How Do Computers Work?

2/26/2024

2

Hardware and Software

• Hardware
– the physical, tangible parts of a computer
– keyboard, monitor, disks, wires, chips, etc.

• Software
– programs and data
– a program is a series of instructions

• A computer requires both hardware and software

• Each is essentially useless without the other

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CPU and Main Memory

Central
Processing
Unit

Main
Memory

Chip that executes
program
commands

Intel Pentium 4
Sun ultraSPARC III

Primary storage area
for programs and
data that are in
active use

Synonymous with
RAM

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4

Secondary Memory Devices

Information is moved
between main memory
and secondary memory
as needed

Central
Processing
Unit

Main
Memory

Floppy Disk

Hard Disk

Secondary memory
devices provide
long-term storage

Hard disks
Floppy disks
ZIP disks
Writable CDs
Writable
DVDs
Tapes

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Input / Output Devices

Central
Processing
Unit

Main
Memory

Floppy Disk

Hard Disk

Monitor

Keyboard

I/O devices facilitate
user interaction

Monitor screen
Keyboard
Mouse
Joystick
Bar code scanner
Touch screen

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Software Categories

• Operating System
– controls all machine activities
– provides the user interface to the computer
– manages resources such as the CPU and memory
– Windows XP, Unix, Linux, Mac OS

• Application program
– generic term for any other kind of software
– word processors, missile control systems, games

• Most operating systems and application programs
have a graphical user interface (GUI)

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Analog vs. Digital

• There are two basic ways to store and manage
data:

• Analog
– continuous, in direct proportion to the data represented
– music on a record album - a needle rides on ridges in the
grooves that are directly proportional to the voltages
sent to the speaker

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Analog vs. Digital

• Digital
– the information is broken down into pieces, and each
piece is represented separately
– music on a compact disc - the disc stores numbers
representing specific voltage levels sampled at specific
times

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Digital Information

• Computers store all information digitally:
– numbers
– text
– graphics and images
– video
– audio
– program instructions

• In some way, all information is digitized - broken
down into pieces and represented as numbers

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Representing Text Digitally

• For example, every character is stored as a
number, including spaces, digits, and punctuation

• Corresponding upper and lower case letters are
separate characters

H i , H e a t h e r .

72 105 44 32 72 101 97 116 104 101 114 46

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Binary Numbers

• Once information is digitized, it is represented
and stored in memory using the binary number
system

• A single binary digit (0 or 1) is called a bit

• Devices that store and move information are
cheaper and more reliable if they have to
represent only two states

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Binary Numbers

• A single bit can represent two possible states,
like a light bulb that is either on (1) or off (0)

• Combinations of bits are used to store values

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Bit Combinations

1 bit

0
1

2 bits

00
01
10
11

3 bits
000
001
010
011
100
101
110
111

4 bits

0000
0001
0010
0011
0100
0101
0110
0111

1000
1001
1010
1011
1100
1101
1110
1111

Each additional bit doubles the number of possible combinations

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A Computer Specification

• Consider the following specification for a personal
computer:

– 2.8 GHz Pentium 4 Processor
– 512 MB RAM
– 80 GB Hard Disk
– 48x CD-RW / DVD-ROM Combo Drive
– 17” Video Display with 1280 x 1024 resolution
– 56 Kb/s Modem

• What does it all mean?

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Storage Capacity

• Every memory device has a storage capacity,
indicating the number of bytes it can hold

• Capacities are expressed in various units:

KB

210 = 1024

MB

220 (over 1 million)

GB

230 (over 1 billion)

TB

240 (over 1 trillion)

Unit

Symbol

Number of Bytes

kilobyte

megabyte

gigabyte

terabyte

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RAM vs. ROM

• RAM - Random Access Memory (direct access)

• ROM - Read-Only Memory

• The terms RAM and main memory are basically
interchangeable

• ROM could be a set of memory chips, or a
separate device, such as a CD ROM

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RAM vs. ROM

• Both RAM and ROM are random (direct) access
devices!

• RAM probably should be called Read-Write
Memory

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Compact Discs

• A CD-ROM is portable read-only memory

• A microscopic pit on a CD represents a binary 1
and a smooth area represents a binary 0

• A low-intensity laser reflects strongly from a
smooth area and weakly from a pit

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The Central Processing Unit (CPU)

•A CPU is on a chip called a microprocessor

•It continuously follows the fetch-decode-execute cycle:

fetch

Retrieve an instruction from main memory

decode

Determine what the
instruction is

execute

Carry out the
instruction

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The Central Processing Unit

• The CPU contains:

Arithmetic / Logic Unit

Registers

Control Unit

Small
storage
areas

Performs
calculations and
makes decisions

Coordinates
processing
steps

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The Central Processing Unit

• The speed of a CPU is controlled by the system
clock

• The system clock generates an electronic pulse at
regular intervals

• The pulses coordinate the activities of the CPU

• The speed is usually measured in gigahertz (GHz)

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Monitor

• The size of a monitor (17") is measured diagonally,
like a television screen

• Most monitors these days have multimedia
capabilities: text, graphics, video, etc.

• A monitor has a certain maximum resolution ,
indicating the number of picture elements, called
pixels, that it can display (such as 1280 by 1024)

• High resolution (more pixels) produces sharper
pictures

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Modem

• Data transfer devices allow information to be
sent and received between computers

• Many computers include a modulator-demodulator
or modem, which allows information to be moved
across a telephone line

• A data transfer device has a maximum data
transfer rate

• A modem, for instance, may have a data transfer
rate of 56,000 bits per second (bps)

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24

How do you connect to the

Internet?

Ethernet vs. Modem
Which is faster? Why?
Requirements?

What is transmission speed measure in?
Modems

Ethernet

56 KBpS

1 – 100 MBpS
112 KBpS