Transcript computed the tax due

CS 101
Chapter 1: Background
Let’s begin
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Goal
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Teach you how to program effectively
Skills and information to be acquired
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Problem solving
Object-oriented design
Java
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What is a computer?
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Not a rhetorical question!
“A device that computes…
especially a programmable electronic machine that
performs high-speed mathematical or logical
operations or that assembles, stores, correlates, or
otherwise processes information”
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From American Heritage® Dictionary of the English
Language, 4th Edition
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So what is a computation?
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The act or process of computing
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Definition of computing:
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Duh!
To determine by the use of a computer
To determine by mathematics, especially by numerical
methods: computed the tax due
My revised definition for computing:
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The act of taking a problem with specific inputs and
determining a specific answer (output)
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Axiom
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By definition, a (properly functioning) computer will
always produce the same output given the same
input
So how do we compute random numbers?
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So what do we do with a computer…
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… now that we have one?
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We have to tell a computer what to do!
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Computers have no intelligence of their own
We tell a computer what to do by writing a
computer program, or “algorithm”
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In this course, we’ll use Java
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Algorithms
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What is an algorithm?
“A step-by-step problem-solving procedure,
especially an established, recursive computational
procedure for solving a problem in a finite number
of steps”
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From American Heritage® Dictionary of the English
Language, 4th Edition
We’ve seen lots of algorithms before…
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Example algorithm: map directions
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Example algorithm: car radio removal
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Example algorithm: Recipes
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Incorrect algorithms
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Not all algorithms are “good”
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So then what makes an algorithm “bad”?
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Can
Can
Can
Can
be wrong
be inefficient
never stop
have other problems as well…
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Incorrect algorithms: just plain wrong
From http://en.for-ua.com/blog/2005/12/09/102028.html
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Inefficient algorithms: Google Maps
directions
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Consider directions to get the IGA
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Note that this is not an incorrect algorithm!
Just a very inefficient one
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Incorrect algorithms: Shampoo directions
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Incorrect algorithms: Shampoo directions
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Lather, rinse, repeat
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This algorithm repeats forever!
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It never halts
Note that humans know to not to spend forever performing the
algorithm
But computers do not!
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Remember, they have zero intelligence
Hence the computer joke:
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How did the computer scientist die in the shower?
S/he read the directions: lather, rinse, repeat
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Incorrect algorithms: Inexact recipes
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Incorrect algorithms: summation
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Consider this algorithm:
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Given an integer n
Keep track of an ongoing sum (starts at 0)
Repeat
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Add n to the ongoing sum
Subtract 1 from n
Until n is zero
What’s wrong with this algorithm?
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Will it ever stop?
Will it always stop?
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Are you sure?
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Our goal
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Is to write correct and efficient algorithms for a
computer to follow
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Remember that computers are dumb!
We aren’t going to worry about the efficient part in
this course
But what does “correct” mean?
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“Correct” algorithms
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Consider an algorithm to display the color blue
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Is this blue?
Definitely
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Is this blue?
Also, yes: two correct results!
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What about this?
Maybe (could be green)
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And this one?
Definitely not
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So what does all this mean?
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Humans specify algorithms without a lot of precision
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Display the color “blue”
Get me from “here” to “there”
When there isn’t much precision, there are often multiple answers
Computers need more precision
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Display the color 0x0000ff (royal blue):
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There is only one possible outcome
Find the shortest route from “here” to “there”
We need to be very specific when we specify things to a
computer
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Computers are dumb!
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How do we tell all this to a computer?
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“Computer: Tea, Earl Gray, hot”
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Jean-Luc Picard from Star Trek
Unfortunately, that doesn’t work so well today…
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Computers don’t understand English
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Programming Languages
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Common programming languages:
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BASIC
COBOL
Pascal
C (1972) by Dennis Ritchie
C++ (1985) by Bjarne Stroustrup
Java (1991) by James Gosling and others at Sun
Microsystems
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Computing units of measure
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A bit is either a 1 or a 0
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On or off, true or false, etc.
A byte is 8 bits:
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01001010
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As there are 8 bits per byte, each byte can hold 28=256 values
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01001010 = 74
All computing measurements are in terms of bytes
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Computing units of measure
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Kilo (K) = 1,000 (thousand)
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Mega (M) = 1,000,000 (million)
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Giga (G) = 1,000,000,000 (billion)
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Tera (T) = 1,000,000,000,000 (trillion)
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Kilo = 210 = 1,024
= Kibi (Ki)
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Mega = (1024)2 = 1,048,576
= Mebi (Mi)
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Giga = (1024)3 = 1,073,741,824
= Gibi (Gi)
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Tera = (1024)4 = 1,099,511,627,776
= Tebi (Ti)
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Computing units of measure
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An unformatted text document (such as a Java program)
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A formatted document (such as a Word file)
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About 1 Mb each (1,000,000 bytes)
An MP3 music file
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About 5k per page with formatting
A digital camera picture
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3 pages per kilobyte (1,000 bytes)
5 Mb for a 5 minute song
A music file on a CD
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50 Mb for a 5 minute song
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10 times the size of an MP3!
A movie clip
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About 10 Mb per minute of (TV-sized) video
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A marketing trick
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This hard drive has
250,059,350,016
bytes
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= 250.06 Gigabytes
= 232.89 Gibibytes
Guess which one
they use when they
are advertising the
drive?
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Our first Java program
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Programming Task
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Display “Hello World!”
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HelloWorld.java
// Purpose: say hello!
public class HelloWorld {
public static void main(String[] args) {
System.out.println (“Hello, world!");
}
}
One
application
statement
program
makes
up
is of
required
the
action
to of
have
method
a
A An
method
is a named
piece
code
that
performs
public
main()
staticorvoid
method named
main().
some
action
implements
a behavior
Method main() is part of class HelloWorld
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Sample output
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Common Language Elements
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There are some concepts that are common to
virtually all programming languages.
Common concepts:
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Key words
Operators
Punctuation
Programmer-defined identifiers
Strict syntactic rules
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Java Documentation
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Familiarize yourself with the Java documentation
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It will save you lots of time!
A link to it is on the website
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We will go over it in a future lab as well
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Key Words
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Key words in the sample program are:
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public
class
static
void
String
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String is not really a key word but is the name of a predefined
class in Java
We’ll go over the difference between these later
Key words
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lower case (Java is a case sensitive language).
cannot be used as a programmer-defined identifier.
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Programming Languages
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Some Java key words have no meaning but are
reserved to prevent their use. (ex. goto, const,
include)
Semi-colons are used to end Java statements;
however, not all lines of a Java program end a
statement.
Part of learning Java is to learn where to properly
use the punctuation.
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Lines vs Statements
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There is a difference between lines and statements
when discussing source code.
System.out.println(
message);
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This is one Java statement written using two lines.
Do you see the difference?
A statement is a complete Java instruction that
causes the computer to perform an action.
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Good commenting
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Necessary so others can re-use your code
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And so the graders can understand it!
A well commented program:
// Authors: J. P. Cohoon and J. W. Davidson
// Purpose: display a quotation in a console window
public class DisplayForecast {
// method main(): application entry point
public static void main(String[] args) {
System.out.print("I think there is a world market for");
System.out.println(" maybe five computers.");
System.out.println("
Thomas Watson, IBM, 1943.");
}
}
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Bad commenting
//
//
//
//
Thomas J. Watson (February 17, 1874 - June 19, 1956) is
considered to be the founder of IBM. He was one of the
richest men of his time and called the world's greatest
salesman when he died.
//
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Watson was born in Campbell, New York. His formal
education consisted of only a course in the Elmira
School of Commerce. His first job was at age 18 as
a bookkeeper in Clarence Risley's Market in Painted
Post, New York. Later he sold sewing machines and
musical instruments before joining the National Cash
Register Company as a salesman in Buffalo. He eventually
worked his way up to general sales manager. Bent on
inspiring the dispirited NCR sales force, Watson
introduced the motto, "THINK," which later became
a widely known symbol of IBM.
//
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Although he is well known for his alleged 1943 statement:
"I think there is a world market for maybe five computers"
there is no evidence he ever made it. The author Kevin
Maney tried to find the origin of the quote. He has been
unable to locate any speeches or documents of Watson's
that contain this, nor is it present in any contemporary
articles about IBM. The earliest known citation is from
1986 on Usenet in the signature of a poster from Convex
Computer Corporation as "I think there is a world market
for about five computers" --Remark attributed to Thomas
J. Watson (Chairman of the Board of International
Business Machines),1943
//
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While at NCR, he was convicted for illegal anticompetitive sales practices (e.g. he used to have
people sell deliberately faulty cash registers, either
second-hand NCR or from competitors; soon after the
second-hand NCR or competitors cash register failed,
an NCR salesperson would arrive to sell them a brand
new NCR cash register). He was sentenced, along with
John H. Patterson (the owner of NCR), to one year of
imprisonment. Their conviction was unpopular with the
public, due to the efforts of Patterson and Watson to
help those affected by the 1913 Dayton, Ohio floods,
but efforts to have them pardoned by President Woodrow
Wilson were unsuccessful. However, the Court of
Appeals overturned the conviction on appeal in 1915,
on the grounds that important defense evidence should
have been admitted.
public class DisplayForecast {
// method main(): application entry point
public static void main(String[] args) {
System.out.print("I think there is a world market for");
System.out.println(" maybe five computers.");
System.out.println("
Thomas Watson, IBM, 1943.");
}
}
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More bad commenting
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From the context-switching code of Unix V6 (file: slp.c)
/*
* If the new process paused because it was
* swapped out, set the stack level to the last call
* to savu(u_ssav). This means that the return
* which is executed immediately after the call to aretu
* actually returns from the last routine which did
* the savu.
*
* You are not expected to understand this.
*/
if(rp->p_flag&SSWAP) {
rp->p_flag =& ~SSWAP;
aretu(u.u_ssav);
}
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Source: http://www.tuhs.org/Archive/PDP-11/Trees/V6/usr/sys/ken/slp.c
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Human stupidity
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Programming overview
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The Programming Process
1. Clearly define what the program is to do.
2. Visualize the program running on the computer.
3. Use design tools to create a model of the program.
4. Check the model for logical errors.
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The Programming Process
5. Enter the code and compile it.
6. Correct any errors found during compilation.
Repeat Steps 5 and 6 as many times as necessary.
7. Run the program with test data for input.
8. Correct any runtime errors found while running the
program.
Repeat Steps 5 through 8 as many times as necessary.
9. Validate the results of the program.
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Software Engineering
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Encompasses the whole process of crafting computer
software.
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Software engineers perform several tasks
development of complex software projects.
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in
the
designing,
writing,
testing,
debugging,
documenting,
modifying, and
maintaining.
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Software Engineering
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Software engineers develop:
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program specifications,
diagrams of screen output,
diagrams representing the program components and the
flow of data,
pseudocode,
examples of expected input and desired output.
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Other buzzwords from the chapter
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I’m not expecting you to fully understand these
after reading chapter 1
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Procedural programming
Object oriented programming
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Data hiding
Code reusability
Classes vs. objects
Inheritance
We will see all of these before the semester ends
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Computer Systems
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Computer Systems
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Hardware
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The central processing unit (CPU)
Main memory
Secondary storage devices
Input and Output devices
Software
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Operating systems
Application software
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Computer Systems: Hardware
Input Devices
ALU
Control Unit
RAM
Output Devices
Input / Output
Devices
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Computer Systems: Hardware
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Computer hardware components are the physical
pieces of the computer.
The major hardware components of a computer
are:
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The central processing unit (CPU)
Main memory
Secondary storage devices
Input and Output devices
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Central Processing Unit
CPU
Instruction (input)
Arithmetic
Logic
Unit
Result (output)
Control
Unit
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Central Processing Unit
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The CPU performs the fetch, decode, execute cycle
in order to process program information.
Fetch
Execute
The CPU’s control unit fetches, from main memory,
the next instruction in the sequence of program
instructions.
The instruction is encoded in the form of a
number. The control unit decodes the
instruction and generates an electronic signal.
Decode
The signal is routed to the appropriate
component of the computer (such as the ALU,
a disk drive, or some other device). The signal
causes the component to perform an
operation.
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Main Memory (aka RAM)
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Commonly known as random-access memory
(RAM)
RAM
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contains:
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currently running programs
data used by those programs
is volatile
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when the computer is turned off, the contents of RAM are
erased.
short-term memory
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Secondary Storage
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Secondary storage devices are capable of storing
information for longer periods of time
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non-volatile
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long-term memory
Examples
•
•
•
•
Hard drive
CD RW drive
DVD RAM drive
Compact Flash card
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Input Devices
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Input is any data the computer collects from the
outside world.
That data comes from devices known as input
devices.
Common input devices:
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Keyboard
Mouse
Scanner
Digital camera
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Output Devices
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Output is any data the computer sends to the
outside world.
That data is displayed on devices known as output
devices
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Common output devices:
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Monitors
Printers
Some devices such as disk drives perform input
and output and are called I/O devices
(input/output).
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Computer Systems: Software
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Software refers to the programs that run on a
computer.
Two classifications of software:
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Operating Systems
Application Software
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Operating Systems
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An operating system has two functions:
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Control the system resources.
Provide the user with a means of interaction with the
computer.
Operating systems can be either
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single tasking (run one program at a time)
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DOS
multi-tasking (run many programs at once)
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Windows
Unix
Apple
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Operating Systems
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Operating systems can also be categorized as
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single user
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only one user to operate the computer at a time
Examples:
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DOS
Windows 95/98/ME
multi-user
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allow several users to run programs and operate the computer at
once.
Examples
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Unix
BSD
Windows NT/2000/XP
OS/X
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Application Software
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Programs that make the computer useful to the
user
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Spreadsheets
Word processors
Accounting software
Tax software
Games
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