CN101

Lecture 1

Introduction to Computers and

Programming

Topics

• Introduction

• Hardware and Software

• How Computers Store Data

• How a Program Works

• Using Python

Introduction

• Computers can be programmed

• Designed to do any job that a program tells them to

• Program: set of instructions that a computer follows to

perform a task

• Commonly referred to as Software

• Programmer: person who can design, create, and test

computer programs

• Also known as software developer

Hardware and Software

• Hardware: The physical devices that make up a

computer

• Computer is a system composed of several components that

all work together

• Typical major components:

• Central processing unit

• Main memory

• Secondary storage devices

• Input and output devices

The CPU

• Central processing unit (CPU): the part of the computer

that actually runs programs

• Most important component

• Without it, cannot run software

• Used to be a huge device

• Microprocessors: CPUs located on small chips

Main Memory

• Main memory: where computer stores a program while

program is running, and data used by the program

• Known as Random Access Memory or RAM

• CPU is able to quickly access data in RAM

• Volatile memory used for temporary storage while program is

running

• Contents are erased when computer is off

Secondary Storage Devices

• Secondary storage: can hold data for long periods of

time

• Programs normally stored here and loaded to main memory

when needed

• Types of secondary memory

• Disk drive: magnetically encodes data onto a spinning circular

disk

• Solid state drive: faster than disk drive, no moving parts,

stores data in solid state memory

• Flash memory: portable, no physical disk

• Optical devices: data encoded optically

Input Devices

• Input: data the computer collects from people and

other devices

• Input device: component that collects the data

• Examples: keyboard, mouse, touchscreen, scanner, camera

• Disk drives can be considered input devices because they load

programs into the main memory

Output Devices

• Output: data produced by the computer for other

people or devices

• Can be text, image, audio, or bit stream

• Output device: formats and presents output

• Examples: video display, printer

• Disk drives and USB drives can be considered output devices

because data is sent to them to be saved

Software

• Everything the computer does is controlled by software

• General categories:

• Application software

• System software

• Application software: programs that make computer

useful for every day tasks

• Examples: word processing, email, games, and Web browsers

Software (cont’d.)

• System software: programs that control and manage

basic operations of a computer

• Operating system: controls operations of hardware

components

• Utility Program: performs specific task to enhance computer

operation or safeguard data

• Software development tools: used to create, modify, and test

software programs

How Computers Store Data

• All data in a computer is stored in sequences of 0s and

• Byte: just enough memory to store letter or small

number

• Divided into eight bits

• Bit: electrical component that can hold positive or negative

charge, like on/off switch

• The on/off pattern of bits in a byte represents data stored in

the byte

Storing Numbers

• Bit represents two values, 0 and 1

• Computers use binary numbering system

• Position of digit j is assigned the value 2

j-1

• To determine value of binary number sum position values of

the 1s

• Byte size limits are 0 and 255

• 0 = all bits off; 255 = all bits on

• To store larger number, use several bytes

Example

Storing Characters

• Data stored in computer must be stored as binary

number

• Characters are converted to numeric code, numeric

code stored in memory

• Most important coding scheme is ASCII

• ASCII is limited: defines codes for only 128 characters

• Unicode coding scheme becoming standard

• Compatible with ASCII

• Can represent characters for other languages

Example

Advanced Number Storage

• To store negative numbers and real numbers,

computers use binary numbering and encoding schemes

• Negative numbers encoded using two’s complement

• Real numbers encoded using floating-point notation

Other Types of Data

• Digital: describes any device that stores data as binary

numbers

• Digital images are composed of pixels

• To store images, each pixel is converted to a binary number

representing the pixel’s color

• Digital music is composed of sections called samples

• To store music, each sample is converted to a binary number

How a Program Works

• CPU designed to perform simple operations on pieces of

data

• Examples: reading data, adding, subtracting, multiplying, and

dividing numbers

• Understands instructions written in machine language and

included in its instruction set

• Each brand of CPU has its own instruction set

• To carry out meaningful calculation, CPU must perform

many operations

How a Program Works (cont’d.)

• Program must be copied from secondary memory to

RAM each time CPU executes it

• CPU executes program in cycle:

• Fetch: read the next instruction from memory into CPU

• Decode: CPU decodes fetched instruction to determine which

operation to perform

• Execute: perform the operation

How a Program Works (cont’d.)

From Machine Language to Assembly

Language

• Impractical for people to write in machine language

• Assembly language: uses short words (mnemonics) for

instructions instead of binary numbers

• Easier for programmers to work with

• Assembler: translates assembly language to machine

language for execution by CPU

High-Level Languages

• Low-level language: close in nature to machine language

• Example: assembly language

• High-Level language: allows simple creation of powerful

and complex programs

• No need to know how CPU works or write large number of

instructions

• More intuitive to understand

Key Words, Operators, and Syntax:

an Overview

• Key words: predefined words used to write program in

high-level language

• Each key word has specific meaning

• Operators: perform operations on data

• Example: math operators to perform arithmetic

• Syntax: set of rules to be followed when writing

program

• Statement: individual instruction used in high-level

language

Python Key words

Compilers and Interpreters

• Programs written in high-level languages must be

translated into machine language to be executed

• Compiler: translates high-level language program into

separate machine language program

• Machine language program can be executed at any time

Compilers and Interpreters (cont’d.)

• Interpreter: translates and executes instructions in high-

level language program

• Used by Python language

• Interprets one instruction at a time

• No separate machine language program

• Source code: statements written by programmer

• Syntax error: prevents code from being translated

Compilers and Interpreters (cont’d.)

Using Python

• Python must be installed and configured prior to use

• One of the items installed is the Python interpreter

• Python interpreter can be used in two modes:

• Interactive mode: enter statements on keyboard

• Script mode: save statements in Python script

Interactive Mode

• When you start Python in interactive mode, you will see

a prompt

• Indicates the interpreter is waiting for a Python statement to

be typed

• Prompt reappears after previous statement is executed

• Error message displayed If you incorrectly type a statement

• Good way to learn new parts of Python

Writing Python Programs and

Running Them in Script Mode

• Statements entered in interactive mode are not saved

as a program

• To have a program use script mode

• Save a set of Python statements in a file

• The filename should have the .py extension

• To run the file, or script, press F5 button

The IDLE Programming Environment

• IDLE (Integrated Development Program): single program

that provides tools to write, execute and test a program

• Automatically installed when Python language is installed

• Runs in interactive mode

• Has built-in text editor with features designed to help write

Python programs

Summary

• This chapter covered:

• Main hardware components of the computer

• Types of software

• How data is stored in a computer

• Basic CPU operations and machine language

• Fetch-decode-execute cycle

• Complex languages and their translation to machine code

• Installing Python and the Python interpreter modes