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Today’s Class

• Course overview

• Introduction to Computer SystemsWhat is both Computer and Computer

Science?Components of a Computer SystemComputer HardwareFetch-Decode-Execute CycleComputer Software

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Computer and Computer Science: Definition

A computer is an electronic device, which can input, process, and output data.

input processing

output

A computer Science is the discipline that seeks to build a scientific foundation for such topics as computer design, computer programming, information processing, and algorithmic solutions of problems.

• A computer is a machine that stores data, interact with devices, and execute programs (provides computing capabilities to its users).

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Major Components of a Computer System

A computer system consists of two main parts: hardware and software.

Hardware is the electronic and mechanical parts of a computer system.

Software is the data and the computer programs of a computer system.

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Computer Hardware

Basic hardware components

Computer hardware is divided into three major Components:

1.  The Central Processing Unit (CPU)2. Computer memory3. Input/Output (I/O) devices   

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CPU The CPU is the "brain" of the computer system.

It does the fundamental computing within the system It directly or indirectly controls all the other components

The CPU has a limited storage capacity. It relies on memory to hold data and programs and to save results.

The CPU consists of:

1.      The Arithmetic and Logic Unit (ALU).

2.      The Control Unit (CU).

3.      Registers. The CPU components are connected by a group of

electrical wires called the CPU bus.

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CPU

PC: Program Counter Register

MAR: Memory Address Register

MDR: Memory Data Register

The CPU is connected to memory and I/O devices by the System busThe System bus consists of: Address-, Control- and Data-buses.

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Computer Memory

The main function of computer memory is to store software. Computer memory is divided into primary memory and secondary

memory. Primary memory is divided into random access memory (RAM) and

read-only memory (ROM):RAM holds the programs and data that the processor is actively

working with. ROM contains software that is used in Input/Output operations. It

also contains software that loads the Operating System in Primary Memory.

The CPU can read and write to RAM but it can only read from ROM.

RAM is volatile while ROM is not. Secondary memory is used for long-term storage of programs and data.

Examples of secondary memory devices are: hard disks, floppy disks and CD ROMs.

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Primary Memory

Primary memory is divided into a number of memory cells (bits) or bytes.

A bit (binary digit) is the smallest storage unit within a computer. It is a tiny electrical circuit that can be in one of two states:

A voltage high represented by the symbol 1

A voltage low represented by the symbol 0

Any system of symbols can be represented by bit or byte patterns.

Each byte has a unique integer address and it is usually 8 bits.

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Primary Memory

UNIT SYMBOL POWER OF 2

Number of bytes

Byte 02

1

Kilobyte KB 10

2

1,024

Megabyte MB 20

2

1,048,576

Gigabyte GB 30

2

1,073,741,824

Terabyte TB 40

2

1,099,511,627,776

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Primary and Secondary Memory Comparison

Primary memory Secondary memory

Fast

Expensive

Low capacity

Connects directly to the processor

Slow

Cheap

Large capacity

Not connected directly to the processor

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I/O (Input/Output)DevicesInput devices are used to enter programs

and data into a computer.Examples: keyboard, mouse, microphone,

scanner, and bar code reader.

Output devices are where program output is shown or is sent. Examples: monitor, printer, and speaker.

An I/O device is directly connected to the System, but through a device controller.

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Fetch Decode Execute Cycle The CPU continuously transfers data to and from the

primary memory Data transfer is done in units called instructions or words When a computer is switched on, the CPU continuously

goes through a process called fetch-decode-execute

cycle: The Control Unit fetches the current instruction from memory,

decodes it and instructs the ALU (Arithmetic Logic Unit) to execute the instruction.

The execution of an instruction may generate further data fetches from memory

The result of executing an instruction is stored in either a register or RAM

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Fetch-Decode-Execute Cycle (cont’d)

Main MemoryMain MemoryControl Unit

Arithmetic/Logic Unit

1

2

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Instruction Cycle

ExecutionCycle

Fetch Decode

ExecuteStore

cpu

RAM

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Computing Environments

Personal computing

Time-sharingDistributed computing

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Software Software is the programs and data that a computer uses.

Programs are lists of instructions for the processor Data can be any information that a program needs:

character data, numerical data, image data, audio data, etc.

Both programs and data are saved in computer memory in the same way.

Computer software is divided into two main categories: 1. Systems software 2. Applications software

System software manages computer resources and makes computers easy to use.

An applications software enables a computer to be used to do a particular task.

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Computer Software

Types of software

Software

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Software

Application Programs Systems Programs

Word processors Game programs

Spreadsheets

Data base systems

Graphics programs

Web browsers

Operating

system.

Networking system.

Programming language software.

Web site server.

Data backup.

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Operating Systems

The most important systems program is the operating system. It is a group of programs that coordinates the

operation of all the hardware and software components of the computer system.

It is responsible for starting application programs running and finding the resources that they need.

Examples of operating systems are: Unix,

Windows NT, Windows XP, MS-DOS, Linux, Solaris, VMS, OS/2 and System 7.

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Computer Languages

Computer language evolution

The only language understood by a computer is machine language.

Note:Note:

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Writing, Editing, Compiling, and Linking Programs

PrimaryMemory

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Disk

Disk

Disk

Editor

Compiler

Class Loader

Program is created in an editor and stored on disk in a file ending

with .java.

Compiler creates bytecodes and stores them on disk in a file ending

with .class.

Class loader reads .class files containing bytecodes from disk and puts those

bytecodes in memory.

Phase 1

Phase 2

Phase 3

PrimaryMemory

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.

.

.

.

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Bytecode Verifier Bytecode verifier confirms that all bytecodes are valid and do

not violate Java’s security restrictions.

Phase 4

PrimaryMemory

.

.

.

.

.

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InterpreterInterpreter reads bytecodes and translates them into a language

that the computer can understand, possibly storing data values as the program

executes.

Phase 5

typical Java environment

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Program Execution

Executing programs

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System Development Model

Figure 1-12 System development model

An old programming proverb:

Resist the temptation to code.

Note:Note:

Pseudocode

A precise algorithmic description of program logic.

Note:Note: