Lecture 2 Introduction to C Programming Arne Kutzner Hanyang University / Seoul Korea.

Lecture 2

Introduction to C Programming

Arne Kutzner

Hanyang University / Seoul Korea

Introduction to C Programming L2.2

This week …

• You will learn:– From the first Computers to the C

Programming Languages• Von Neumann Architecture• The Idea of “binary computing”• Origins of Programming Languages

– About Flowcharts and Algorithms

Von Neumann’s Computer Architecture

Foundations of modern Computers and their

Programming


Computers until the second world war and before

• The earliest computing machines had fixed programs. – The earliest computers were not so much

"programmed" as they were "designed".– "Reprogramming", when it was possible at all, was

a laborious process, starting with flow charts and paper notes, followed by detailed engineering designs, and then the often arduous process of physically rewiring and rebuilding the machine.


Examples of “fixed program computers”

• Colossus Mark I (Dec. 1943) / Colossus Mark II (June 1944)

INFO in the Web: http://en.wikipedia.org/wiki/Colossus_computer

Purpose:Message Decoding during World War 2 (was used to help decipher teleprinter messages which had been encrypted using the Lorenz SZ40/42 machine (Enigma))

http://en.wikipedia.org/wiki/Colossus_computer

http://en.wikipedia.org/wiki/Colossus_computer


Von Neumann Architecture (proposed 1945)

Memory

Control UnitArithmetic Logic

Unit

Accumulator

Input Output


Von Neumann Architecture (2)

• The Control Unit “understands” instructions like e.g. copy data from the memory to the accumulator => Instruction Set Architecture

• The Arithmetic Logic Unit performs arithmetic operations and Boolean operations.

• The Memory stores data and Program (Series of control unit instructions

• The Input takes data in form of an data stream from some medium

• The Output receives data in form of data stream and delivers them to some output medium


Von Neumann Architecture (3)

• Von Neumann’s approach was revolutionary because:He created an instruction set architecture and detailed the computation as the execution of a series of instructions (the program) stored in the memory.– Program and data share the memory !!

• Highly flexible computer design


Structure of Memory

• Ordered sequence of storage locations called memory cells.

• Each memory cell is identified by a unique address

43

87

06

00

12

0000

0001

0002

0003

0004

000005

Addresses Memory Cells

Content of a memory cell


Instruction representation

• Instruction are encoded using sequences of bits

• Example PDP-11(single operand instructions):

15

6 5 3 2 0

OP-Code Mode Register

3 bit3 bit10 bit16 bit = 2 byte

INFO in the Web: http://en.wikipedia.org/wiki/PDP-11

http://en.wikipedia.org/wiki/PDP-11


Computer nowadays?• Almost all modern computer have still a

Von Neumann Architecture

Main Memory

Input Output

CPU

http://www.kmelektronik.de/root/shop4/auswahl.php?ArtNr=2431

How to program “von Neumann” computer ?


Assembler Code

• Sequence of processor instructions in human readable text-form – this is called Assembler Code

• Example for PDP-11 Assembler Code:.TITLE HELLO WORLD .MCALL .TTYOUT,.EXIT

HELLO:: MOV #MSG,R1 ;STARTING ADDRESS OF STRING1$: MOVB (R1)+,R0 ;FETCH NEXT CHARACTER BEQ DONE ;IF ZERO, EXIT LOOP .TTYOUT ;OTHERWISE PRINT IT BR 1$ ;REPEAT LOOPDONE: .EXIT

MSG: .ASCIZ /Hello, world!/ .END HELLO

This PDP-11 Assembler Code prints “Hello World”


Assembler and Programming Languages

• Assembler was the first form of some primitive (low-level) programming language

• Definition:A programming language is a set of rules that provides a way of telling a computer what operations to perform.


Disadvantages of Assembler Code

• Low level of abstraction– Code is processor (vendor)-specific

Therefore: Non portable

• The code is unstructured– Difficult to read– Risk of “spaghetti-structure”

• Time intensive to learn and to program


High-level Programming Languages

• The wish for some programming medium that is more abstract than assembler lead to the development of High-level Programming Languages

• Definition:A high-level programming language is a programming language that, in comparison to low-level programming languages, may be more abstract, easier to use, or more portable across platforms.


Early high-level programming languages

• Fortran:For programming in the scientific area (number crunching)

• Cobol:For programming of business applications (e.g. banking applications)

• Lisp:Development originally motivated be research in the Artificial Intelligence area


Fortran Code Examplec23456789012345678901234567890123456789012345678901234567890123456789012

PROGRAM spherec This program determines the surface area and volume of a sphere, given c its radius.c Variable declarations

REAL rad, area, volume, pic Definition of variablesc rad = radius, area = surface area, volume = volume of the spherec Assign a value to the variable pi.

pi = 3.141593c Input the value of the radius and echo the inputted value.

PRINT *, 'Enter the radius of the sphere. 'READ *, radPRINT *, rad, ' is the value of the radius. '

c Compute the surface area and volume of the sphere.area = 4.0 * pi * rad**2volume = (4.0/3.0) * pi * rad**3

c Print the values of the radius (given in cm), the surface area (sq cm),c and the volume (cubic cm).

PRINT *,'In a sphere of radius', rad, ' , the surface area is', + area, ' and its volume is', volume, '. '

STOPEND


COBOL Code Example $ SET SOURCEFORMAT"FREE"IDENTIFICATION DIVISION.PROGRAM-ID. Multiplier.AUTHOR. Michael Coughlan.* Example program using ACCEPT, DISPLAY and MULTIPLY to* get two single digit numbers from the user and multiply them together

DATA DIVISION.

WORKING-STORAGE SECTION.01 Num1 PIC 9 VALUE ZEROS.01 Num2 PIC 9 VALUE ZEROS.01 Result PIC 99 VALUE ZEROS.

PROCEDURE DIVISION.

DISPLAY "Enter first number (1 digit) : " WITH NO ADVANCING. ACCEPT Num1. DISPLAY "Enter second number (1 digit) : " WITH NO ADVANCING. ACCEPT Num2. MULTIPLY Num1 BY Num2 GIVING Result. DISPLAY "Result is = ", Result.

STOP RUN.


Programming Languages


Why was C created ?

• The origin of C is closely tied to the development of the Unix operating system – Originally Unix was implemented in Assembler for

the PDP-7 (1970)– 1973 Unix was “rewritten” for the PDP-11

architecture using the newly developed C Programming Language

• An older language “B” was unsuitable for this purpose due to some language design lacks


Characteristics of C

• Primitive but fast

• Quite “low-level” compared to e.g. Java– Some people say: “More or less like

assembler but independent of any specific instruction set”

• Strange and quite cryptic syntax

• Difficult to grasp

The Idea of Binary Computing

Binary Number, Hexadecimal Number and the encoding of

characters


Binary Numbers

• A sequence of 1s and 0s can be interpreted as a decimal numbere.g. 101 = 1*22 +0*21 +1*20 = 5

• These type of number (0 and 1 represented) is called Binary Number


8 bits = 1 Byte

• Todays computer architectures are based on a 8 bits (= 1 Byte) approach.– E.g. one memory cell has nowadays 1 Byte

1 0 1 0 1 1 0 0 = 172

7 6 5 4 3 2 1 0


Hexadecimal Numbers (1)

• Counting on the foundation of 16 digits (0 – F)

• Reasonable in the context of the decomposition of binary number into blocks of 4 bits

decimal binary hex

0 0000 0

1 0001 1

2 0010 2

3 0011 3

4 0100 4

5 0101 5

6 0110 6

7 0111 7

8 1000 8

9 1001 9

10 1010 A

11 1011 B

12 1100 C

13 1101 D

14 1110 E

15 1111 F


Hexadecimal Numbers (2)

• Example:

0 1 1 0 1 1 1 0 1 1 1 0 0 1 1 1

6 E E 7

28391

binary

hex

decimal

8 bit (1 Byte) 8 bit (1 Byte)

16 bit


Exercise

• What is the number 01000111 in decimal notation?

• What is the number 156 in hexadecimal notation?


Characters and Numbers

• Using a Code Table numbers can be mapped to characters of some alphabet.Example (part of the ASCII table):

Character Code (decimal)

‘A’ 65

‘B’ 66

‘C’ 67

‘D’ 68


Sequences of Characters (Strings)

• Text is represented as a sequence of characters (numbers)

• Such sequences are called Strings

• Example:

H E L L O

72 69 76 76 79The Text "Hello" as sequence of characters (numbers)

Algorithms and Flowcharts

Binary Number, Hexadecimal Number and the encoding of

characters


Algorithms - Introduction

• In order to solve some problem, we need some description how to solve it.E.g. Cooking some eggs:How to cook eggs?1. Put some water into a pot2. Heat the water until it cooks3. Put the eggs into the water4. Wait 3 Minutes 5. Take the eggs out of the pot and cool them under floating

water

• Such a description of a problem solution is called an Algorithm


Algorithms described as Flowcharts

put water into pot

Heat water

Water cooking ?

Wait 3 Minutes

No

Yes

Water in pot ?

Yes

No

Take eggs out of pot

Cool the eggs

START

END

Our “Cook-eggs”-Algorithm as Flowchart:


Flowchart Atoms

• Statement – Some operation as e.g. “put the eggs into water”

put water into pot

Statement


Flowchart Atoms (cont.)

• Condition: Some yes/no question (condition). Depending on the answer Yes (True) or No (False) we go different ways.

ConditionNo

Yes


Combined Flowchart Constructs

• If/else – alternative: Construct where select on of two alternative sequences of statements, depending on the result of the condition

ConditionYes No

Statement

Statement

Statement

Statement

if side else side


Combined Flowchart Constructs (cont.)

• If/else example: (Here we do nothing on one side, but this is ok)

put water into pot

Water in pot ?

Yes

No



• Loop: Special construct with a Condition where one alternative of the Condition creates a circular flow

Statement n

Question

Statement 1



• Loop Example:

Heat water

Water not cooking ?YES


Algorithmic Description

• Solutions for all realistic Problems can be algorithmically described by combing the introduced elements:

–Statements

–If/else alternatives

–Loops


Variables

• Variables are named containers that store some type of data, e.g. a number.Example:

read x from user input

x 0

user input is 93

x 93

Variable x Value of x

User input is typically something typed on the keyboard


Variables in Flowcharts

• Example: Calculation of the maximum of two numbers:

read x from user input

read y from user input

x > y

set max to xset max to y

print max on output

YesNo


Exercise

• Take a piece of paper and draw a flowchart that:

1. reads the value of three variables x and y and z

2. calculates the minimum of the three variables read before

Lecture 2 Introduction to C Programming Arne Kutzner Hanyang University / Seoul Korea.

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Transcript of Lecture 2 Introduction to C Programming Arne Kutzner Hanyang University / Seoul Korea.