computer science stuff, chapter 0

8/8/2019 Computer Science Stuff, Chapter 0

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COSC 1410

Chapter 0: Introduction


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Contents

Computer Organization

Programming Languages Binary Numbers

Compilers


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COMPUTER ORGANIZATION

A computer has the following functionalcomponents:

CPU (Central Processing Unit)

Control Unit

ALU (Arithmetic and Logic Unit)

Main Memory

Secondary Memory

Input Devices

Output Devices


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Major Components of a Computer

MainMemory

SecondaryMemory

OutputDevice

InputDevice

ControlUnit

ALU

CPU


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The Control Unit

The control unit of the CPU controls alloperations of the computer.

It works in a cycle. Each cycle fetches the next instruction of the program currently

being executed,

interprets (decodes) the instruction to determine whatshould be done,

executes the instruction.

This cycle is sometimes summarized asfetch/decode/execute.


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CPU

If the instruction being executed in this cycle requires 2numbers to be added together then the control unitcauses copies of the 2 numbers to be sent from thememory cells to the ALU.

The control unit then sends a signal to the ALU telling itto perform the addition.

If the next instruction executed says the resultingnumber should be stored in main memory then thecontrol unit sends signals that cause a copy of the

number to be sent to and stored in some specifiedmemory cell.

If the instruction being executed says to perform input oroutput then the control unit sends signals to the input or

output devices causing this to be done.


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Example: Fetch/Decode/Execute

MainMemory

SecondaryMemory

OutputDevice

InputDevice

ControlUnit

ALU

CPU

LOAD A

LOAD B

ADD

STORE

15 3


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MainMemory

SecondaryMemory

OutputDevice

InputDevice

ControlUnit

ALU

CPU

LOAD A

LOAD B

ADD

STORE 15

3


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MainMemory

SecondaryMemory

OutputDevice

InputDevice

ControlUnit

ALU

CPU

LOAD A

LOAD B

ADD

STORE 15 + 3 = 18


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MainMemory

SecondaryMemory

OutputDevice

InputDevice

ControlUnit

ALU

CPU

LOAD A

LOAD B

ADD

STORE 18


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The Control Unit

The control unit uses

a program counter to store the address of the next instruction

to be fetched and an instruction register to store the current instruction that is

being decoded and executed.

The faster this basic cycle can be performed the faster

the computer can execute a program.

The speed of a computer is usually stated as somenumber of

megahertz (million cycles per second) or

gigahertz (billion cycles per second).


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ALU

The arithmetic and logic unit, ALU, isresponsible for performing

Arithmetic calculations involving addition, subtraction,multiplication and division and

Logical operations such as the test i < n.

The ALU uses arithmetic registers to store thenumbers involved in a calculation or logical

operation. The number of such registers variesfor different computers.


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The Main Memory

The main memory is where the data and instructions ofthe program being executed are stored.

Main memory consists of a large number of memorycells each of which can store a sequence of binary digits(bits) that represent an instruction or data value.

The amount of memory necessary to store a single

character is called a byte and memory size is usuallystated in terms of the number of bytes of memory(megabytes or gigabytes).

Each byte consists of 8 bits (binary digits).


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Memory Address

The bytes of memory are numbered from 0 to n-1 wheren is the number of bytes in the main memory of thecomputer.

This number, referred to as the address of the byte,serves to identify the memory location and is used tospecify which bytes of memory are being referred to in

machine language instructions. Memory addresses are also used in some C++

instructions. The information stored in memory (data or

instructions) can be sensed by the computer withoutbeing changed.

However if the computer stores new information inmemory, this destroys the old information that was in

that position in memory.


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Main Memory

010001000000010001010100

0100011101100100 0110010101011100

0000000000000000

01000100

0 202 203 208

300 301 302 303

n-2 n-1

bit Byte

WordAddress


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Main Memory

010001000000010001010100

0100011101100100 0110010101011100

0000000000000000

01000100

0 202 203 208

300 301 302 303

n-2 n-1

Integer 1092 Character D

An instruction


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Addresses

High-level programming languages like C++ use variablenames to refer to values stored in memory. This makesreferring to stored values much easier than having toremember an address.

For example if a programmer in C++ declares a variable,say i, to be of type integer then the compiler associates

this name with some bytes of memory, say bytes 202-203 in the above picture.

Then instead of having to refer to its address, we canuse the variable name in the C++ program.

There are however some C++ instructions that useaddresses (pointers) and you need to be familiar with thenotion of a memory address.


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Memory Addresses

0000010000000000

01010100

01000111

01100100

01100101

01011100

00000000

01000100

01000100

0

201

202

203

204205

206

207

208

n-1

00000000

00000000

i = i + 1;

print ch;

1092

D

variable type Startingaddress

i int 202

ch char 208


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Secondary Memory

Secondary memory provides permanent and large-scale storage of information. The most commonsecondary storage devices are magnetic disks thatrecord information in a magnetic form. Floppy disks provide a cheap way of storing data and programs

in a form that is easily transportable.

Hard disks provide for much faster writing and or reading ofinformation on the disk, but are not easily transportable.

CD-ROMs are another secondary storage device. They providestorage for a large amount of information represented by asequence of tiny pits recorded by laser. The ROM part of thename stands for Read Only Memory.

Flash memory is still another increasingly popular form ofsecondary memory.


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Input Devices

The input devices allow information to be inputted intothe computer.

The most common input devices are the keyboard andmouse.

The keyboard allows typed information (instructions or data) tobe inputted into the computer.

The mouse allows input into the computer by moving the cursordisplayed on the monitor to a particular icon (picture) or wordand clicking one of the mouse buttons. This inputs to the

computer a choice of some operation to perform.


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Output Devices

The output devices allow information to beoutputted from the computer to the user. Themost common output devices are the monitorand printer. The monitor of a computer has a screen where

information is displayed. When the user types

information at the keyboard this information isdisplayed (echoed) on the monitor as well as beinginputted into the computer. When the computer doesoutput from the program that is currently being

executed, this is also displayed on the monitor. A printer is used to print computer output on paper sothat the user can have a printed copy of theinformation.


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Representation of Information

All information stored in the memory of a computer is insome binary form, i.e. a sequence of binary digits (bits).

We will discuss some of the details for one type ofinformation, namely integers. We are used to writingnumbers in decimal.

Decimal numbers are written as a sequence of decimal

digits, 0-9. The position of a digit determines what itstands for. The right most digit is thought of as multipliedby 1, the next digit from the right is thought of asmultiplied by 10, the next by 100, etc. In other wordseach digit from right to left is multiplied by the next powerof 10.


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Example: Decimal Number

0 0 0 0 3 8 5 1

107

106

105

104

103

102

101

100

10000000

1000000

100000

10000

1000

100

10

1

For examplethe decimalinteger 3851stands for thenumber 1*1 +

5*10 + 8*100+ 3*1000.


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Example: Decimal Number

0 0 0 0 3 8 5 1

107

106

105

104

103

102

101

100

00000000

0000000

000000

00000

3000

800

50

1

For examplethe decimalinteger 3851stands for thenumber 1*1 +

5*10 + 8*100+ 3*1000.

3851


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Binary Numbers

The binary number system represents an integer by asequence of bits, 0, 1.

The right most bit is thought of as multiplied by 1, the

next bit from the right is thought of as multiplied by 2, thenext by 4, etc.

In other words each bit from right to left is multiplied bythe next power of 2.

For example the binary integer 100101 stands for thenumber 1*1 + 0*2 + 1*4 + 0*8 + 0*16 + 1*32 which is 37in decimal.

In a computer that stores integers in 2 bytes (= 16 bits)this number would be stored as 00000000 00100101.


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Example: Binary Number

0 0 1 0 0 1 0 1

27

26

25

24

23

22

21

20

128

64

32

16

84

2

1

37


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Binary Arithmetic

Addition:

0+0 = 0, with no carry,

1+0 = 1, with no carry, 0+1 = 1, with no carry,

1+1 = 0, with a carry of 1.

Complement 0 -> 1

1 -> 0


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Binary Arithmetic

+ 0 1

0 0 1

1 1 10


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Binary Arithmetic

+ 0 1

0 0 1

1 1 10

0 1 1 0

+ 0 1 1 1

011

Carry over

11 1 0


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Negative Numbers

There are several schemes to store negative integers.The most commonly used one on computers is twoscomplement notation.

This notation has several advantages that are difficult toexplain in a brief discussion. The twos complementversion of a negative number is formed by finding thebinary number representation of the positive number.

Then each bit is changed to its complement, i.e. each 1is changed to 0 and each 0 to 1. This gives the so calledones complement of the number.

To get the twos complement version one adds 1 to this

number.


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Example: Negative Integers

37

Complementeach bit

Add 1

00000000 00100101

11111111 11011010

11111111 11011011

2scomplement

1scomplement


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2s Complement

This is the twoscomplement version of -37

and is the way mostcomputers would store -37.

Note that if we do anaddition of +37 and -37we do get 0.

Note that 1 + 1 results in 0plus a carry of 1.

00000000 00100101

11111111 11011011

00000000 00000000


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2s Complement

Problem with 1s Complement Zero is coded (000000)

+3 (000011) and -3 (111100) should add up to zero,but it add up to 111111.

So there are two zeros (000000 and 111111) in this

system!


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2s Complement

000 001 010 011

111 110 101 100

0 1 2 3

100 101 110 111

101 110 111 000

-3 -2 -1 -0

Problem with 1s Complement Zero is coded (000000)

+3 (000011) and -3 (111100) should add up to zero,but it add up to 111111, not 000000.

So there are two zeros!

D i l Bi


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Decimal to Binary

How do you convert a decimal integer into binary? One method is torepeatedly divide the decimal number by 2. The remainder of thedivision is the next bit from right to left. The quotient is then used inthe next division. For example

Thus the binary number representation of the decimal number 37 is100101. If this is stored in 2 bytes=16 bits then the leading bits are

all 0 giving the result 00000000 00100101.

Number

(Divide by 2)

Quotient Remainder

37/2 18 1

18/2 9 0

9/2 4 1

4/2 2 0

2/2 1 0

1/0 0 1

Stop

Si l Fl h


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Simple Flowchart

p = 0 ?

q = p/2

r = p mod 2r is the next bit

p=q

Input p

Outputthe bits

no

yes

Bi I f ti


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Binary Information

To convert a negative decimal integer (such as -37) tothe twos complement version in binary one just convertsthe positive decimal integer (+37) to the binary form

shown and then complements each bit and adds 1 asillustrated before.

All other information is also stored in some binary form.

For example a character like A is stored using charactercodes. The ASCII character code for A is 65. This isstored as a one byte binary integer. 01000001 (= 1*1 + 0*2 + 0*4 + 0*8 + 0*16 + 0*32 + 1*64 = 65)

Floating point numbers are also stored in a binary form.Even machine language instructions are stored usingbinary codes.

P i L


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Programming Languages

There are 3 classes of programming languages,

machine languages,

assembly languages and

high-level languages.

M hi L


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Machine Languages

Each computer has its own machine language which isvery detailed and specific for the exact details of thecomputer architecture.

All instructions in a machine language are in binarycodes.

Since machine languages are machine dependent,very detailed and use binary codes it is very difficult towrite programs in them.

A mbl L g g


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Assembly Languages

Assembly languages are symbolic versions of machinelanguages.

The instructions use symbolic codes rather than binary

codes and thus are easier to remember. But assemblylanguages are still machine dependent and very detailedand thus difficult to use. As an example an assemblylanguage might have an instruction like

Load R4,Xwhich means load the value of variable X into the register

R4 in the ALU. In machine language this sameinstruction might be 00010111 0100 00100110 wherethe first 8 bits is a binary code standing for load and thenext 4 bits give the register number in binary and the lastbit sequence gives the address of the variable X inbinary.

High Level Languages


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High Level Languages

High-level languages are machine independent andmuch less detailed than machine/assembly languages.

This makes it easier to write programs and allows thesame program to be run on different computers. Mostlanguages have standards defined.

There are many different high-level languages including

Fortran, Cobol, Basic, C, C++, C# and Java. We will learn C++/C in this course.

C


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C

C is a programming language that was developed in the1972 at Bell Labs by Dennis Ritchie and was initiallyused to implement the UNIX operating system.

It has since gained wide use in scientific and systemsprogramming.

It is a high-level language with some low-level featuressuch as bit manipulation that most high level languagesdo not have.

A sample C program


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A sample C program

/* Hello World program */

#include

main()

{

printf("Hello World");}

Exam1-0.cpp, Exam1-1.cpp

From C to C++
http://../CPP/ch01/exam1-0.cpphttp://../CPP/ch01/exam1-1.cpphttp://../CPP/ch01/exam1-1.cpphttp://../CPP/ch01/exam1-0.cpp


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From C to C++

C was later extended to C++ by adding many newfeatures related to object-oriented programming andimproving some other aspects of the language.

Backward compatibility issue: (almost) all syntax of theoriginal C are included in the C++. A program written instandard C should compile with a C++ compiler (thereare exceptions in which you have to make some minormodification).

C with Some C++


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C++

C with Some C++

In this course you will learn the basic parts of C++ thatare common to C, procedural programming and someaspects of C++ that are new, including an introduction toclasses and object-oriented programming.

C

A sample C++ program


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A sample C++ program

/* Hello World program */

#include

using namespace std;

int main()

{

cout


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C++ Compilers

When the computer is actually executing a program it isalways in the machine language of that computer.

However it is difficult and tedious to program in machinelanguage. Most programs are written in some high-level,machine independent programming language such asC++.

Before the computer can execute a program written insuch a language it must be translated into the machinelanguage of the computer.

The Role of a Compiler


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High LevelLanguage

MachineLanguage

Computer

Programmer

Compiler



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A compiler is a program that does the translation from ahigh-level language into the computer's machinelanguage. An instruction in C++ such as

i = i + 1;that is C++'s way of saying to assign to variable i the

current value of i plus 1 might be translated into severalmachine language instructions something like the

following. Send a copy of the integer in bytes 202-203 to the ALU

(assuming is value is stored in bytes 202-203) Add 1 to this value in the ALU

Store the new value back in memory in bytes 202-203

Steps of Compilation


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A C++ program must be compiled before thecomputer can execute it.

The C++ compiler first invokes a program called thepreprocessor that does such things as copy the file (which contains information about

the input and output functions used in C++) into theC++ source file before compilation starts.

The compiler next translates the instructions in a C++

program into the machine language of the computeras suggested above.

Finally it links in the code for functions such as theinput and output functions.



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The resulting file contains machine language instructionsthat can be understood and executed by the computer.

Files Involved


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Files Involved

A file containing a C++ program is called a C++ sourcefile (.cpp).

After being compiled the resulting file is called anexecutable file (.exe).

If a file is compiled, but not linked then the resulting file iscalled an object file (.obj).



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Source File

Object File

Executable File

Library File

Translator Linker

Variations


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Variations

The exact details of how to compile and execute a C++program vary slightly from system to system and youmust learn these from documentation about your systemor from someone who already knows the details. ForDev-C++ these are given in Appendix 1.

Different programming languages may have different

compilation process. The next slide show how a Javaprogram compiles and executes in an HP OpenVMSenvironment. Its just an example to show how

complicated it can get. You dont have to understand thechart.

HP OpenVMS (you dont have to know this)


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p (y )

Source: HP web Site

Abstraction


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In discussing C++ programs we will always describe howC++ instructions are executed by the computer.

We act as if the computer directly executes the C++

instructions. In fact a single C++ instruction may havebeen translated into several machine languageinstructions that are actually the instructions which areexecuted by the computer.

However since we don't have to do the translation, weshould think entirely at the higher level and understandC++ programs in terms of C++ instructions.

About Your C++ Compiler


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p

If you program in C++ you must have a C++ compiler totranslate your program into machine language before itcan be executed.

You can obtain a free version of Dev-C++, the C/C++compiler used in the lab, by down loading it from thefollowing web site.

http://prdownloads.sourceforge.net/dev-cpp/devcpp4960.exe

See Appendix 1 for more details on how to down loadand use Dev-C++.

Which C?


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So, which version of the C are we learning?

Mostly C,

Some C++, especially those features that areimprovements of C,

Minimum C++, just to get by,

It is still important to know the old C because thereare a lot of the C codes out there.

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