CHAPTER 1 INTRODUCTION

 Data Processor: ◦ The basic definition of a computer is as a data processor. ◦ A black box that: 1.Accepts input data, 2.Processes the data, 3.And creates output data. Computer Input dataOutput data 2 nbajunaid.kau.edu. sa

 Drawback of previous definition: ◦ It is too general. ◦ It does not specify the type of processing. ◦ It is not clear how many types or sets of operations a machine based on this model can perform. ◦ Is it a specific-purpose machine or a general –purpose machine? 3 nbajunaid.kau.edu. sa

 The Turing Model adds an extra element to the computing machine: the program.  A program is a set of instructions that tells the computer what to do with data. Computer Input dataOutput data Program 4 nbajunaid.kau.edu. sa

 Output data depends on the input data and the program.  With same program, we can generate different outputs if we change the input data  With same input, we can generate different outputs if we change the program  We expect the same result each time if both input data and the program are the same. 5 nbajunaid.kau.edu. sa

6 1.1 Turing Model Computer 3, 12, 8, 22 Input data 3, 8, 12, 22 Output data Sort Program Computer 14, 6, 8, 12 Input data 6, 8, 12, 14 Output data Sort Program

nbajunaid.kau.edu. sa Turing Model Computer 3, 12, 8, 22 Input data 3, 8, 12, 22 Output data Sort Program Computer 3, 12, 8, 22 Input data 45 Output data Add Program

 The universal Turing machine ◦ A universal Turing machine is a machine that can do any computation if the appropriate program is provided. ◦ A very powerful computer and a universal Turing machine can compute the same thing. 8 nbajunaid.kau.edu. sa

 Computers built on the universal Turing machine store data in their memory.  Around , John von Neumann proposed that since programs and data are the same, programs should also be stored in the memory of a computer. 9 nbajunaid.kau.edu. sa

 Computers built on the von Neumann model divide the computer hardware into four subsystems: ◦ Memory, ◦ Arithmetic logic unit, ◦ Control unit, ◦ Input/output. 10 nbajunaid.kau.edu. sa

Von Neumann Model Input / output Arithmetic logic unit (ALU) Control unit Memory Computer Input data Output data

 The Four Subsystems of Von Neumann Model 1. Memory: the storage area where programs and data are stored during processing. 2. The arithmetic logic unit (ALU): where calculations and logical operations takes place. 12 nbajunaid.kau.edu. sa

 The Four Subsystems of Von Neumann Model (cont.) 3. The control unit: controls the operations of the memory, ALU, and the input/ output system. 4. Input/ output: the input subsystem accepts data and program from outside the computer, while the output subsystem sends the results of processing to the outside world. 13 nbajunaid.kau.edu. sa

 The stored program concept ◦ The von Neumann model states that the program must be stored in memory. ◦ Totally different from the architecture of early computers in which only the data was stored in memory. ◦ In early computers, the programs were implemented by manipulating a set of switches or by changing the wiring system. 14 nbajunaid.kau.edu. sa

 The stored program concept ◦ The memory in new computers hosts both program and data. ◦ → both program and data must have the same format, because they both stored in memory. ◦ They are stored as binary patterns in memory−a sequence of 0s and 1s. 15 nbajunaid.kau.edu. sa

 Sequential execution of instructions ◦ A program in the von Neumann model is made of a finite number of instructions. ◦ The control unit fitches one instruction from memory, decodes it, then executes it. ◦ Instruction are executed sequentially, i.e. one after the other. 16 nbajunaid.kau.edu. sa

 Sequential execution of instructions (cont.) ◦ One instruction may request the control unit to jump to some previous or following instruction. ◦ Sequential execution of a program where the initial requirement of a computer based on von Neumann model. 17 nbajunaid.kau.edu. sa

 We can think of a computer as being made up of three components ◦ Computer Hardware ◦ Data ◦ Computer Software 18 nbajunaid.kau.edu. sa

 Computer hardware ◦ Has four components under the von Neumann model, although we can have different types of memory, different types of input/ output subsystems, and so on. ◦ Discussed in more detail in Chapter nbajunaid.kau.edu. sa

 Data ◦ The von Neumann model defines a computer as a data processing machine. However, it does not define how data must be stored in a computer. ◦ Storing Data  If a computer is an electronic device, the best way to store data is in the form of an electrical signal.  A computer can store data in one of two states, the presence or absence of an electrical signal. 20 nbajunaid.kau.edu. sa

 Data ◦ Storing Data (cont.)  Obviously, the data we use in daily life is not just in one of two states.  For example: numbering system (10 digits from 0 to 9) How can we store numbers in computers then? It needs to be changed in another system that uses only two states (0 and 1). We call such a system, a binary system.  All other types of data (text, image, audio, video) cannot be stored in a computer directly, but need to be changed to the appropriate form (0s and 1s). 21 nbajunaid.kau.edu. sa

 Data ◦ Organizing data  Although data should be stored in only one form inside a computer, a binary pattern, data outside a computer can take many forms.  Data organization is a new field of study concerned about organizing data into different entities and formats before storing it inside a computer.  Instead of treated as a flat sequence, data is organized ito small units, small units are organized into larger units, and so on. 22 nbajunaid.kau.edu. sa

 Computer software ◦ Although early computers did not store the program in the computer’s memory, they did use the concept of programs. ◦ Programming in early computers meant changing the wiring systems or turning a set of switches on or off. ◦ It was done by an operator or engineer before the actual data processing began. 23 nbajunaid.kau.edu. sa

 Computer software ◦ In von Neumann model we need memory to hold data and the program. ◦ Another requirement of the model is that the program must consists of a sequence of instructions. 24 nbajunaid.kau.edu. sa

 Computer software ◦ Example: a program that inputs two numbers, adds them, and prints the result. 25 nbajunaid.kau.edu. sa 1.Input the first number into memory. 2.Input the second number into memory. 3.Add the two together and store the result in memory. 4.Output the result. 1.Input the first number into memory. 2.Input the second number into memory. 3.Add the two together and store the result in memory. 4.Output the result.

 Computer software ◦ Why a program must be composed of instructions? ◦ The answer is: Reusability. ◦ The Turing and von Neumann models make programming easier by defining the different instructions that can be used by computers. ◦ A programmer can then combine these instructions to make any number of programs. ◦ Each program can be a different combination of a different instructions. 26 nbajunaid.kau.edu. sa

 Computer software ◦ Algorithms: ◦ A programmer must not only learn the task performed by instructions, but also learn how to combine these instructions to do a particular task. ◦ The programmer must first solve the problem in a step-by-step manner, then find the appropriate instructions to implement those steps. ◦ The step-by-step solution is called an algorithm. 27 nbajunaid.kau.edu. sa

 Computer software ◦ Languages: ◦ At the beginning of the computer age, programmer used machine languages. Instructions where wrote in binary patterns. ◦ As programs became larger, writing long programs using these patterns became tedious. Programming languages are symbols to represent these patterns. ◦ A computer language has a more limited number of symbols and also a limited number of words. 28

 Computer software ◦ Software Engineering:  Rules and principles of designing and writing structured programs. ◦ Operating Systems:  Some instruction series are common in all programs.  For example: instructions to tell a computer where to receive data and where to send data are needed by almost all programs.  An operating system works as a manager to facilitate access to the computer components. 29

◦ The history of computers divided mainly into three periods:  Mechanical machines (before 1930)  The birth of electronic computers (1930 – 1950)  Computer generations (1950 – present) 30

Mechanical machines (before 1930)  Pascaline, a mechanical calculator for addition and subtraction operations.  Leibnitz’ Wheel, a mechanical calculator that could do multiplication and division as well as addition and subtraction.  The Jacquard loom was the first machine that used the idea of storage and programming. 31

Mechanical machines (cont…)  Analytical Engine that parallels the idea of modern computers, it had four components:  A mill (corresponding to a modern ALU).  A store (memory).  An operator (control unit).  And output (input/output). 32

The birth of electronic computers (1930 – 1950) Externally programmed computers (early electronic computers) :  The first special-purpose computer that encoded information electrically was the ABC. It was designed to solve a system of linear equations.  Mark I used both electrical and mechanical components.  Colossus was designed to break the German Enigma code.  The first general-purpose, totally electronic computer was called ENIAC. 33

The birth of electronic computers (cont…) Computers based on the von Neumann model:  The first computer based on von Neumann’s idea was called EDVAC.  At the same time, a similar computer called EDSAC was built. 34

Computer generations (1950 – present) Computers built after 1950 have become faster, smaller, and cheaper but the principle is almost the same. First generation  Computer were locked in rooms with access limited only to the operator or computer specialist.  They were bulky and used vacuum tubes as electronic switches.  Computers were affordable only by big organizations. Second generation  Computers used transistors instead of vacuum tubes.  This reduced the size and the cost of computers  Made them affordable to small and medium size corporations.  Two high-level programming languages, FORTRAN and COBOL were invented.  High-level languages separated the programming task from the computer operation task. 35

Computer generations (cont…) Third generation  The invention of the integrated circuit reduced the cost and size of computers  Minicomputers appeared.  Software packages became available. Fourth generation  The appearance microcomputers and computer networks. Fifth generation:  The appearance of laptop and palmtop computers.  Improvements in secondary storage media (CD-ROM, DVD and so on).  The use of multimedia and virtual reality. 36

 Social Issues Dependency: Some people think that computers have created a kind of dependency, which makes people’s lives more difficult. Social justice: Using computers at home is a luxury benefit that not all people can afford. The cost of a computer and a monthly charge for Internet access is an extra burden on low-income people. Digital divide: The concept of digital divide covers both the issues of dependency and social justice discussed above. The concept divides society into two groups: those who are electronically connected to the rest of society and those who are not. 37

 Ethical issues:  Privacy: Computers allow communication between two parties to be done electronically. However, much needs to be done to make this type of communication private. Society is paying a big price for private electronic communication. Network security may create this type of privacy, but it needs effort and costs a lot.  Copyright: Who owns data? The Internet has created opportunities to share ideas, but has also brought with it a further ethical issue: electronic copyright.  Computer crime : Computers and information technology have created new types of crime: Hackers and Virus creators. Society is paying a big price for this type of crime. 38