extremely small (usually
between 50 and 60 microns in diameter), so
small that they are tinier than the diameter of
a human hair (70 microns)!
The dots are positioned very precisely, with
resolutions of up to 1440x720 dots per inch
(dpi).
The dots can have different colors combined
together to create photo-quality images.
In this edition of HowStuffWorks, you will learn about the various
parts of an inkjet printer and how these parts work together to
create an image. You will also learn about the ink cartridges and
the special paper some inkjet printers use.
First, let's take a quick look at the various printer technologies.
So, let's take a closer look at what's inside an inkjet printer.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Inside an Inkjet Printer
Parts of a typical inkjet printer include:
 Print head assembly
 Print head - The core of an inkjet printer, the
print head contains a series of nozzles that
are used to spray drops of ink.
The print head assembly
 Ink
cartridges - Depending on the
manufacturer and model of the printer, ink
cartridges come in various combinations,
such as separate black and color
cartridges, color and black in a single
cartridge or even a cartridge for each ink
color. The cartridges of some inkjet printers
include the print head itself.
 Print head stepper motor - A stepper motor
moves the print head assembly (print head
and ink cartridges) back and forth across
the paper. Some printers have another
stepper motor to park the print head
assembly when the printer is not in use.
Parking means that the print head
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
assembly is restricted from accidentally
moving, like a parking brake on a car.
Stepper motors like this one control the movement of
most parts of an inkjet printer.
 Belt - A belt is used to attach the print head
assembly to the stepper motor.
 Stabilizer bar - The print head assembly
uses a stabilizer bar to ensure that
movement is precise and controlled.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Here you can see the stabilizer bar and belt.

Paper feed assembly
 Paper tray/feeder - Most inkjet printers have
a tray that you load the paper into. Some
printers dispense with the standard tray for
a feeder instead. The feeder typically
snaps open at an angle on the back of the
printer, allowing you to place paper in it.
Feeders generally do not hold as much
paper as a traditional paper tray.
 Rollers - A set of rollers pull the paper in
from the tray or feeder and advance the
paper when the print head assembly is
ready for another pass.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
The rollers move the paper through the printer.
Paper feed stepper motor - This stepper
motor powers the rollers to move the paper
in the exact increment needed to ensure a
continuous image is printed.
Power supply - While earlier printers often had an
external transformer, most printers sold today use a
standard power supply that is incorporated into the
printer itself.
Control circuitry - A small but sophisticated amount
of circuitry is built into the printer to control all the
mechanical aspects of operation, as well as decode
the information sent to the printer from the computer.



The mechanical operation of the printer is controlled by a small
circuit board containing a microprocessor and memory.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬

‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Interface port(s) - The parallel port is still used by
many printers, but most newer printers use the USB
port. A few printers connect using a serial port or
small computer system interface (SCSI) port.
While USB taking over, many printers still use a parallel port.
Heat vs. Vibration
Different types of inkjet printers form their droplets of ink in
different ways. There are two main inkjet technologies currently
used by printer manufacturers:
View of the nozzles on a thermal bubble inkjet print head

Thermal bubble - Used by manufacturers such as
Canon and Hewlett Packard, this method is
commonly referred to as bubble jet. In a thermal
inkjet printer, tiny resistors create heat, and this heat
vaporizes ink to create a bubble. As the bubble
expands, some of the ink is pushed out of a nozzle
onto the paper. When the bubble "pops" (collapses),
a vacuum is created. This pulls more ink into the
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
print head from the cartridge. A typical bubble jet
print head has 300 or 600 tiny nozzles, and all of
them can fire a droplet simultaneously.

Click the button to see how a thermal
bubble inkjet printer works.
Piezoelectric - Patented by Epson, this technology
uses piezo crystals. A crystal is located at the back
of the ink reservoir of each nozzle. The crystal
receives a tiny electric charge that causes it to
vibrate. When the crystal vibrates inward, it forces a
tiny amount of ink out of the nozzle. When it vibrates
out, it pulls some more ink into the reservoir to
replace the ink sprayed out.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Click on the button to see how a piezoelectric inkjet printer
works.
Let's walk through the printing process to see just what happens.
Paper and Ink
Inkjet printers are fairly inexpensive. They cost less than a typical
black-and-white laser printer, and much less than a color laser
printer. In fact, quite a few of the manufacturers sell some of their
printers at a loss. Quite often, you can find the printer on sale for
less than you would pay for a set of the
ink cartridges!
Why would they do this? Because they
count on the supplies you purchase to
provide their profit. This is very similar to
the way the video game business works.
The hardware is sold at or below cost. This printer sells for
Once you buy a particular brand of less than $100.
hardware, then you must buy the other
products that work with that hardware. In other words, you can't
buy a printer from Manufacturer A and ink cartridges from
Manufacturer B. They will not work together.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Another way that they have reduced
costs is by incorporating much of the
actual print head into the cartridge itself.
The manufacturers believe that since the
print head is the part of the printer that is
most likely to wear out, replacing it every
time you replace the cartridge increases
the life of the printer.
The paper you use on an inkjet printer
greatly determines the quality of the
image. Standard copier paper works, but
doesn't provide as crisp and bright an
image as paper made for an inkjet printer.
There are two main factors that affect
image quality:


A typical color ink
cartridge:
This cartridge has
cyan, magenta and
yellow
inks
in
separate reservoirs.
Brightness
Absorption
The brightness of a paper is normally determined by how rough
the surface of the paper is. A course or rough paper will scatter
light in several directions, whereas a smooth paper will reflect
more of the light back in the same direction. This makes the paper
appear brighter, which in turn makes any image on the paper
appear brighter. You can see this yourself by comparing a photo in
a newspaper with a photo in a magazine. The smooth paper of the
magazine page reflects light back to your eye much better than the
rough texture of the newspaper. Any paper that is listed as being
bright is generally a smoother-than-normal paper.
The other key factor in image quality is absorption. When the ink
is sprayed onto the paper, it should stay in a tight, symmetrical dot.
The ink should not be absorbed too much into the paper. If that
happens, the dot will begin to feather. This means that it will
spread out in an irregular fashion to cover a slightly larger area
than the printer expects it to. The result is an page that looks
somewhat fuzzy, particularly at the edges of objects and text.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Imagine that the dot on the left is on coated paper and the dot
on the right is on low-grade copier paper. Notice how irregular
and larger the right dot is compared to the left one.
As stated, feathering is caused by the paper absorbing the ink. To
combat this, high-quality inkjet paper is coated with a waxy film
that keeps the ink on the surface of the paper. Coated paper
normally yields a dramatically better print than other paper. The
low absorption of coated paper is key to the high resolution
capabilities of many of today's inkjet printers. For example, a
typical Epson inkjet printer can print at a resolution of up to
720x720 dpi on standard paper. With coated paper, the resolution
increases to 1440x720 dpi. The reason is that the printer can
actually shift the paper slightly and add a second row of dots for
every normal row, knowing that the image will not feather and
cause the dots to blur together.
Inkjet printers are capable of printing on a variety of media.
Commercial inkjet printers sometimes spray directly on an item like
the label on a beer bottle. For consumer use, there are a number
of specialty papers, ranging from adhesive-backed labels or
stickers to business cards and brochures. You can even get ironon transfers that allow you to create an image and put it on a Tshirt! One thing is for certain, inkjet printers definitely provide an
easy and affordable way to unleash your creativity.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Refilling Cartridges
Because of the expense of inkjet cartridges, a huge business has
grown around the idea of refilling them. For most people, refilling
makes good sense, but there are a few things to be aware of:
 Make sure the refill kit is for your printer model. As
you learned in the previous section, different
printers use different technologies for putting the
ink on the paper. If the wrong type of ink is used,
it can degrade the output or possibly damage the
printer. While some commercial inkjets use oilbased inks, virtually all desktop inkjets for home
or office use have water-based ink. The exact ink
composition
varies
greatly
between
manufacturers. For example, thermal bubble
inkjets need ink that is stable at higher
temperatures than piezoelectric printers.
 Most manufacturers require that you use only
their approved ink. Refill kits normally will void
your warranty.
 While you can refill cartridges, be very careful of
the ones that have the print head built into the
cartridge. You do not want to refill these more
than two or three times, or the print head will
begin to deteriorate and could damage your
printer.
Check out this site for some good links and information about inkjet
refills.
Laser Printers
The term inkjet printer is very descriptive of the process at work -these printers put an image on paper using tiny jets of ink. The
term laser printer, on the other hand, is a bit more mysterious -how can a laser beam, a highly focused beam of light, write letters
and draw pictures on paper?
In this article, we'll unravel the mystery behind the laser printer,
tracing a page's path from the characters on your computer screen
to printed letters on paper. As it turns out, the laser printing
process is based on some very basic scientific principles applied in
an exceptionally innovative way.
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‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
Shopping for a laser printer?
Check out our Laser Printer reviews and
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The Basics: Static Electricity
The primary principle at work in a laser printer is static electricity,
the same energy that makes clothes in the dryer stick together or a
lightning bolt travel from a thundercloud to the ground. Static
electricity is simply an electrical charge built up on an insulated
object, such as a balloon or your body. Since oppositely charged
atoms are attracted to each other, objects with opposite static
electricity fields cling together.
The path of a piece of paper through a laser printer
A laser printer uses this phenomenon as a sort of "temporary
glue." The core component of this system is the photoreceptor,
typically a revolving drum or cylinder. This drum assembly is
made out of highly photoconductive material that is discharged
by light photons.
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The basic components of a laser printer
The Basics: Drum
Initially, the drum is given a total positive charge by the charge
corona wire, a wire with an electrical current running through it.
(Some printers use a charged roller instead of a corona wire, but
the principle is the same.) As the drum revolves, the printer shines
a tiny laser beam across the surface to discharge certain points. In
this way, the laser "draws" the letters and images to be printed as
a pattern of electrical charges -- an electrostatic image. The
system can also work with the charges reversed -- that is, a
positive electrostatic image on a negative background.
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The laser "writes" on a photoconductive revolving drum.
After the pattern is set, the printer coats the drum with positively
charged toner -- a fine, black powder. Since it has a positive
charge, the toner clings to the negative discharged areas of the
drum, but not to the positively charged "background." This is
something like writing on a soda can with glue and then rolling it
over some flour: The flour only sticks to the glue-coated part of the
can, so you end up with a message written in powder.
With the powder pattern affixed, the drum rolls over a sheet of
paper, which is moving along a belt below. Before the paper rolls
under the drum, it is given a negative charge by the transfer
corona wire (charged roller). This charge is stronger than the
negative charge of the electrostatic image, so the paper can pull
the toner powder away. Since it is moving at the same speed as
the drum, the paper picks up the image pattern exactly. To keep
the paper from clinging to the drum, it is discharged by the detac
corona wire immediately after picking up the toner.
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The basic components of a laser printer
The Basics: Fuser
Finally, the printer passes the paper through the fuser, a pair of
heated rollers. As the paper passes through these rollers, the
loose toner powder melts, fusing with the fibers in the paper. The
fuser rolls the paper to the output tray, and you have your finished
page. The fuser also heats up the paper itself, of course, which is
why pages are always hot when they come out of a laser printer or
photocopier.
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So what keeps the paper from burning up? Mainly, speed -- the
paper passes through the rollers so quickly that it doesn't get very
hot.
After depositing toner on the paper, the drum surface passes the
discharge lamp. This bright light exposes the entire photoreceptor
surface, erasing the electrical image. The drum surface then
passes the charge corona wire, which reapplies the positive
charge.
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The basic components of a laser printer
Conceptually, this is all there is to it. Of course, actually bringing
everything together is a lot more complex. In the following
sections, we'll examine the different components in greater detail
to see how they produce text and images so quickly and precisely.
The Controller: The Conversation
Before a laser printer can do anything else, it needs to receive the
page data and figure out how it's going to put everything on the
paper. This is the job of the printer controller.
The printer controller is the laser printer's main onboard computer.
It talks to the host computer (for example, your PC) through a
communications port, such as a parallel port or USB port. At the
start of the printing job, the laser printer establishes with the host
computer how they will exchange data. The controller may have to
start and stop the host computer periodically to process the
information it has received.
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A typical laser printer has a few different types of
communications ports.
In an office, a laser printer will probably be connected to several
separate host computers, so multiple users can print documents
from their machine. The controller handles each one separately,
but may be carrying on many "conversations" concurrently. This
ability to handle several jobs at once is one of the reasons why
laser printers are so popular.
The Controller: The Language
For the printer controller and the host computer to communicate,
they need to speak the same page description language. In
earlier printers, the computer sent a special sort of text file and a
simple code giving the printer some basic formatting information.
Since these early printers had only a few fonts, this was a very
straightforward process.
These days, you might have hundreds of different fonts to choose
from, and you wouldn't think twice about printing a complex
graphic. To handle all of this diverse information, the printer needs
to speak a more advanced language.
The primary printer languages these days are Hewlett Packard's
Printer Command Language (PCL) and Adobe's Postscript.
Both of these languages describe the page in vector form -- that
is, as mathematical values of geometric shapes, rather than as a
series of dots (a bitmap image). The printer itself takes the vector
images and converts them into a bitmap page. With this system,
the printer can receive elaborate, complex pages, featuring any
sort of font or image. Also, since the printer creates the bitmap
image itself, it can use its maximum printer resolution.
Some printers use a graphical device interface (GDI) format
instead of a standard PCL. In this system, the host computer
creates the dot array itself, so the controller doesn't have to
process anything -- it just sends the dot instructions on to the laser.
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But in most laser printers, the controller must organize all of the
data it receives from the host computer. This includes all of the
commands that tell the printer what to do -- what paper to use, how
to format the page, how to handle the font, etc. For the controller to
work with this data, it has to get it in the right order.
The Controller: Setting up the Page
Once the data is structured, the controller begins putting the page
together. It sets the text margins, arranges the words and places
any graphics. When the page is arranged, the raster image
processor (RIP) takes the page data, either as a whole or piece
by piece, and breaks it down into an array of tiny dots. As we'll see
in the next section, the printer needs the page in this form so the
laser can write it out on the photoreceptor drum.
In most laser printers, the controller saves all print-job data in its
own memory. This lets the controller put different printing jobs into
a queue so it can work through them one at a time. It also saves
time when printing multiple copies of a document, since the host
computer only has to send the data once.
The Laser Assembly
Since it actually draws the page, the printer's laser system -- or
laser scanning assembly -- must be incredibly precise. The
traditional laser scanning assembly includes:
 A laser
 A movable mirror
 A lens
The laser receives the page data -- the tiny dots that make up the
text and images -- one horizontal line at a time. As the beam
moves across the drum, the laser emits a pulse of light for every
dot to be printed, and no pulse for every dot of empty space.
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The laser doesn't actually move the beam itself. It bounces the
beam off a movable mirror instead. As the mirror moves, it shines
the beam through a series of lenses. This system compensates
for the image distortion caused by the varying distance between
the mirror and points along the drum.
Writing the Page
The laser assembly moves in only one plane, horizontally. After
each horizontal scan, the printer moves the photoreceptor drum up
a notch so the laser assembly can draw the next line. A small
print-engine computer synchronizes all of this perfectly, even at
dizzying speeds.
Some laser printers use a strip of light emitting diodes (LEDs) to
write the page image, instead of a single laser. Each dot position
has its own dedicated light, which means the printer has one set
print resolution. These systems cost less to manufacture than true
laser assemblies, but they produce inferior results. Typically, you'll
only find them in less expensive printers.
Photocopiers
Laser printers work the same basic way as photocopiers, with a few
significant differences. The most obvious difference is the source of
the image: A photocopier scans an image by reflecting a bright light
off of it, while a laser printer receives the image in digital form.
Another major difference is how the electrostatic image is created.
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When a photocopier bounces light off a piece of paper, the light
reflects back onto the photoreceptor from the white areas but is
absorbed by the dark areas. In this process, the "background" is
discharged, while the electrostatic image retains a positive charge.
This method is called "write-white."
In most laser printers, the process is reversed: The laser discharges
the lines of the electrostatic image and leaves the background
positively charged. In a printer, this "write-black" system is easier to
implement than a "write-white" system, and it generally produces
better results.
Toner Basics
One of the most distinctive things about a laser printer (or
photocopier) is the toner. It's such a strange concept for the paper
to grab the "ink" rather than the printer applying it. And it's even
stranger that the "ink" isn't really ink at all.
So what is toner? The short answer is: It's an electrically-charged
powder with two main ingredients: pigment and plastic.
The role of the pigment is fairly obvious -- it provides the coloring
(black, in a monochrome printer) that fills in the text and images.
This pigment is blended into plastic particles, so the toner will melt
when it passes through the heat of the fuser. This quality gives
toner a number of advantages over liquid ink. Chiefly, it firmly
binds to the fibers in almost any type of paper, which means the
text won't smudge or bleed easily.
Photo courtesy Xerox A developer bead coated with small toner particles
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Applying Toner
So how does the printer apply this toner to the electrostatic image
on the drum? The powder is stored in the toner hopper, a small
container built into a removable casing. The printer gathers the
toner from the hopper with the developer unit. The "developer" is
actually a collection of small, negatively charged magnetic beads.
These beads are attached to a rotating metal roller, which moves
them through the toner in the toner hopper.
Because they are negatively charged, the developer beads collect
the positive toner particles as they pass through. The roller then
brushes the beads past the drum assembly. The electrostatic
image has a stronger negative charge than the developer beads,
so the drum pulls the toner particles away.
In a lot of printers, the toner hopper, developer and drum
assembly are combined in one replaceable cartridge.
The drum then moves over the paper, which has an even stronger
charge and so grabs the toner. After collecting the toner, the paper
is immediately discharged by the detac corona wire. At this point,
the only thing keeping the toner on the page is gravity -- if you
were to blow on the page, you would completely lose the image.
The page must pass through the fuser to affix the toner. The fuser
rollers are heated by internal quartz tube lamps, so the plastic in
the toner melts as it passes through.
But what keeps the toner from collecting on the fuser rolls, rather
than sticking to the page? To keep this from happening, the fuser
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rolls must be coated with Teflon, the same non-stick material that
keeps your breakfast from sticking to the bottom of the frying pan.
Color Printers
Initially, most commercial laser printers were limited to
monochrome printing (black writing on white paper). But now,
there are lots of color laser printers on the market.
Essentially, color printers work the same way as monochrome
printers, except they go through the entire printing process four
times -- one pass each for cyan (blue), magenta (red), yellow and
black. By combining these four colors of toner in varying
proportions, you can generate the full spectrum of color.
Inside a color laser printer
There are several different ways of doing this. Some models have
four toner and developer units on a rotating wheel. The printer lays
down the electrostatic image for one color and puts that toner unit
into position. It then applies this color to the paper and goes
through the process again for the next color. Some printers add all
four colors to a plate before placing the image on paper.
Some more expensive printers actually have a complete printer
unit -- a laser assembly, a drum and a toner system -- for each
color. The paper simply moves past the different drum heads,
collecting all the colors in a sort of assembly line.
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Advantages of a Laser
So why get a laser printer rather than a cheaper inkjet printer? The
main advantages of laser printers are speed, precision and
economy. A laser can move very quickly, so it can "write" with
much greater speed than an ink jet. And because the laser beam
has an unvarying diameter, it can draw more precisely, without
spilling any excess ink.
Laser printers tend to be more expensive than inkjet printers, but it
doesn't cost as much to keep them running -- toner powder is
cheap and lasts a long time, while you can use up expensive ink
cartridges very quickly. This is why offices typically use a laser
printer as their "work horse," their machine for printing long text
documents. In most models, this mechanical efficiency is
complemented by advanced processing efficiency. A typical laserprinter controller can serve everybody in a small office.
When they were first introduced, laser printers were too expensive
to use as a personal printer. Since that time, however, laser
printers have gotten much more affordable. Now you can pick up a
basic model for just a little bit more than a nice inkjet printer.
As technology advances, laser-printer prices should continue to
drop, while performance improves. We'll also see a number of
innovative design variations, and possibly brand-new applications
of electrostatic printing. Many inventors believe we've only
scratched the surface of what we can do with simple static
electricity!
For more information on laser printers and related topics, check
out the links on the next page.
Serial dot matrix printer
There are several printer technologies used in today's home, office
and banking printers. Dot matrix printers, known also as impact
printers, represent the oldest printing technology, are still the
widespread today, grace of it's best cost per page ratio. Dot
matrix printers are divided on two main groups: serial dot matrix
printers and line dot matrix printers (or simply line printers). In
serial dot matrix printers the characters are formed by the print
head (or printhead). Such a print head has a number of print
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wires (pins) arranged in vertical columns and electro-magnetic
mechanism
able
to
shoot
these
wires.
There are two main printhead technologies - in the first one
electromagnetic field shoots the print head's wire. In the second
one, the so called permanent magnet printheads, a spring shoots
the printhead wire and the magnetic field just holds the spring in
stressed and ready to shoot position. When the electromagnetic
field equalizes the magnetic field, the spring is released to shoot
the wire. Both print head mechanisms are shown in action at the
picture
bellow.
Dot matrix printer head mechanisms in action:
Classical printhead mechanism is showed from the left side.
The permanent magnet printer head mechanism you may see
at right.
In general the permanent magnet printheads are faster and are
used in heavy-duty printers. Some of the most popular print
heads of this type are: Epson DFX, IBM 4226, Fujitsu 5600 and
6400, and all Oki print heads.
How the serial dot matrix printers work?
As the printer head moves in horizontal direction, the printhead
controller sends electrical signals which forces the appropriate
wires to strike against the inked ribbon, making dots on the paper
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and forming the desired characters. The most commonly used
printer heads has 9 print wires in one column (9-pin printheads)
or 24 print wires in two columns (24-pin printheads), for better
print quality. In some heavy-duty dot matrix printers there are also
used 18 wire print heads (18-pin printheads) which have 2
columns, 9 wires in each. The printing process of a 9-pin printer
head is shown at the picture bellow:
Serial 9-pin dot matrix printer in action
The distance between wires in column may give us the vertical
printing resolution. For example: 9 wire print head with distance
0.35 mm between adjacent wires will result in 25.4/0.35=72.5
dots/inch (dots per inch DPI) vertical printing resolution for one
pass printed line of characters. 24 wire print heads has 2 columns
- 12 wires in each, with a vertical displacement of ½ step. So if the
distance between adjacent wires is 0.21 mm, then one column will
print with 25.4/0.21=120.9 dots/inch (DPI) vertical resolution, but
since the second column print between the dots printed from the
first one, the overall vertical resolution will be 240 DPI. Please note
that the first laser printers released on the market had the same
resolution.
Daisy wheel printer
From Wikipedia, the free encyclopedia
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Daisy Wheel
A daisy wheel printer is a type of computer printer that produces
high-quality type, and was often referred to during the 1980s as a
letter-quality printer (in contrast to high-quality dot matrix printers,
capable of so-called near letter quality (NLQ) output). There were
also, and still are, daisy wheel typewriters, based on the same
principle.
The system used a small wheel with each letter printed on it in
raised metal or plastic. The printer turns the wheel to line up the
proper letter under a single pawl which then strikes the back of the
letter and drives it into the paper. In many respects the daisy wheel
is similar to a standard typewriter in the way it forms its letters on
the page, differing only in the details of the mechanism (daisy
wheel vs. typebars or the typeball used on IBM Selectric
typewriters).
As with daisy wheel typewriters and typeball-based Selectrics,
different fonts could be supported through replacing the daisy
wheel. Appropriately-written software would stop the printer at the
font change, space to the center of the carriage, and prompt the
user to change the wheel before proceeding. The Xerox Diablo
D25 included this functionality in the printer's hardware. While
practical for most needs, printing a document which frequently
alternated between italics and plain text would become an arduous
task.
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Bold face could typically be supported, though found mostly on
later and high-end daisywheel printers. When instructed to print in
bold, some printers would double or triple strike a given character,
and some servo-based printers would very slightly advance the
carriage for a wider (and therefore blacker) character. Still others
(typically inexpensive daisy wheel printers) would perform a
carriage return (without a line feed) to return to the beginning of
the line, space through all non-bold text, and restrike each bolded
character - the inherent imprecision in attempting to restrike on
exactly the same spot after a carriage return providing the same
effect as a far more expensive servo-based printer, though with the
unique consequence that as the printer aged, bold text would
become bolder.
Daisy wheel printers were fairly common in the 1980s, but were
always less popular than dot matrix printers (ballistic wire printers)
due to higher cost and the dot-matrix's ability to print graphics and
different fonts. Most dot-matrix printers were also substantially
faster than competing daisy wheel printers, as each character
required that the wheel be rotated to a new position.
As with all other impact printers, daisy wheel printers are loud.
Unlike the more familiar whine of a dot-matrix printer, a high speed
daisy wheel printer sounded like intermittent machine gun fire.
With the introduction of high-quality laser printers and inkjet
printers in the later 1980s, daisy wheel systems quickly
disappeared but for the small remaining typewriter market.
Some remaining daisy wheel printers are used by aficionados for
line-based (as opposed to page-based) text like logging data, while
others use them to print high-quality labels. Both are tasks for
which a laser or inkjet printer aren't well suited. Laser printers are
page (not line) printers, meaning that they cannot print a line of
text without ejecting a full page. As with dot-matrix printers, laser
and inkjet printers are also highly vulnerable to damage when
printing labels.
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Dye-sublimation printer
From Wikipedia, the free encyclopedia
Samsung SPP-2040 working.
A disassembled dye sublimation cartridge.
A dye-sublimation printer (or dye-sub printer) is a computer
printer which employs a printing process that uses heat to transfer
dye to a medium such as a plastic card, printer paper or poster
paper. The process is usually to lay one color at a time using a
ribbon that has color panels. Most dye-sublimation printers use
CMYO colors which differs from the more recognised CMYK colors
in that the black dye is eliminated in favour of a clear overcoating.
This overcoating (which has numerous names depending on the
manufacturer) is effectively a thin laminate which protects the print
from discoloration from UV light and the air while also rendering
the print water-resistant. Many consumer and professional dyesublimation printers are designed and used for producing
photographic prints.
Sublimation is when a substance transitions between the solid and
gas states without going through a liquid stage; dry ice is an
example. In a dye-sublimation printer the printing dye is heated up
until it turns into a gas, at which point it diffuses onto the printing
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media and solidifies. Prior to printing, the dye is stored on a
cellophane ribbon. The ribbon is made up of three colored panels
(cyan, magenta, and yellow) and one clear panel which holds the
lamination material for the overcoating. Each colored panel is the
size of the media that is being printed on; for example, a 6" by 4"
dye sub printer would have four 6" by 4" panels. During the printing
cycle, the printer rollers will move the media and one of the colored
panels together under a thermal printing head, which is usually the
same width as the shorter dimension of the print media. Tiny
heating elements on the head change temperature rapidly, laying
different amounts of dye depending on the amount of heat applied.
After the printer finishes covering the media in one color, it winds
the ribbon on to the next color panel and partially ejects the media
from the printer to prepare for the next cycle. The entire process is
repeated four times in total: the first three lay the colors onto the
media to form a complete image, while the last one lays the
laminate over top. This layer protects the dye from resublimating
when handled or
Thermal printer
From Wikipedia, the free encyclopedia
For the type of printer which uses sparks and aluminised
paper (and is sometimes referred to as a "thermal printer"),
see spark printer.
See also: Thermal transfer printer
A thermal printer (or direct thermal printer) produces a
printed image by selectively heating coated thermochromic
paper, or thermal paper as it is commonly known, when the
paper passes over the thermal print head. The coating
turns black in the areas where it is heated, producing an
image. Two-color direct thermal printers are capable of
printing both black and an additional color (often red), by
applying heat at two different temperatures.
Thermal transfer printing is a related method that uses a
heat-sensitive ribbon instead of heat-sensitive paper[1].
[edit] Essential mechanisms
A thermal printer comprises these key components:
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Thermal head — generates heat; prints on paper
Platen — a rubber roller that feeds paper
Spring — applies pressure to the thermal head, causing
it to contact the thermo-sensitive paper
Controller boards — for controlling the mechanism
In order to print, one inserts thermo-sensitive paper
between the thermal head and the platen. The printer
sends an electrical current to the heating resistor of the
thermal head, which in turn generates heat in a prescribed
pattern. The heat activates the thermo-sensitive coloring
layer of the thermo-sensitive paper, which manifests a
pattern of color change in response. Such a printing
mechanism is known as a thermal system or direct
system.
The paper is impregnated with a solid-state mixture of a
dye and a suitable matrix; a combination of a fluoran leuco
dye and an octadecylphosphonic acid is an example.
When the matrix is heated above its melting point, the dye
reacts with the acid, shifts to its colored form, and the
changed form is then conserved in metastable state when
the matrix solidifies back quickly enough. See
thermochromism.
Controller boards are embedded with firmware to manage
the thermal printer mechanisms. These controller boards’
features are designed to meet the needs in terms of
functionalities and specifications.
The Firmware can manage multiple bar code types,
graphics and logos. They enable the user to choose
between different resident fonts (also including Asian fonts)
and character sizes.
Controller boards can drive various sensors like paper low,
paper out, door open, top of form etc., and they are
available with the most commonly used interfaces (RS232,
Parallel, USB, wireless). For POS application some boards
can also control the cash drawer.
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‫ ذاكرة القراءة فقط وتطورها عبر التاريخ‬ROM
Read-only memory (ROM), also known as firmware, is an
integrated circuit programmed with specific data when it is
manufactured. ROM chips are used not only in computers, but in
most other electronic items as well.
In this article, you will learn about the different types of ROM and
how each works.
ROM Types
There are five basic ROM types:
 ROM
 PROM
 EPROM
 EEPROM
 Flash memory
Each type has unique characteristics, which you'll learn about in
this article, but they are all types of memory with two things in
common:
 Data stored in these chips is nonvolatile -- it is not lost
when power is removed.
 Data stored in these chips is either unchangeable or
requires a special operation to change (unlike RAM,
which can be changed as easily as it is read).
This means that removing the power source from the chip will not
cause it to lose any data.
ROM at Work
Similar to RAM, ROM chips (Figure 1) contain a grid of columns
and rows. But where the columns and rows intersect, ROM chips
are fundamentally different from RAM chips. While RAM uses
transistors to turn on or off access to a capacitor at each
intersection, ROM uses a diode to connect the lines if the value is
1. If the value is 0, then the lines are not connected at all.
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Figure 1. BIOS uses Flash memory, a type of ROM.
A diode normally allows current to flow in only one direction and
has a certain threshold, known as the forward breakover, that
determines how much current is required before the diode will
pass it on. In silicon-based items such as processors and memory
chips, the forward breakover voltage is approximately 0.6 volts. By
taking advantage of the unique properties of a diode, a ROM chip
can send a charge that is above the forward breakover down the
appropriate column with the selected row grounded to connect at a
specific cell. If a diode is present at that cell, the charge will be
conducted through to the ground, and, under the binary system,
the cell will be read as being "on" (a value of 1). The neat part of
ROM is that if the cell's value is 0, there is no diode at that
intersection to connect the column and row. So the charge on the
column does not get transferred to the row.
As you can see, the way a ROM chip works necessitates the
programming of perfect and complete data when the chip is
created. You cannot reprogram or rewrite a standard ROM chip. If
it is incorrect, or the data needs to be updated, you have to throw it
away and start over. Creating the original template for a ROM chip
is often a laborious process full of trial and error. But the benefits
of ROM chips outweigh the drawbacks. Once the template is
completed, the actual chips can cost as little as a few cents each.
They use very little power, are extremely reliable and, in the case
of most small electronic devices, contain all the necessary
programming to control the device. A great example is the small
chip in the singing fish toy. This chip, about the size of your
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fingernail, contains the 30-second song clips in ROM and the
control codes to synchronize the motors to the music.
PROM
Creating ROM chips totally from scratch is time-consuming and
very expensive in small quantities. For this reason, mainly,
developers created a type of ROM known as programmable
read-only memory (PROM). Blank PROM chips can be bought
inexpensively and coded by anyone with a special tool called a
programmer.
PROM chips (Figure 2) have a grid of columns and rows just as
ordinary ROMs do. The difference is that every intersection of a
column and row in a PROM chip has a fuse connecting them. A
charge sent through a column will pass through the fuse in a cell to
a grounded row indicating a value of 1. Since all the cells have a
fuse, the initial (blank) state of a PROM chip is all 1s. To change
the value of a cell to 0, you use a programmer to send a specific
amount of current to the cell. The higher voltage breaks the
connection between the column and row by burning out the fuse.
This process is known as burning the PROM.
Figure 2
PROMs can only be programmed once. They are more fragile than
ROMs. A jolt of static electricity can easily cause fuses in the
PROM to burn out, changing essential bits from 1 to 0. But blank
PROMs are inexpensive and are great for prototyping the data for
a ROM before committing to the costly ROM fabrication process.
EPROM
Working with ROMs and PROMs can be a wasteful business.
Even though they are inexpensive per chip, the cost can add up
over time. Erasable programmable read-only memory (EPROM)
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addresses this issue. EPROM chips can be rewritten many times.
Erasing an EPROM requires a special tool that emits a certain
frequency of ultraviolet (UV) light. EPROMs are configured using
an EPROM programmer that provides voltage at specified levels
depending on the type of EPROM used.
Once again we have a grid of columns and rows. In an EPROM,
the cell at each intersection has two transistors. The two
transistors are separated from each other by a thin oxide layer.
One of the transistors is known as the floating gate and the other
as the control gate. The floating gate's only link to the row
(wordline) is through the control gate. As long as this link is in
place, the cell has a value of 1. To change the value to 0 requires
a curious process called Fowler-Nordheim tunneling. Tunneling
is used to alter the placement of electrons in the floating gate. An
electrical charge, usually 10 to 13 volts, is applied to the floating
gate. The charge comes from the column (bitline), enters the
floating gate and drains to a ground.
This charge causes the floating-gate transistor to act like an
electron gun. The excited electrons are pushed through and
trapped on the other side of the thin oxide layer, giving it a
negative charge. These negatively charged electrons act as a
barrier between the control gate and the floating gate. A device
called a cell sensor monitors the level of the charge passing
through the floating gate. If the flow through the gate is greater
than 50 percent of the charge, it has a value of 1. When the charge
passing through drops below the 50-percent threshold, the value
changes to 0. A blank EPROM has all of the gates fully open,
giving each cell a value of 1.
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Figure 3
To rewrite an EPROM, you must erase it first. To erase it, you
must supply a level of energy strong enough to break through the
negative electrons blocking the floating gate. In a standard
EPROM, this is best accomplished with UV light at a frequency of
253.7. Because this particular frequency will not penetrate most
plastics or glasses, each EPROM chip has a quartz window on top
of it. The EPROM must be very close to the eraser's light source,
within an inch or two, to work properly.
An EPROM eraser is not selective, it will erase the entire EPROM.
The EPROM must be removed from the device it is in and placed
under the UV light of the EPROM eraser for several minutes. An
EPROM that is left under too long can become over-erased. In
such a case, the EPROM's floating gates are charged to the point
that they are unable to hold the electrons at all.
EEPROMs and Flash Memory
Though EPROMs are a big step up from PROMs in terms of
reusability, they still require dedicated equipment and a laborintensive process to remove and reinstall them each time a change
is necessary. Also, changes cannot be made incrementally to an
EPROM; the whole chip must be erased. Electrically erasable
programmable read-only memory (EEPROM) chips remove the
biggest drawbacks of EPROMs.
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In EEPROMs:
The chip does not have to removed to be rewritten.
 The entire chip does not have to be completely erased to
change a specific portion of it.
 Changing the contents does not require additional
dedicated equipment.
Instead of using UV light, you can return the electrons in the cells
of an EEPROM to normal with the localized application of an
electric field to each cell. This erases the targeted cells of the
EEPROM, which can then be rewritten. EEPROMs are changed 1
byte at a time, which makes them versatile but slow. In fact,
EEPROM chips are too slow to use in many products that make
quick changes to the data stored on the chip.

Manufacturers responded to this limitation with Flash memory, a
type of EEPROM that uses in-circuit wiring to erase by applying
an electrical field to the entire chip or to predetermined sections of
the chip called blocks. Flash memory works much faster than
traditional EEPROMs because it writes data in chunks, usually 512
bytes in size, instead of 1 byte at a time. See How Flash Memory
Works to learn more about this type of ROM and its applications.
For more information on ROM and other types of computer
memory, check out the links on the next page!
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‫ نظام الدخل والخرج األساسي‬BIOS
One of the most common uses of Flash memory is for the basic
input/output system of your computer, commonly known as the
BIOS (pronounced "bye-ose"). On virtually every computer
available, the BIOS makes sure all the other chips, hard drives,
ports and CPU function together.
Every desktop and laptop computer in common use today contains
a microprocessor as its central processing unit. The
microprocessor is the hardware component. To get its work done,
the microprocessor executes a set of instructions known as
software. You are probably very familiar with two different types of
software:
The operating system - The operating system provides
a set of services for the applications running on your
computer, and it also provides the fundamental user
interface for your computer. Windows 98 and Linux are
examples of operating systems. (See How Operating
Systems Work for lots of details.)

The applications - Applications are pieces of software
that are programmed to perform specific tasks. On your
computer right now you probably have a browser
application, a word processing application, an e-mail
application and so on. You can also buy new applications
and install them.
It turns out that the BIOS is the third type of software your
computer needs to operate successfully. In this article, you'll learn
all about BIOS -- what it does, how to configure it and what to do if
your BIOS needs updating.

What BIOS Does
The BIOS software has a number of different roles, but its most
important role is to load the operating system. When you turn on
your computer and the microprocessor tries to execute its first
instruction, it has to get that instruction from somewhere. It cannot
get it from the operating system because the operating system is
located on a hard disk, and the microprocessor cannot get to it
without some instructions that tell it how. The BIOS provides those
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instructions. Some of the other common tasks that the BIOS
performs include:
 A power-on self-test (POST) for all of the different
hardware components in the system to make sure
everything is working properly
 Activating other BIOS chips on different cards
installed in the computer - For example, SCSI and
graphics cards often have their own BIOS chips.
 Providing a set of low-level routines that the
operating system uses to interface to different
hardware devices - It is these routines that give the
BIOS its name. They manage things like the
keyboard, the screen, and the serial and parallel
ports, especially when the computer is booting.
 Managing a collection of settings for the hard disks,
clock, etc.
The BIOS is special software that interfaces the major hardware
components of your computer with the operating system. It is
usually stored on a Flash memory chip on the motherboard, but
sometimes the chip is another type of ROM.
When you turn on your computer, the BIOS does several things.
This is its usual sequence:
1. Check the CMOS Setup for custom settings
2. Load the interrupt handlers and device drivers
3. Initialize registers and power management
4. Perform the power-on self-test (POST)
5. Display system settings
6. Determine which devices are bootable
7. Initiate the bootstrap sequence
The first thing the BIOS does is check the information stored in a
tiny (64 bytes) amount of RAM located on a complementary
metal oxide semiconductor (CMOS) chip. The CMOS Setup
provides detailed information particular to your system and can be
altered as your system changes. The BIOS uses this information to
modify or supplement its default programming as needed. We will
talk more about these settings later.
Interrupt handlers are small pieces of software that act as
translators between the hardware components and the operating
system. For example, when you press a key on your keyboard, the
signal is sent to the keyboard interrupt handler, which tells the
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CPU what it is and passes it on to the operating system. The
device drivers are other pieces of software that identify the base
hardware components such as keyboard, mouse, hard drive and
floppy drive. Since the BIOS is constantly intercepting signals to
and from the hardware, it is usually copied, or shadowed, into
RAM to run faster.
Booting the Computer
Whenever you turn on your computer, the first thing you see is the
BIOS software doing its thing. On many machines, the BIOS
displays text describing things like the amount of memory installed
in your computer, the type of hard disk and so on. It turns out that,
during this boot sequence, the BIOS is doing a remarkable amount
of work to get your computer ready to run. This section briefly
describes some of those activities for a typical PC.
After checking the CMOS Setup and loading the interrupt handlers,
the BIOS determines whether the video card is operational. Most
video cards have a miniature BIOS of their own that initializes the
memory and graphics processor on the card. If they do not, there
is usually video driver information on another ROM on the
motherboard that the BIOS can load.
Next, the BIOS checks to see if this is a cold boot or a reboot. It
does this by checking the value at memory address 0000:0472. A
value of 1234h indicates a reboot, and the BIOS skips the rest of
POST. Anything else is considered a cold boot.
If it is a cold boot, the BIOS verifies RAM by performing a
read/write test of each memory address. It checks the PS/2 ports
or USB ports for a keyboard and a mouse. It looks for a peripheral
component interconnect (PCI) bus and, if it finds one, checks all
the PCI cards. If the BIOS finds any errors during the POST, it will
notify you by a series of beeps or a text message displayed on the
screen. An error at this point is almost always a hardware problem.
The BIOS then displays some details about your system. This
typically includes information about:


The processor
The floppy drive and hard drive
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Memory

BIOS revision and date

Display
Any special drivers, such as the ones for small computer system
interface (SCSI) adapters, are loaded from the adapter, and the
BIOS displays the information. The BIOS then looks at the
sequence of storage devices identified as boot devices in the
CMOS Setup. "Boot" is short for "bootstrap," as in the old phrase,
"Lift yourself up by your bootstraps." Boot refers to the process of
launching the operating system. The BIOS will try to initiate the
boot sequence from the first device. If the BIOS does not find a
device, it will try the next device in the list. If it does not find the
proper files on a device, the startup process will halt. If you have
ever left a floppy disk in the drive when you restarted your
computer, you have probably seen this message.

This is the message you get if a floppy disk is in the
drive when you restart your computer.
The BIOS has tried to boot the computer off of the floppy disk left
in the drive. Since it did not find the correct system files, it could
not continue. Of course, this is an easy fix. Simply pop out the disk
and press a key to continue.
Configuring BIOS
In the previous list, you saw that the BIOS checks the CMOS
Setup for custom settings. Here's what you do to change those
settings.
To enter the CMOS Setup, you must press a certain key or
combination of keys during the initial startup sequence. Most
systems use "Esc," "Del," "F1," "F2," "Ctrl-Esc" or "Ctrl-Alt-Esc" to
enter setup. There is usually a line of text at the bottom of the
display that tells you "Press ___ to Enter Setup."
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Once you have entered setup, you will see a set of text screens
with a number of options. Some of these are standard, while
others vary according to the BIOS manufacturer. Common options
include:









System Time/Date - Set the system time and date
Boot Sequence - The order that BIOS will try to load
the operating system
Plug and Play - A standard for auto-detecting
connected devices; should be set to "Yes" if your
computer and operating system both support it
Mouse/Keyboard - "Enable Num Lock," "Enable the
Keyboard," "Auto-Detect Mouse"...
Drive Configuration - Configure hard drives, CDROM and floppy drives
Memory - Direct the BIOS to shadow to a specific
memory address
Security - Set a password for accessing the
computer
Power Management - Select whether to use power
management, as well as set the amount of time for
standby and suspend
Exit - Save your changes, discard your changes or
restore default settings
CMOS Setup
Be very careful when making changes to setup. Incorrect settings
may keep your computer from booting. When you are finished with
your changes, you should choose "Save Changes" and exit. The
BIOS will then restart your computer so that the new settings take
effect.
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The BIOS uses CMOS technology to save any changes made to
the computer's settings. With this technology, a small lithium or NiCad battery can supply enough power to keep the data for years.
In fact, some of the newer chips have a 10-year, tiny lithium
battery built right into the CMOS chip!
Updating Your BIOS
Occasionally, a computer will need to have its BIOS updated. This
is especially true of older machines. As new devices and
standards arise, the BIOS needs to change in order to understand
the new hardware. Since the BIOS is stored in some form of ROM,
changing it is a bit harder than upgrading most other types of
software.
To change the BIOS itself, you'll probably need a special program
from the computer or BIOS manufacturer. Look at the BIOS
revision and date information displayed on system startup or check
with your computer manufacturer to find out what type of BIOS you
have. Then go to the BIOS manufacturer's Web site to see if an
upgrade is available. Download the upgrade and the utility
program needed to install it. Sometimes the utility and update are
combined in a single file to download. Copy the program, along
with the BIOS update, onto a floppy disk. Restart your computer
with the floppy disk in the drive, and the program erases the old
BIOS and writes the new one. You can find a BIOS Wizard that will
check your BIOS at BIOS Upgrades.
Major BIOS manufacturers include:




American Megatrends Inc. (AMI)
Phoenix Technologies
ALi
Winbond
As with changes to the CMOS Setup, be careful when upgrading
your BIOS. Make sure you are upgrading to a version that is
compatible with your computer system. Otherwise, you could
corrupt the BIOS, which means you won't be able to boot your
computer. If in doubt, check with your computer manufacturer to
be sure you need to upgrade.
For more information on BIOS and related topics, check out the
links on the next page.
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Power Supply ‫مزود الطاقة‬
If there is any one component that is absolutely vital to the
operation of a computer, it is the power supply. Without it, a
computer is just an inert box full of plastic and metal. The power
supply converts the alternating current (AC) line from your home to
the direct current (DC) needed by the personal computer. In this
article, we'll learn how PC power supplies work and what the
wattage ratings mean.
Power Supply
In a personal computer (PC), the power supply is the metal box
usually found in a corner of the case. The power supply is visible
from the back of many systems because it contains the power-cord
receptacle and the cooling fan.
This is a power supply removed from its PC case. The small, red switch
at right, above the power-cord connector, is for changing line voltages in
various countries.
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The interior of a power supply.
Power supplies, often referred to as "switching power supplies",
use switcher technology to convert the AC input to lower DC
voltages. The typical voltages supplied are:



3.3 volts
5 volts
12 volts
The 3.3- and 5-volts are typically used by digital circuits, while the
12-volt is used to run motors in disk drives and fans. The main
specification of a power supply is in watts. A watt is the product of
the voltage in volts and the current in amperes or amps. If you
have been around PCs for many years, you probably remember
that the original PCs had large red toggle switches that had a good
bit of heft to them. When you turned the PC on or off, you knew
you were doing it. These switches actually controlled the flow of
120 volt power to the power supply.
Today you turn on the power with a little push button, and you turn
off the machine with a menu option. These capabilities were added
to standard power supplies several years ago. The operating
system can send a signal to the power supply to tell it to turn off.
The push button sends a 5-volt signal to the power supply to tell it
when to turn on. The power supply also has a circuit that supplies
5 volts, called VSB for "standby voltage" even when it is officially
"off", so that the button will work.
Switcher Technology
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Prior to 1980 or so, power supplies tended to be heavy and bulky.
They used large, heavy transformers and huge capacitors (some
as large as soda cans) to convert line voltage at 120 volts and 60
hertz into 5 volts and 12 volts DC.
The switching power supplies used today are much smaller and
lighter. They convert the 60-Hertz (Hz, or cycles per second)
current to a much higher frequency, meaning more cycles per
second. This conversion enables a small, lightweight transformer
in the power supply to do the actual voltage step-down from 110
volts (or 220 in certain countries) to the voltage needed by the
particular computer component. The higher-frequency AC current
provided by a switcher supply is also easier to rectify and filter
compared to the original 60-Hz AC line voltage, reducing the
variances in voltage for the sensitive electronic components in the
computer.
In this photo you can see three small transformers (yellow) in the
center. To the left are two cylindrical capacitors. The large finned
pieces of aluminum are heat sinks. The left heat sink has
transistors attached to it. These are the transistors in charge of
doing the switching -- they provide high-frequency power to the
transformers. Attached to the right heat sink are diodes that
rectify AC signals and turn them into DC signals.
A switcher power supply draws only the power it needs from the
AC line. The typical voltages and current provided by a power
supply are shown on the label on a power supply.
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Personal computer power supply label.
VSB is the standby voltage provided to the power
switch.
Switcher technology is also used to make AC from DC, as found in
many of the automobile power inverters used to run AC appliances
in an automobile and in uninterruptible power supplies. Switcher
technology in automotive power inverters changes the direct
current from the auto battery into alternating current. The
transformer uses alternating current to make the transformer in the
inverter step the voltage up to that of household appliances (120
VAC).
Power Supply Standardization
Over time, there have been at least six different standard power
supplies for personal computers. Recently, the industry has settled
on using ATX-based power supplies. ATX is an industry
specification that means the power supply has the physical
characteristics to fit a standard ATX case and the electrical
characteristics to work with an ATX motherboard.
PC power-supply cables use standardized, keyed connectors that
make it difficult to connect the wrong ones. Also, fan
manufacturers often use the same connectors as the power cables
for disk drives, allowing a fan to easily obtain the 12 volts it needs.
Color-coded wires and industry standard connectors make it
possible for the consumer to have many choices for a replacement
power supply.
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A PC power supply removed from its PC case. Cables and
connectors at right supply DC voltages.
Advanced Power Management
Advanced Power Management (APM) offers a set of five
different states that your system can be in. It was developed by
Microsoft and Intel for PC users who wish to conserve power.
Each system component, including the operating system, basic
input/output system (BIOS), motherboard and attached devices all
need to be APM-compliant to be able to use this feature. Should
you wish to disable APM because you suspect it is using up
system resources or causing a conflict, the best way to do this is in
the BIOS. That way, the operating system won't try to reinstall it,
which could happen if it were disabled only in the software.
Power Supply Wattage
A 400-watt switching power supply will not necessarily use more
power than a 250-watt supply. A larger supply may be needed if
you use every available slot on the motherboard or every available
drive bay in the personal computer case. It is not a good idea to
have a 250-watt supply if you have 250 watts total in devices,
since the supply should not be loaded to 100 percent of its
capacity.
According to PC Power & Cooling, Inc., some power consumption
values (in watts) for common items in a personal computer are:
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PC Item
Watts
Accelerated Graphics Port (AGP)
20 to 30W
card
Peripheral Component Interconnect
5W
(PCI) card
small computer system interface
20 to 25W
(SCSI) PCI card
floppy disk drive
5W
network interface card
4W
50X CD-ROM drive
10 to 25W
RAM
10W per
128M
5200
RPM
Integrated
Drive
5 to 11W
Electronics (IDE) hard disk drive
7200 RPM IDE hard disk drive
5 to 15W
Motherboard (without CPU or RAM) 20 to 30W
550 MHz Pentium III
30W
733 MHz Pentium III
23.5W
300 MHz Celeron
18W
600 MHz Athlon
45W
Power supplies of the same form factor ("form factor" refers to the
actual shape of the motherboard) are typically differentiated by the
wattage they supply and the length of the warranty.
Power Supply Problems
The PC power supply is probably the most failure-prone item in a
personal computer. It heats and cools each time it is used and
receives the first in-rush of AC current when the PC is switched on.
Typically, a stalled cooling fan is a predictor of a power supply
failure due to subsequent overheated components. All devices in a
PC receive their DC power via the power supply.
A typical failure of a PC power supply is often noticed as a burning
smell just before the computer shuts down. Another problem could
be the failure of the vital cooling fan, which allows components in
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the power supply to overheat. Failure symptoms include random
rebooting or failure in Windows for no apparent reason.
For any problems you suspect to be the fault of the power supply,
use the documentation that came with your computer. If you have
ever removed the case from your personal computer to add an
adapter card or memory, you can change a power supply. Make
sure you remove the power cord first, since voltages are present
even though your computer is off.
Power Supply Improvements
Recent motherboard and chipset improvements permit the user to
monitor the revolutions per minute (RPM) of the power supply fan
via BIOS and a Windows application supplied by the motherboard
manufacturer. New designs offer fan control so that the fan only
runs the speed needed, depending on cooling needs.
Recent designs in Web servers include power supplies that offer a
spare supply that can be exchanged while the other power supply
is in use. Some new computers, particularly those designed for
use as servers, provide redundant power supplies. This means
that there are two or more power supplies in the system, with one
providing power and the other acting as a backup. The backup
supply immediately takes over in the event of a failure by the
primary supply. Then, the primary supply can be exchanged while
the other power supply is in use.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
‫الفأرة‬mouse
Mice first broke onto the public stage with the introduction of the
Apple Macintosh in 1984, and since then they have helped to
completely redefine the way we use computers.
Every day of your computing life, you reach out for your mouse
whenever you want to move your cursor or activate something.
Your mouse senses your motion and your clicks and sends them
to the computer so it can respond appropriately.
This Microsoft Intellimouse uses optical technology. See
more computer mouse pictures.
In this article we'll take the cover off of this important part of the
human-machine interface and see exactly what makes it tick.
Evolution
It is amazing how simple and effective a mouse is, and it is also
amazing how long it took mice to become a part of everyday life.
Given that people naturally point at things -- usually before they
speak -- it is surprising that it took so long for a good pointing
device to develop. Although originally conceived in the 1960s, a
couple of decades passed before mice became mainstream.
In the beginning, there was no need to point because computers
used crude interfaces like teletype machines or punch cards for
data entry. The early text terminals did nothing more than emulate
a teletype (using the screen to replace paper), so it was many
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years (well into the 1960s and early 1970s) before arrow keys
were found on most terminals. Full screen editors were the first
things to take real advantage of the cursor keys, and they offered
humans the first way to point.
Light pens were used on a variety of machines as a pointing
device for many years, and graphics tablets, joy sticks and various
other devices were also popular in the 1970s. None of these really
took off as the pointing device of choice, however.
When the mouse hit the scene -- attached to the Mac, it was an
immediate success. There is something about it that is completely
natural. Compared to a graphics tablet, mice are extremely
inexpensive and they take up very little desk space. In the PC
world, mice took longer to gain ground, mainly because of a lack of
support in the operating system. Once Windows 3.1 made
Graphical User Interfaces (GUIs) a standard, the mouse became
the PC-human interface of choice very quickly.
Inside a Mouse
The main goal of any mouse is to translate the motion of your hand
into signals that the computer can use. Let's take a look inside a
track-ball mouse to see how it works:
The guts of a mouse
1. A ball inside the mouse touches the desktop and rolls
when the mouse moves.
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The underside of the mouse's logic board: The exposed
portion of the ball touches the desktop.
2. Two rollers inside the mouse touch the ball. One of the
rollers is oriented so that it detects motion in the X
direction, and the other is oriented 90 degrees to the first
roller so it detects motion in the Y direction. When the ball
rotates, one or both of these rollers rotate as well. The
following image shows the two white rollers on this
mouse:
The rollers that touch the ball and detect X and Y motion
3. The rollers each connect to a shaft, and the shaft spins a
disk with holes in it. When a roller rolls, its shaft and disk
spin. The following image shows the disk:
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A typical optical encoding disk: This disk has 36 holes around
its outer edge.
4. On either side of the disk there is an infrared LED and an
infrared sensor. The holes in the disk break the beam of
light coming from the LED so that the infrared sensor
sees pulses of light. The rate of the pulsing is directly
related to the speed of the mouse and the distance it
travels.
A close-up of one of the optical encoders that track mouse
motion: There is an infrared LED (clear) on one side of the disk
and an infrared sensor (red) on the other.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
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5. An on-board processor chip reads the pulses from the
infrared sensors and turns them into binary data that the
computer can understand. The chip sends the binary data
to the computer through the mouse's cord.
The logic section of a mouse is dominated by an encoder chip, a
small processor that reads the pulses coming from the infrared
sensors and turns them into bytes sent to the computer. You can also
see the two buttons that detect clicks (on either side of the wire
connector).
In this optomechanical arrangement, the disk moves
mechanically, and an optical system counts pulses of light. On this
mouse, the ball is 21 mm in diameter. The roller is 7 mm in
diameter. The encoding disk has 36 holes. So if the mouse moves
25.4 mm (1 inch), the encoder chip detects 41 pulses of light.
You might have noticed that each encoder disk has two infrared
LEDs and two infrared sensors, one on each side of the disk (so
there are four LED/sensor pairs inside a mouse). This
arrangement allows the processor to detect the disk's direction of
rotation. There is a piece of plastic with a small, precisely located
hole that sits between the encoder disk and each infrared sensor.
It is visible in this photo:
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
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A close-up of one of the optical encoders that track mouse
motion: Note the piece of plastic between the infrared sensor
(red) and the encoding disk.
This piece of plastic provides a window through which the infrared
sensor can "see." The window on one side of the disk is located
slightly higher than it is on the other -- one-half the height of one of
the holes in the encoder disk, to be exact. That difference causes
the two infrared sensors to see pulses of light at slightly different
times. There are times when one of the sensors will see a pulse of
light when the other does not, and vice versa. This page offers a
nice explanation of how direction is determined.
Data Interface
Most mice on the market today use a
USB connector to attach to your
computer. USB is a standard way to
connect all kinds of peripherals to your
computer, including printers, digital
cameras, keyboards and mice. See How
USB Ports Work for more information
about this technology.
Some older mice, many of which are still in use today, have a PS/2
type connector, as shown here:
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A typical PS/2 connector.
Instead of a PS/2 connector, a few other older mice use a serial
type of connector to attach to a computer. See How Serial Ports
Work for more information.
Optical Mice
Developed by Agilent Technologies and introduced to the world in
late 1999, the optical mouse actually uses a tiny camera to take
thousands of pictures every second.
Able to work on almost any surface without a mouse pad, most
optical mice use a small, red light-emitting diode (LED) that
bounces light off that surface onto a complimentary metal-oxide
semiconductor (CMOS) sensor. In addition to LEDs, a recent
innovation are laser-based optical mice that detect more surface
details compared to LED technology. This results in the ability to
use a laser-based optical mouse on even more surfaces than an
LED mouse.
Here's how the sensor and other parts of an optical mouse work
together:




The CMOS sensor sends each image to a digital signal
processor (DSP) for analysis.
The DSP detects patterns in the images and examines
how the patterns have moved since the previous image.
Based on the change in patterns over a sequence of
images, the DSP determines how far the mouse has
moved and sends the corresponding coordinates to the
computer.
The computer moves the cursor on the screen based on
the coordinates received from the mouse. This happens
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hundreds of times each second, making the cursor
appear to move very smoothly.
In this photo, you can see the sensor on the bottom of the
mouse.
Optical mice have several benefits over track-ball mice:




No moving parts means less wear and a lower chance of
failure.
There's no way for dirt to get inside the mouse and
interfere with the tracking sensors.
Increased tracking resolution means a smoother response.
They don't require a special surface, such as a mouse pad.
Apple has transformed its optical mouse into a modern work of art.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
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Accuracy
A number of factors affect the accuracy of an optical mouse. One
of the most important aspects is resolution. The resolution is the
number of pixels per inch that the optical sensor and focusing lens
"see" when you move the mouse. Resolution is expressed as dots
per inch (dpi). The higher the resolution, the more sensitive the
mouse is and the less you need to move it to obtain a response.
Most mice have a resolution of 400 or 800 dpi. However, mice
designed for playing electronic games can offer as much as 1600
dpi resolution. Some gaming mice also allow you to decrease the
dpi on the fly to make the mouse less sensitive in situations when
you need to make smaller, slower movements.
Historically, corded mice have been more responsive than wireless
mice. This fact is changing, however, with the advent of
improvements in wireless technologies and optical sensors. Other
factors that affect quality include:




Size of the optical sensor -- larger is generally better,
assuming the other mouse components can handle the
larger size. Sizes range from 16 x 16 pixels to 30 x 30
pixels.
Refresh rate -- it is how often the sensor samples images
as you move the mouse. Faster is generally better,
assuming the other mouse components can process
them. Rates range from 1500 to 6000 samples per
second.
Image processing rate -- is a combination of the size of
the optical sensor and the refresh rate. Again, faster is
better and rates range from 0.486 to 5.8 megapixels per
second.
Maximum speed -- is the maximum speed that you can
move the mouse and obtain accurate tracking. Faster is
better and rates range from 16 to 40 inches per second.
Wireless Mice
Most wireless mice use radio frequency (RF) technology to
communicate information to your computer. Being radio-based, RF
devices require two main components: a transmitter and a
receiver. Here's how it works:
 The transmitter is housed in the mouse. It sends an
electromagnetic (radio) signal that encodes the
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information about the mouse's movements and the
buttons you click.
The receiver, which is connected to your computer,
accepts the signal, decodes it and passes it on to the
mouse driver software and your computer's operating
system.
The receiver can be a separate device that plugs into your
computer, a special card that you place in an expansion
slot, or a built-in component.
Photo courtesy Logitech
MX900 and docking station
Many electronic devices use radio frequencies to communicate.
Examples include cellular phones, wireless networks, and garage
door openers. To communicate without conflicts, different types of
devices have been assigned different frequencies. Newer cell
phones use a frequency of 900 megahertz, garage door openers
operate at a frequency of 40 megahertz, and 802.11b/g wireless
networks operate at 2.4 gigahertz. Megahertz (MHz) means "one
million cycles per second," so "900 megahertz" means that there
are 900 million electromagnetic waves per second. Gigahertz
(GHz) means "one billion cycles per second." To learn more about
RF and frequencies, see How the Radio Spectrum Works.
Benefits
Unlike infrared technology, which is commonly used for shortrange wireless communications such as television remote controls,
RF devices do not need a clear line of sight between the
transmitter (mouse) and receiver. Just like other types of devices
that use radio waves to communicate, a wireless mouse signal can
pass through barriers such as a desk or your monitor.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
RF technology provides a number of additional benefits for
wireless mice. These include:
RF transmitters require low power and can run on batteries
 RF components are inexpensive
 RF components are light weight
As with most mice on the market today, wireless mice use optical
sensor technology rather than the earlier track-ball system. Optical
technology improves accuracy and lets you use the wireless
mouse on almost any surface -- an important feature when you're
not tied to your computer by a cord.

Pairing and Security
In order for the transmitter in the mouse to communicate with its
receiver, they must be paired. This means that both devices are
operating at the same frequency on the same channel using a
common identification code. A channel is simply a specific
frequency and code. The purpose of pairing is to filter out
interference from other sources and RF devices.
Pairing methods vary, depending on the mouse manufacturer.
Some devices come pre-paired. Others use methods such as a
pairing sequence that occurs automatically, when you push
specific buttons, or when you turn a dial on the receiver and/or
mouse.
To protect the information your mouse transmits to the receiver,
most wireless mice include an encryption scheme to encode data
into an unreadable format. Some devices also use a frequency
hopping method, which causes the mouse and receiver to
automatically change frequencies using a predetermined pattern.
This provides additional protection from interference and
eavesdropping.
Wireless Mice
Most wireless mice use radio frequency (RF) technology to
communicate information to your computer. Being radio-based, RF
devices require two main components: a transmitter and a
receiver. Here's how it works:
 The transmitter is housed in the mouse. It sends an
electromagnetic (radio) signal that encodes the
information about the mouse's movements and the
buttons you click.
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
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The receiver, which is connected to your computer,
accepts the signal, decodes it and passes it on to the
mouse driver software and your computer's operating
system.
The receiver can be a separate device that plugs into your
computer, a special card that you place in an expansion
slot, or a built-in component.
Photo
courtesy
MX900 and docking station
Logitech
Many electronic devices use radio frequencies to communicate.
Examples include cellular phones, wireless networks, and garage
door openers. To communicate without conflicts, different types of
devices have been assigned different frequencies. Newer cell
phones use a frequency of 900 megahertz, garage door openers
operate at a frequency of 40 megahertz, and 802.11b/g wireless
networks operate at 2.4 gigahertz. Megahertz (MHz) means "one
million cycles per second," so "900 megahertz" means that there
are 900 million electromagnetic waves per second. Gigahertz
(GHz) means "one billion cycles per second." To learn more about
RF and frequencies, see How the Radio Spectrum Works.
Benefits
Unlike infrared technology, which is commonly used for shortrange wireless communications such as television remote controls,
RF devices do not need a clear line of sight between the
transmitter (mouse) and receiver. Just like other types of devices
that use radio waves to communicate, a wireless mouse signal can
pass through barriers such as a desk or your monitor.
RF technology provides a number of additional benefits for
wireless mice. These include:
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‫ كلية االتصاالت – جدة‬- ‫قسم تقنية الحاسب‬
‫معتصم الحمدان‬.‫ أ‬:‫جمع وترتيب‬
RF transmitters require low power and can run on batteries
 RF components are inexpensive
 RF components are light weight
As with most mice on the market today, wireless mice use optical
sensor technology rather than the earlier track-ball system. Optical
technology improves accuracy and lets you use the wireless
mouse on almost any surface -- an important feature when you're
not tied to your computer by a cord.

Pairing and Security
In order for the transmitter in the mouse to communicate with its
receiver, they must be paired. This means that both devices are
operating at the same frequency on the same channel using a
common identification code. A channel is simply a specific
frequency and code. The purpose of pairing is to filter out
interference from other sources and RF devices.
Pairing methods vary, depending on the mouse manufacturer.
Some devices come pre-paired. Others use methods such as a
pairing sequence that occurs automatically, when you push
specific buttons, or when you turn a dial on the receiver and/or
mouse.
To protect the information your mouse transmits to the receiver,
most wireless mice include an encryption scheme to encode data
into an unreadable format. Some devices also use a frequency
hopping method, which causes the mouse and receiver to
automatically change frequencies using a predetermined pattern.
This provides additional protection from interference and
eavesdropping.
RF Mice
The other common type of wireless mouse is an RF device that
operates at 27 MHz and has a range of about 6 feet (2 meters).
More recently, 2.4 GHz RF mice have hit the market with the
advantage of a longer range -- about 33 feet (10 meters) and
faster transmissions with less interference. Multiple RF mice in one
room can result in cross-talk, which means that the receiver
inadvertently picks up the transmissions from the wrong mouse.
Pairing and multiple channels help to avoid this problem.
Typically, the RF receiver plugs into a USB port and does not
accept any peripherals other than the mouse (and perhaps a
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keyboard, if sold with the mouse). Some portable models designed
for use with notebook computers come with a compact receiver
that can be stored in a slot inside the mouse when not in use.
Mouse Innovations
As with many computer-related devices, mice are being combined
with other gadgets and technologies to create improved and
multipurpose devices. Examples include multi-media mice,
combination mice/remote controls, gaming mice, biometric mice,
tilting wheel mice and motion-based mice. To learn more about
innovations in mouse technology, let's start with multi-media mice
and combination mice/remote controls.
Multi-Media Mouse and Combination Mouse/Remote
These types of mice are used with multimedia systems such as the
Windows XP Media Center Edition computers. Some combine
features of a mouse with additional buttons (such as play, pause,
forward, back and volume) for controlling media. Others resemble
a television/media player remote control with added features for
mousing. Remote controls generally use infrared sensors but
some use a combination of infrared and RF technology for greater
range.
Gaming Mice
Gaming mice are high-precision, optical mice designed for use
with PCs and game controllers. Features may include:



Multiple buttons for added flexibility and functions such as
adjusting dpi rates on the fly
Wireless connectivity and an optical sensor
Motion feedback and two-way communication
Motion-Based Mice
Yet another innovation in mouse technology is motion-based
control. With this feature, you control the mouse pointer by waving
the mouse in the air.
The technology patented by one manufacturer, Gyration,
incorporates miniature gyroscopes to track the motion of the
mouse as you wave it in the air. It uses an electromagnetic
transducer and sensors to detect rotation in two axes at the same
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time. The mouse operates on the principle of the Coriolis Effect,
which is the apparent turning of an object that's moving in relation
to another rotating object. The device and accompanying software
converts the mouse movements into movements on the
computer's screen. The mice also include an optical sensor for use
on a desktop.
Biometric Mice
Biometric mice add security to your computer system by permitting
only authorized users to control the mouse and access the
computer. Protection is accomplished with an integrated fingerprint
reader either in the receiver or the mouse. This feature enhances
security and adds convenience because you can use your
fingerprint rather than passwords for a secure login.
Photo
courtesy
Microsoft
Corporation
The Wireless IntelliMouse Explorer with Fingerprint
Reader is a biometric mouse.
To use the biometric feature, a software program that comes with
the mouse registers fingerprints and stores information about
corresponding authorized users. Some software programs also let
you encrypt and decrypt files. For more information about
biometric fingerprint technology, see How Fingerprint Scanners
Work.
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Tilting Scroll Wheel
A recent innovation in mouse scrolling is a tilting scroll wheel that
allows you to scroll onscreen both horizontally (left/right) and
vertically (up/down). The ability to scroll both ways is handy when
you are viewing wide documents like a Web page or spreadsheet.
To navigate both horizontally and vertically, the scroll wheel is
positioned on a combination fulcrum and lever. This is the design
used by the Logitech Cordless Click! Plus mouse.
Photo courtesy Logitech
Logitech Cordless Click! Plus
Another method for vertical and horizontal scrolling is a touch
scroll panel that responds to your finger sliding horizontally and
vertically, as employed by the Logitech V500 Cordless Notebook
Mouse.
Photo courtesy Logitech
Logitech V500 Cordless Notebook Mouse
For more information on mice and related topics, check out the
links on the next page.
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‫ لوحة المفاتيح‬Keyboard
When you look at all the extras and options that are available for
new computer keyboards, it can be hard to believe that their
original design came from mechanical typewriters that didn't even
use electricity. Now, you can buy ergonomic keyboards that bear
little resemblance to flat, rectangular models with ordinary square
keys. Some flashier models light up, roll up or fold up, and others
offer options for programming your own commands and shortcuts.
An average Windows keyboard.
See more keyboard pictures.
But no matter how many bells and whistles they offer, most
keyboards operate using similar technology. They use switches
and circuits to translate a person's keystrokes into a signal a
computer can understand. In this article we will explore keyboard
technology along with different key layouts, options and designs.
Keyboard Basics
A keyboard's primary function is to act as an input device. Using a
keyboard, a person can type a document, use keystroke shortcuts,
access menus, play games and perform a variety of other tasks.
Keyboards can have different keys depending on the
manufacturer, the operating system they're designed for, and
whether they are attached to a desktop computer or part of a
laptop. But for the most part, these keys, also called keycaps, are
the same size and shape from keyboard to keyboard. They're also
placed at a similar distance from one another in a similar pattern,
no matter what language or alphabet the keys represent.
Most keyboards have between 80 and 110 keys, including:
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Typing keys
 A numeric keypad
 Function keys
 Control keys
The typing keys include the letters of the alphabet, generally laid
out in the same pattern used for typewriters. According to legend,
this layout, known as QWERTY for its first six letters, helped keep
mechanical typewriters' metal arms from colliding and jamming as
people typed. Some people question this story – whether it’s true
or not, the QWERTY pattern had long been a standard by the time
computer keyboards came around.

Photo courtesy HSW Shopper
This Logitech wireless keyboard uses a QWERTY layout.
Keyboards can also use a variety of other typing key
arrangements. The most widely known is Dvorak, named for its
creator, August Dvorak. The Dvorak layout places all of the vowels
on the left side of the keyboard and the most common consonants
on the right. The most commonly used letters are all found along
the home row. The home row is the main row where you place
your fingers when you begin typing. People who prefer the Dvorak
layout say it increases their typing speed and reduces fatigue.
Other layouts include ABCDE, XPeRT, QWERTZ and AZERTY.
Each is named for the first keys in the pattern. The QWERTZ and
AZERTY arrangements are commonly used in Europe.
The numeric keypad is a more recent addition to the computer
keyboard. As the use of computers in business environments
increased, so did the need for speedy data entry. Since a large
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part of the data was numbers, a set of 17 keys, arranged in the
same configuration found on adding machines and calculators,
was added to the keyboard.
The Apple keyboard's control keys include the
"Command" key.
In 1986, IBM further extended the basic keyboard with the addition
of function and control keys. Applications and operating systems
can assign specific commands to the function keys. Control keys
provide cursor and screen control. Four arrow keys arranged in an
inverted T formation between the typing keys and numeric keypad
move the cursor on the screen in small increments.
Photo courtesy www.artlebedev.com
Optimus keyboard OLED arrow keys
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Other common control keys include:









Home
End
Insert
Delete
Page Up
Page Down
Control (Ctrl)
Alternate (Alt)
Escape (Esc)
Photo courtesy www.artlebedev.com
This Optimus keyboard has programmable hot keys.
The Windows keyboard adds some extra control keys: two
Windows or Start keys, and an Application key. Apple
keyboards, on the other hand, have Command (also known as
"Apple") keys. A keyboard developed for Linux users features
Linux-specific hot keys, including one marked with "Tux" the
penguin -- the Linux logo/mascot.
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Photo courtesy www.artlebedev.com
Optimus keyboard OLED Windows key
Inside the Keyboard
A keyboard is a lot like a miniature computer. It has its own
processor and circuitry that carries information to and from that
processor. A large part of this circuitry makes up the key matrix.
The microprocessor and controller circuitry of a keyboard
The key matrix is a grid of circuits underneath the keys. In all
keyboards (except for capacitive models, which we'll discuss in
the next section), each circuit is broken at a point below each key.
When you press a key, it presses a switch, completing the circuit
and allowing a tiny amount of current to flow through. The
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mechanical action of the switch causes some vibration, called
bounce, which the processor filters out. If you press and hold a
key, the processor recognizes it as the equivalent of pressing a
key repeatedly.
When the processor finds a circuit that is closed, it compares the
location of that circuit on the key matrix to the character map in its
read-only memory (ROM). A character map is basically a
comparison chart or lookup table. It tells the processor the position
of each key in the matrix and what each keystroke or combination
of keystrokes represents. For example, the character map lets the
processor know that pressing the a key by itself corresponds to a
small letter "a," but the Shift and a keys pressed together
correspond to a capital "A."
The key matrix
A computer can also use separate character maps, overriding the
one found in the keyboard. This can be useful if a person is typing
in a language that uses letters that don't have English equivalents
on a keyboard with English letters. People can also set their
computers to interpret their keystrokes as though they were typing
on a Dvorak keyboard even though their actual keys are arranged
in a QWERTY layout. In addition, operating systems and
applications have keyboard accessibility settings that let people
change their keyboard's behavior to adapt to disabilities.
Keyboard Switches
Keyboards use a variety of switch technologies. Capacitive
switches are considered to be non-mechanical because they do
not physically complete a circuit like most other keyboard
technologies. Instead, current constantly flows through all parts of
the key matrix. Each key is spring-loaded and has a tiny plate
attached to the bottom of it. When you press a key, it moves this
plate closer to the plate below it. As the two plates move closer
together, the amount of current flowing through the matrix
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changes. The processor detects the change and interprets it as a
key press for that location. Capacitive switch keyboards are
expensive, but they have a longer life than any other keyboard.
Also, they do not have problems with bounce since the two
surfaces never come into actual contact.
All of the other types of switches used in keyboards are
mechanical in nature. Each provides a different level of audible
and tactile response -- the sounds and sensations that typing
creates. Mechanical key switches include:
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Rubber dome
Membrane
Metal contact
Foam element
This keyboard uses rubber dome switches.
Rubber dome switches are very common. They use small, flexible
rubber domes, each with a hard carbon center. When you press a
key, a plunger on the bottom of the key pushes down against the
dome, and the carbon center presses against a hard, flat surface
beneath the key matrix. As long as the key is held, the carbon
center completes the circuit. When the key is released, the rubber
dome springs back to its original shape, forcing the key back up to
its at-rest position. Rubber dome switch keyboards are
inexpensive, have pretty good tactile response and are fairly
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resistant to spills and corrosion because of the rubber layer
covering the key matrix.
Rather than having a switch for each key, membrane keyboards
use a continuous membrane that stretches from one end to
another. A pattern printed in the membrane completes the circuit
when you press a key. Some membrane keyboards use a flat
surface printed with representations of each key rather than
keycaps. Membrane keyboards don't have good tactile response,
and without additional mechanical components they don't make
the clicking sound that some people like to hear when they're
typing. However, they're generally inexpensive to make.
Metal contact and foam element keyboards are increasingly less
common. Metal contact switches simply have a spring-loaded key
with a strip of metal on the bottom of the plunger. When the key is
pressed, the metal strip connects the two parts of the circuit. The
foam element switch is basically the same design but with a small
piece of spongy foam between the bottom of the plunger and the
metal strip, providing a better tactile response. Both technologies
have good tactile response, make satisfyingly audible "clicks," and
are inexpensive to produce. The problem is that the contacts tend
to wear out or corrode faster than on keyboards that use other
technologies. Also, there is no barrier that prevents dust or liquids
from coming in direct contact with the circuitry of the key matrix.
Different manufacturers have used these standard technologies,
and a few others, to create a wide range of non-traditional
keyboards. We'll take a look at some of these non-traditional
keyboards in the next section.
Non-Traditional Keyboards
A lot of modifications to the traditional keyboard design are an
attempt to make them safer or easier to use. For example, some
people have associated increased keyboard use with repetitive
stress injuries like carpal tunnel syndrome, although scientific
studies have produced conflicting results. Ergonomic keyboard
designs are intended to keep a person's hands in a more natural
position while typing in an attempt to prevent injuries. While these
keyboards can certainly keep people from holding their hands in a
"praying mantis" position, studies disagree on whether they
actually prevent injury.
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Photo courtesy www.safetype.com
The SafeType keyboard places the two halves of the
keyboard perpendicular to the desk surface.
The simplest ergonomic keyboards look like traditional keyboards
that have been divided down the middle, keeping a person's hands
farther apart and aligning the wrists with the forearms. More
complex designs place the two halves of the keyboard at varying
angles to one another and to the surface on which the keyboard
rests. Some go even further, placing the two halves of the
keyboard on the armrests of chairs or making them completely
perpendicular to the desk surface. Others, like the Datahand, don't
look much like keyboards at all.
Photo courtesy www.saitek.com
Saitek Truview backlit keyboard buttons
Some modifications, while not necessarily ergonomic, are
designed to make keyboards more portable, more versatile or just
cooler:
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Das Keyboard is a completely black keyboard with
weighted keys that require more pressure from a person's
strongest fingers and less pressure from the weaker
ones.
The Virtual Laser Keyboard projects a representation of a
keyboard onto a flat surface. When used successfully, a
person's fingers pass through the beam of infrared light
above the projected surface, and a sensor interprets it as
a keystroke.
The True-touch Roll-up keyboard is flexible and can be
rolled up to fit in a backpack or bag.
Blue backlit keyboard 'on'
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Blue backlit keyboard 'off'
Illuminated keyboards, like the Ion Illuminated Keyboard,
use light-emitting diodes or electroluminescent film to
send light through the keys or the spaces between keys.
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Photo courtesy www.artlebedev.com
Optimus keyboard programmable hot keys
The Optimus keyboard has organic light-emitting diodes
(OLEDs) in the keys. Users can change what letter,
command or action each key represents, and the OLED
can change to display the new information.
Photo courtesy www.artlebedev.com
This Optimus keyboard is set for keystrokes used to play Quake.
With the exception of the Virtual Laser Keyboard, which has its
own sensing system, each of these keyboards uses the same type
of technology as traditional models do to communicate with the
computer. We'll look at that technology next.
From the Keyboard to the Computer
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As you type, the processor in the keyboard analyzes the key
matrix and determines what characters to send to the computer. It
maintains these characters in its memory buffer and then sends
the data.
A PS/2 type keyboard connector.
Many keyboards connect to the computer through a cable with a
PS/2 or USB (Universal Serial Bus) connector. Laptops use
internal connectors. Regardless of which type of connector is
used, the cable must carry power to the keyboard, and it must
carry signals from the keyboard back to the computer.
Wireless keyboards, on the other hand, connect to the computer
through infrared (IR), radio frequency (RF) or Bluetooth
connections. IR and RF connections are similar to what you'd find
in a remote control. Regardless of which sort of signal they use,
wireless keyboards require a receiver, either built in or plugged in
to the USB port, to communicate with the computer. Since they
don't have a physical connection to the computer, wireless
keyboards have an AC power connection or use batteries for
power.
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Microsoft wireless keyboard
This Microsoft wireless keyboard is batterypowered.
Whether it's through a cable or wireless, the signal from the
keyboard is monitored by the computer's keyboard controller.
This is an integrated circuit (IC) that processes all of the data that
comes from the keyboard and forwards it to the operating system.
When the operating system (OS) is notified that there is data from
the keyboard, it checks to see if the keyboard data is a system
level command. A good example of this is Ctrl-Alt-Delete on a
Windows computer, which reboots the system. Then, the OS
passes the keyboard data on to the current application.
The application determines whether the keyboard data is a
command, like Alt-f, which opens the File menu in a Windows
application. If the data is not a command, the application accepts it
as content, which can be anything from typing a document to
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entering a URL to performing a calculation. If the current
application does not accept keyboard data, it simply ignores the
information. This whole process, from pressing the key to entering
content into an application, happens almost instantaneously.
To learn more about computers and keyboards, check out the links
on the next page.
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