1 February

How Computers Work

Buying BugsA 21st Century Entrepreneurship

How do you find bugs in your software? Offer a bounty! How do malicious hackers gain an

advantage? Buy it! NY Times article

Computers as a Tool

What is a tool? A device used to perform or facilitate manual or

mechanical work Something regarded as necessary to the carrying

out of one's occupation or profession Something used in the performance of an

operation An instrument

Thinking about Tools

Write down a favorite tool or device Write down what it is intended to do Write down an alternative use Switch with a neighbor Write down another alternative Switch back

Computers: The Most Adaptable of Tools

Why? We can PROGRAM the computer Instructions are called software

Embedded computers Programmed by the manufacturer

General purpose computers Programmed by the consumer

How Does a Computer Work?

It is actually a very simple machine It executes exactly what it is told to do Forrest Gump

“Run, Forrest, run”

Forrest

Computers execute algorithms

Simplified Model of a Computer

processor

instructions data

the information that it works

on

defines an

algorithm

retrieves theinstruction directs data movement

Control Unit

Arithmetic Logic Unit

MEMORY

Performs the operations

Binary System

Everyone knows that computers store bits, right?

What does it mean? All data is stored as a series of zeroes and ones

Why?

10

=

Representing Numbers

Additive system ||||| ||||| Every item represents 1 Examples of additive systems?

Positional system Value = face * place 37 = 3*10 + 7*1

||||

Positional System

Base = number of different values in a position Base 10 = 10 values: 0-9 Base 2 = 2 values: 0-1

Value of each position = power of base b4 b3 b2 b1 b0

Binary: 24 23 22 21 20

Binary Positional System

10

=0 0 0 0=

0 0 1 0=

8 4 2 1

=0 0 0 1

0 1 0 0=

=0 0 1 1

0

1

2

3

4

Examples

24 23 22 21 20

16 8 4 2 1 Examples

1111 = 15

1000 = 8

01100100 = 100

Data Types

Computer doesn’t know what the bits represents or what format is being used

Computer assumes that the instructions know the format of the data

What are the types of data? Numbers, text, pictures, sound, instructions

Data Types Numbers: integers and floating point numbers (scientific notation)

Why do we need floating point numbers? Text: Unicode, double byte

Languages and symbols (Word insert symbol) Pictures: pixels

A very fine needlepoint How to represent color?

Sound: different formats Instructions

Alphanumeric Text

8-bit ASCII sample0 0101 00001 0101 00012 0101 0010

A 0100 0001B 0100 0010C 0100 0011

01001000

01100101

01101100

01101100

01101111

H

e

l

l

o

LOGICAL _______________Hello

How are pictures stored?

Monochrome

A pixel can look BLACK,or it can look WHITE.

ASCII Images

Instead of pixels, use characters Web site that converte pictures

http://asciiconvert.com/ An example

Or it can display various shades of grey:

000 001 010 011 100 101 110 111

0 1 2 3 4 5 6 7

For example:

000 001 010 011 100 101 110 111

Mixes various amounts of R G and B light to produce other colors.

Color

Or a pixel display various colors: