CS101 - Introduction To Computer Concepts1 Purpose of Computer programs The purpose of writing a program is to solve a problem, perform a repetitious task

CS101 - Introduction To Computer Concepts 1

Purpose of Computer programs

• The purpose of writing a program is to solve a problem, perform a repetitious task or manage data

• The general steps in computer programming and general problem solving are:

– Understand the problem or task requirements

– Break down the problem or requirements into manageable pieces

– Design a solution addressing all aspects of the task

– Consider alternatives to the solution and refine it

– Implement the solution

– Test the solution and fix any problems that exist


Computer Program Life Cycle


• Software projects can fail because the developer didn't really understand the problem or task at hand

• The software development did not gather all the details and cases relevant to the task

• The complexity of the system caused the solution to become confusing and disconnected.

• Software development team did not use the correct technology tools

Common Program Development Problems


Computer Program Structure Chart


• Flowchart

Computer Program Flow Chart


• Computer– Performs computations and makes logical decisions– Millions / billions times faster than human beings

• Computer programs– Sets of instructions by which a computer processes data

• Hardware– Physical devices of computer system

• Software– Programs that run on computers

2003 Prentice Hall, Inc. All rights reserved. (modified by Evan Korth)

Components of a Computer System


• 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 theinstruction is

execute

Carry out theinstruction

Central Processing Unit


• The CPU contains:

Arithmetic / Logic Unit

Registers

Control Unit

Small storage areas

Performs calculations and makes decisions

Coordinates processing steps

CPU Components


• System Software

– Interface between hardware and computer users

– Does not serve the needs of solving user’s problem

• Application Software

– Accessed directly by users to serve their needs

Computer Programs - Software Types


• System Software

– Operating System

• Windows, Linux, Unix, Mac OS

• User Interface (UI)

• File and database management

• Communication protocols (e.g., TCP/IP)

– System support software

• System Utilities

• Disk utilities (format, error-checking, defragmentation)

System Software


Application Software

• Application Software– General-purpose software

• Used for more than one application• Word processors, spreadsheets, DBMS, CAD systems• Solve a variety of problems

– Application-specific software • Used for only one application• Accounting systems• Space-shuttle control systems


Programming Languages

• A programming language specifies the words and symbols that we can use to write a program

• A programming language employs a set of rules that dictate how the words and symbols can be put together to form valid program statements

• The actual statements in a language , something like sentences, are referred to as “source code”


Language Levels

• There are four programming language levels:

– machine language

– assembly language

– high-level language (many specific to a academic discipline)

– fourth-generation language

• Each type of CPU has its own specific machine language

• The other levels were created to make it easier for people to work with programs.


Programming Language Example

Source Code Statement Lanaguage

Add one to my-total (COBOL)

ADD 1,@my-total (Basic Assemble Language)

mytotal = mytotal + 1 (Visual Basic, FORTRAN)

mytotal++ (C++)

set mytotal [expr mytotal +1] (PERL)

All of the above statements accomplish the same thing


Running Programming Languages

• A program must be translated into machine language before it can be executed on a particular type of CPU

• A compiler is a software tool which translates source code into a specific target language

• Often, that target language is the machine language for a particular CPU type


The Program Translation Process

Terms to Know:

Source Code

Object Code

Executable

Source

Object

Executable

TranslatorLinker

Loaderother programs


Programming Translation Example

Source Code Statement Level

Add one to my-total Application

ST 1, R5 AssemblyST my-total, R6ADD R5,R6ST R5,my-total

10011011 0001 0101 Machine Language for “ST 1,R5”

Instruction Code for ST (Store)

The number 1 in base 2

The number 5 in base 2 indicating CPU Register 5


Syntax and Semantics

• The syntax rules of a language define how we can put together symbols, reserved words, and identifiers to make a valid program

• The semantics of a program statement define what that statement means (its purpose or role in a program)

• A program that is syntactically correct is not necessarily logically (semantically) correct

• A program will always do what we tell it to do, not what we meant to tell it to do


Language Components

• Lexicon

– All legal words in the language

– Meaning and type

• Syntax– grammar rules

• Semantics

– meaning of command


Basic Program Development

errors

errors

Edit andsave program

Compile program

Execute program andevaluate results


Errors

A program can have three types of errors

1. The compiler will find syntax errors and other basic problems (compile-time errors)

1. If compile-time errors exist, an executable version of the program is not created

2. A problem can occur during program execution, such as trying to divide by zero, which causes a program to terminate abnormally (run-time errors)

3. A program may run, but produce incorrect results, perhaps using an incorrect formula (logical errors)


Debugging Techniques

• Step through programs

• Check variable values

• Use tools to stop program running when a value is reached

• Print out information from within your program


• EditorsEditors

• AssemblersAssemblers

• DebuggersDebuggers

• CompilersCompilers

• LinkersLinkers

• LoadersLoaders

• InterpretersInterpreters

Integrated Development Environments (IDEs) Integrated Development Environments (IDEs) combine several of the above programming toolscombine several of the above programming tools

Programming Tools and Support


Example of High-Level IDE (Visual Basic .NET)

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Program Text Editors

• Word processors format the appearance of the text

• Text editors

– Format the spacing between words for legibility

– Ideal for structured languages

– Text is the same font size

• Examples

– DOS – Edit

– Windows – Notepad, Wordpad

– Unix / Linux – ed, vi, emacs

• IDEs

– MS Visual C++, Symantec Visual Cafe


Interpreters

• Translates source code instructions into machine language and executes it one statement at a time

• Disadvantages

– Longer to execute, particularly bad for loops

– Uses more memory

• Advantage

– Faster testing and code modification

• Examples of interpreted languages

– Java, BASIC, LISP


Interpreter vs. Compiler

Resources during execution Interpreter Compiler

Contents in memory

Interpreter/compiler Yes No

Source code Partial No

Executable code Yes Yes

CPU cycles

Translation operations Yes No

Library linking Yes No

Application program Yes Yes


Where does memory fit in?

Computer Processing Components

• Major components of a computer

– CPU or Central Processing Unit

– Primary (internal) memory– Secondary (external memory)– Control Unit– Arithmetic Logic Unit– Input devices– Output devices

Control Unit

Arithmetic-Logic Unit

Main Memory

Input Devices

Output Devices

Network External Memory

CPU


CPU and Main Memory

CentralProcessing

Unit

MainMemory

Chip that executes program commands

Intel Pentium 4 orSun ultraSPARC III Processor

Primary storage area for

programs and data that are in

active use

Synonymous with RAM (Random

Access Memory)


Purposes of Memory

• Hold instructions so the CPU can fetch your program and execute it

• Keep your data available to the CPU to use when executing an instruction

• Work with the CPU to perform some really fast arithmetic


Memory Areas

• Main Memory

– Internal, primary, random access memory (RAM)

– Stores instructions and data

• Cache memory

– Smaller quantity of high speed memory, speed meaning quicker for the CPU to access

• Registers

– Specific high speed memory locations used repeatedly by instructions

– Helps in arithemetic


Memory Structure

Main memory is divided into many memory locations (or cells)

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Each memory cell has a numeric address, which uniquely identifies it


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

• 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


Digital Information is Stored in Memory

• 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


Representing Text Data 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

These numbers can be found in a decoding table


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

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

• Permutations of bits are used to store values


Bit Permutations1 bit

01

2 bits

00011011

3 bits

000001010011100101110111

4 bits

00000001001000110100010101100111

10001001101010111100110111101111

Each additional bit doubles the number of possible permutations


Bit Permutations

• Each permutation can represent a particular item

• There are 2N permutations of N bits

• Therefore, N bits are needed to represent 2N unique items

21 = 2 items

22 = 4 items

23 = 8 items

24 = 16 items

25 = 32 items

1 bit ?

2 bits ?

3 bits ?

4 bits ?

5 bits ?

How manyitems can be

represented by


Storing Information

927892799280928192829283928492859286

Large values arestored in consecutivememory locations

1001101010011010Each memory cell stores a set number of bits (usually 8 bits, or one byte)


Look Back at our Example

927892799280928192829283928492859286

These two locations represent oneinstruction ST 1,R5 0001101000011010

1001101110011011

0001----0001----1011101110111011

These three locations represent oneinstruction LOAD R1,mytotal

00000101000001010000000000000000 location of mytotal

value = 5

Symbol Location

mytotal 9285

Memory Symbol Table

Memory Used for Instructions

Memory Used for Data

----------------

The ---- in 9281 and 0282 would be the string of 0,1 that equal 9285 in base 2

R1..R9 are special memory areas of the CPU much like your MEM button on a calculator


Programs and Memory Summary

• The computer will actually be looking at a set of zeroes and ones when executing instructions or working with data

• Programmers think of data in terms of numbers (to add / multiply) and characters (letters , or numbers that are not added -like zip codes)

• Programmers use variables as a way to set aside memory and keep data there during the program

• When setting up a variable, the programmer will say how much much memory is needed and whether the memory will hold numbers or letters

• Each variable you used in a program will be assigned a real memory address when the program runs or executes


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

Documents

CS101 - Introduction To Computer Concepts1 Purpose of Computer programs The purpose of writing a program is to solve a problem, perform a repetitious task