Computing Fundamentals
Lecture No 2
Layers of a Computing System
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Hardware Layer
Computers and Electricity
How do we describe the behavior of gates and circuits?
Boolean expressions
Uses Boolean algebra, a mathematical notation for expressing
two-valued logic
Logic diagrams
A graphical representation of a circuit; each gate has its
own symbol
Truth tables
A table showing all possible input values and the associated
output values
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Gates
Six types of gates
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NOT
AND
OR
XOR
NAND
NOR
Typically, logic diagrams are black and white with gates
distinguished only by their shape
We use color for clarity (and fun)
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NOT Gate
A NOT gate accepts one input signal (0 or 1) and returns
the complementary (opposite) signal as output
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AND Gate
An AND gate accepts two input signals
If both are 1, the output is 1; otherwise,
the output is 0
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OR Gate
An OR gate accepts two input signals
If both are 0, the output is 0; otherwise,
the output is 1
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XOR Gate
An XOR gate accepts two input signals
If both are the same, the output is 0; otherwise,
the output is 1
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XOR Gate
Note the difference between the XOR gate
and the OR gate; they differ only in one
input situation
When both input signals are 1, the OR gate produces a 1
and the XOR produces a 0
XOR is called the exclusive OR because its output is 1 if
(and only if):
• either one input or the other is 1,
• excluding the case that they both are
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NAND Gate
The NAND (“NOT of AND”) gate accepts two input signals
If both are 1, the output is 0; otherwise,
the output is 1
NOR Gate
The NOR (“NOT of OR”) gate accepts two inputs
If both are 0, the output is 1; otherwise,
the output is 0
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Gates with More Inputs
Some gates can be generalized to accept three or more input
values
A three-input AND gate, for example, produces an output of 1
only if all input values are 1
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Review of Gate Processing
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Gate
Behavior
NOT
Inverts its single input
AND
Produces 1 if all input values are 1
OR
Produces 0 if all input values are 0
XOR
Produces 0 if both input values are the same
NAND
Produces 0 if all input values are 1
NOR
Produces 1 if all input values are 0
Constructing Gates
Transistor
A device that acts either as a wire that conducts electricity or as a
resistor that blocks the flow of electricity, depending on the voltage
level of an input signal
A transistor has no moving parts, yet acts like
a switch
It is made of a semiconductor material, which is neither a particularly
good conductor of electricity nor a particularly good insulator
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Constructing Gates
Note: If an electrical signal is grounded, it
is “pulled low” to zero volts
A transistor has three terminals
• A collector (or source)
• A base
• An emitter
What’s the Output in Figure 4.8?
• If the Base signal is low, the
transistor acts like an open switch,
so the Output is the same as the
Source
• If the Base signal is high, the
transistor acts like a closed switch,
so the Output is pulled low
What gate did we just describe?
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Constructing Gates
The easiest gates to create are the NOT, NAND, and
NOR gates
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Circuits
Combinational circuit
The input values explicitly determine the output
Sequential circuit
The output is a function of the input values and the existing state of the
circuit
We describe the circuit operations using
Boolean expressions
Logic diagrams
Truth tables
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Are you surprised?
Combinational Circuits
Gates are combined into circuits by using the output of one gate as the
input for another
This same circuit using
a Boolean expression
is AB + AC
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Combinational Circuits
Three inputs require eight rows to describe all possible input
combinations
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Combinational Circuits
Consider the following Boolean expression A(B + C)
Does this truth table look familiar?
Compare it with previous table
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Combinational Circuits
Circuit equivalence
Two circuits that produce the same output for identical input
Boolean algebra
Allows us to apply provable mathematical principles to help design
circuits
A(B + C) = AB + BC (distributive law) so circuits must be equivalent
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Properties of Boolean Algebra
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Adders
At the digital logic level, addition is performed in binary
Addition operations are carried out
by special circuits called, appropriately, adders
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Adders
The result of adding two
binary digits could
produce a carry value
Recall that 1 + 1 = 10
in base two
Half adder
A circuit that computes the
sum of two bits
and produces the correct
carry bit
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Truth table
Adders
Circuit diagram
representing
a half adder
Boolean expressions
sum = A  B
carry = AB
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Adders
Full adder
A circuit that takes the carry-in value into account
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Stored-Program Concept
Instructions and
data both stored in
memory unit
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Memory
Memory
A collection of cells, each with a
unique physical address
Most computers are byte-addressable
Cell at address 11111110 contains
10101010
“Little endian” bit numbering:
Memory
Memory
What does 10101010 mean?
No way to answer that
Could be an instruction, a natural
number, a signed integer, a
character, part of an image, …
Arithmetic/Logic Unit
Performs basic arithmetic operations such as addition and subtraction
Performs logical operations such as AND, OR, and NOT
Most modern ALUs have a small amount of special storage units called
registers that can be accessed faster than main memory
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Input/Output Units
Input Unit
A device through which data and programs from
the outside world enter the computer system;
Can you name three?
Output unit
A device through which results stored in the
computer memory are made available outside the computer system
Can you name two?
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Control Unit
Control unit
The organizing force in the computer
Instruction register (IR)
Contains the instruction that is being executed
Program counter (PC)
Contains the address of the next instruction to be
executed
Central Processing Unit (CPU)
ALU and the control unit called the Central Processing
Unit, or CPU
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Flow of Information
Bus
In general: A communication system that transfers data between components inside
a computer or between computers; the medium (wires, optical fiber, etc.) and the
protocols (rules for sharing the medium nicely)
“The” bus: Connects the CPU, main memory, I/O devices, and possibly other
components (e.g. hard disk drive)
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What is an N-bit Processor?
It could mean any of these:
• N-bit general registers
• N-bit ALU
• N-bit addresses
• N-bit data bus
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The Fetch-Execute Cycle
Fetch the next instruction
Decode the instruction
Get data if needed
Execute the instruction
Why is it called a cycle?
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The Fetch-Execute Cycle
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RAM and ROM
Random Access Memory (RAM)
Memory in which each location can be accessed and changed
Read Only Memory (ROM)
Memory in which each location can be accessed but not changed
RAM is volatile, ROM is not
What does volatile mean?
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Secondary Storage Devices
Why is it necessary to have secondary storage devices?
Can you name some of these devices?
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Magnetic Disks
Magnetic Disks
Seek time
Time for read/write head to be over right track
Latency
Time for sector to be in position
Access time
Can you define it?
Transfer rate
Rate at which data moves from the disk to memory
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Optical Disks
CD
A compact disk that uses a laser to read information stored optically on a plastic-coated disk; data is
evenly distributed around spiral track
CD-ROM read-only memory
CD-DA digital audio
CD-WORM write once, read many
RW or RAM both read from and written to
DVD
Digital Versatile Disk, used for storing audio and video
Blu-ray
Higher capacity DVD allowing higher resolution video, etc.
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Flash Drives
Flash Memory
Nonvolatile
Can be erased and rewritten
Supports USB mass storage standard
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Touch Screens
Touch screen
A computer monitor that can respond to the user, touching the screen
with a stylus or finger
There are four types
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Resistive
Capacitive
Infrared
Surface acoustic wave (SAW)
Touch Screens
Resistive touch screen
A screen made up of two layers of electrically
conductive material
• One layer has vertical lines, the other has horizontal lines
• When the top layer is pressed, it comes in contact with
the second layer which allows electrical current to flow
• The specific vertical and horizontal lines that make
contact dictate the location on the screen that was
touched
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Touch Screens
Capacitive touch screen
A screen made up of a laminate applied over a glass screen
• Laminate conducts electricity in all directions; a very small current is applied
equally on the four corners
• When the screen is touched, current flows to the finger or stylus
• The location of the touch on the screen is determined by comparing how
strong the flow of electricity is from each corner
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Touch Screens
Infrared touch screen
A screen with crisscrossing horizontal and vertical beams of infrared
light
• Sensors on opposite sides of the screen detect the beams
• When the user breaks the beams by touching the screen, the location of the
break can be determined
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Touch Screens
Surface acoustic wave (SAW)
A screen with crisscrossing high frequency sound waves across the
horizontal and vertical axes
• When a finger touches the surface, corresponding sensors detect the
interruption and determine location of the touch
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Embedded Systems
Embedded systems
Computers that are dedicated to perform
a narrow range of functions as part of a
larger system
Empty your pockets or backpacks.
How many embedded systems do you
have?
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Operating System
Software Categories
Application software
Software written to address specific needs—to solve problems in the
real world
System software
Software that manages a computer system at a fundamental level
Can you name examples of each?
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Computer Gaming
•Computer gaming is a computer simulation of a
virtual world
•Game designers must have knowledge of the following
to make people, objects, and environments behave
realistically in a virtual world:
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Computer graphics
Artificial intelligence
Human-computer interactions and simulation
Software engineering
Computer security
Fundamentals of mathematics
Laws of physics relating to gravity, elasticity, light, and sound
Roles of an Operating System
Operating system
System software that
• manages computer resources, such as memory and input/output devices
• provides an interface through which a human can interact with the computer
• allows an application program to interact with these other system resources
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Roles of an Operating System
What operating
systems have
you used?
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Roles of an Operating System
The various roles of an operating system generally revolve around the
idea of “sharing nicely”
An operating system manages resources, and these resources are often
shared in one way or another among programs that want to use them
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Application
Gameplay
•Gameplay: The type of interactions and experiences a
player has during the game
•Game genres, based on gameplay, include:
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Action games
Shooter games
Action-adventure games
Life-simulation games
Role-playing games
Strategy games
Creating the Virtual World
•Game engine--a software system within which games
can be created
•Following functionality provided by tools of a game
engine:
• A rendering engine for graphics
• A physics engine to provide a collision detection system and
dynamics simulation
• A sound-generating component
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Creating the Virtual World
• Additional functionality resulting from tools of a game
engine:
• A scripting language apart from the code driving the
game
• Animation
• Artificial intelligence algorithms (e.g., path-finding
algorithms)
• A scene graph that holds the spatial representation in a
graphical sense
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Soft Skills
•High Quality Game Design and Development Requires
Effective Use of “Soft Skills”:
• Effective collaboration with designers, programmers, and artists
on various technical ideas throughout the entire game design and
development process
• Flexibility and adaptability as the game design constantly
evolves and changes throughout the development and production
process
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Communication
Networking
Computer network
A collection of computing devices connected in order to communicate
and share resources
Connections between computing devices can be physical using wires or
cables or wireless using radio waves or infrared signals
Can you name some of the devices in a computer network?
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Networking
Node (host)
Any device on a network
Data transfer rate (bandwidth)
The speed with which data is moved from one place to another on a
network
Why is bandwidth so key?
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Networking
Computer networks have opened up an entire frontier
in the world of computing called the client/server
model
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Networking
Protocol
A set of rules that defines how data is formatted and processed on a network
File server
A computer dedicated to storing and managing files for network users
Web server
A computer dedicated to responding to requests for web pages
P2P model
A decentralized approach that shares resources and responsibilities among many
“peer” computers
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Types of Networks
Local-area network (LAN)
A network that connects a relatively small number of machines in a
relatively close geographical area
Ring topology connects all nodes in a closed loop on which messages travel in
one direction
Star topology centers around one node to which all others are connected and
through which all messages are sent
Bus topology nodes are connected to a single communication line that
carries messages in both directions
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Types of Networks
Ethernet
The industry standard bus technology for localarea networks
Types of Networks
Wide-area network (WAN)
A network that connects local-area networks over a potentially large
geographic distance
Metropolitan-area network (MAN)
The communication infrastructures that have been developed in and
around large cities
Gateway
One particular set up to handle all communication going between that
LAN and other networks
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Types of Networks
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Types of Networks
Internet
A wide area network that spans the planet
So, who owns the Internet?
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Internet Connections
Wireless network
A network in which devices communicate with other
nodes through a wireless access point
Bluetooth
A technology used for wireless communication over
short distances
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Internet Connections
Internet backbone
A set of high-speed networks that carry Internet traffic,
provided by companies such as AT&T, Verizon, GTE,
British Telecom, and IBM
Internet service provider (ISP)
An organization providing access to the Internet
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Internet Connections
Various technologies available to connect a home
computer to the Internet
Phone modem converts computer data into an analog audio
signal for transfer over a telephone line, and then a modem
at the destination converts it back again into data
Digital subscriber line (DSL) uses regular copper phone lines
to transfer digital data to and from the phone company’s
central office
Cable modem uses the same line that your cable TV signals
come in on to transfer the data back and forth
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Internet Connections
Broadband
A connection in which transfer speeds are faster than 768 kilobits per
second
• DSL connections and cable modems are broadband connections
• The speed for downloads (getting data from the Internet to your home
computer) may not be the same as uploads (sending data from your home
computer to the Internet)
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Packet Switching
Packet
A unit of data sent across a network
Router
A network device that directs a packet between networks
toward its final destination
Packet switching
Messages are divided into fixed-sized, numbered packets;
packets are individually routed to their destination, then
reassembled
Packet Switching
Take a message, break it into three packets, and
simulate this process
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Open Systems
A logical progression...
Proprietary system
A system that uses technologies kept private by a particular commercial vendor
Interoperability
The ability of software and hardware on multiple machines and from multiple
commercial vendors to communicate
Open systems
Systems based on a common model of network architecture and a suite of
protocols used in its implementation
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Open Systems
Open Systems Interconnection
Reference Model
A seven-layer logical break down
of network interaction to facilitate
communication standards
Each layer deals with a particular
aspect of network communication
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Network Protocols
• Network protocols are layered such that each one
relies on the protocols that underlie it
• Sometimes referred to as a protocol stack
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TCP/IP
Transmission Control Protocol (TCP)
Software that breaks messages into packets, hands them off to the IP
software for delivery, and then orders and reassembles the packets at
their destination
Internet Protocol (IP)
Software that deals with the routing of packets through the maze of
interconnected networks to their final destination
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