Computer Hardware

Today many jobs mainly depend on the either the creation or collection, and distribution of information. This topic addresses the different advancements in computer technology.

Computer Generations

As each new computer generation improves on its previous generation, there have been a corresponding reduction in size (or miniaturization) of computer circuitry. In contrast to this miniaturization, the speed, power, and memory of computers have proportionally increased.

Although this Topic will describe the major developments of each computer generation, it is more important to consider how the technology has influenced the development of current computer systems.

The computer generations are summarized in Table below :

Generation	Denoted by	Approximate range of years
1	Vacuum tube	1940s – 1950s
2	Transistor	Mid 1950s – early 1960s
3	Integrated circuit	Early 1960s – Early 1970s
4	Very Large–Scale Integration/Microprocessor	Early 1970s – Mid 1980s
5	Parallel processing	Mid 1980s – Early 1990s
6	Reduced Instruction Set Chip (RISC) and more inventions to date	Mid 1990s to Present

The First–Generation Computer (1940s – 1950s) Vacuum Tubes

These early computers comprised huge vacuum tubes (similar to light bulbs) whose main purpose was to strengthen weak signals for transmission. The vacuum tubes acted as switches to start and stop the instant flow of electricity. The concept underlying the on/off switch was the idea of ones and zeros.

The combination of ones and zeros or on and off created a type of machine language that could be used to process information. The first computer, the Mark 1, was so huge that it filled a typical stadium field and was said to have used more than 500 miles of wire. Its success however was its processing power, such as a mathematical problem that normally took 40 hours to solve, only took the Mark 1, 20 seconds to produce a result. It was used by the U.S.

Navy during World War II, to help solve strategic military mathematical problems such as cracking secret codes and creating complex fire tables for naval guns and artillery. Scientists used the computer to predict weather patterns and for analyzing problems in airplane design.

As noted earlier the language of this first generation was called machine language, which comprised zeros and ones for programming the computer. The next computer, the ENIAC (Electronic Numerical Integrator and Computer) was an improvement on the Mark 1, since its processing power was about 1,000 times faster.

The ENIAC was about 80 feet (24.4 m) long by 3 feet (0.9 m) wide, and used about 18,000 vacuum tubes. Unfortunately, the tubes were very hot, had to be replaced frequently, and were costly in using much electricity to keep them cool.

The UNIVAC (Universal Automatic Computer) soon replaced the ENIAC by containing a compact 5000 tubes, a reduction from the 18,000 used in the ENIAC.

The Second–Generation Computer (Mid 1950s – early 1960s) Transistors

During this generation the significantly smaller transistors (a new technology) eventually replaced vacuum tubes as the processing component of the computer. This allowed engineers to dramatically reduce the size and space needed to house a computer.

Transistors proved to be cheaper to produce and emitted no heat, thus reducing the costs of manufacturing and making the technology more reliable since they conducted electricity faster vacuum tubes and failed less often.

This generation was noted for making space travel possible, and for the development of more sophisticated programming languages that allowed programmers to use short specific terms (e.g., ADD) to represent certain computer processing instructions. Higher level programming languages such as FORTRAN, ALGOL and COBOL, which use short English–like words, were developed during this generation. During this generation computer hardware and software was created that allowed users to store and retrieve programming instructions.

The Third Generation Computer (Early 1960s – Early 1970s) Integrated Circuits

This generation improved on the previous one by miniaturizing the transistor and building several onto a single chip of silicon.

This compacting of millions of transistors in a small space, also called an integrated circuit, greatly increased the power of computer, the processing speed, lowered its production costs even more and reduced the size of the computer. During this generation the creation of new input and output devices were created.

Keyboards, monitors, and the mouse made the computer a more user–friendly environment and eliminated the need for huge stacks of punch cards to input data into a main frame computer. Output of the computer calculations could be read on screen vice printing out large amounts of paper with graphic symbols on them.

The Fourth Generation Computer (Early 1970s – Mid 1980s) The Microprocessor

Again, an improvement on the previous generation, this generation continued to reduce the size of the silicon chip comparable to the size of a pencil eraser while vastly increasing its computational speeds.

This single chip with the components and processing speeds of a computer was called a micro–processor and was initially intended for use in calculators, not computers. During this generation the silicon chip technology was harnessed by the chip manufacturer Intel.

Intel in partnership with IBM created the first microcomputer, or personal computer. A few years later Apple in partnership with Motorola introduced the Macintosh to compete against the IBM personal computer.

Only recently has Apple recognized the need for standardization and has embraced the Pentium technologies currently supported by Intel. During this periods new and more powerful programming languages developed.

This included Pascal, C and compilers which converted the program to a lower–level language that the machine could process. The UNIX operating system was also developed during this time to manage system resources.

The Fifth Computer Generation Computer (Mid 1980s – Early 1990s)

Artificial intelligence and Parallel Processing

The goal of this generation is to develop devices that are capable of responding sensibly to a user’s natural language and also capable of learning and self–organization.

It is characterized by the introduction of computers with hundreds of processors that could be assigned to work on different sections of a single program.

This is called parallel processing, and is being used to develop communication between a computer in natural spoken language and its user, through access to a vast knowledge database to make intelligent inferences and draw logical conclusions in the way that humans do.

Several programming languages known as artificial intelligence (AI) languages have been developed simply because they are used almost exclusively for AI applications. The two most commonly used languages are LISP and Prolog.

The linking of computers together, called computer networks, and the increasing use of workstations (explained in table 2.1) were also developed during this generation.

The Sixth Generation (Mid 1990s to Present) Computers and Video Games

This era refers to the generation of computer and video games which became popular around 1998. The Nintendo Game Cube, Sony PlayStation 2, and Microsoft’s Xbox are noted to have dominated game playing in all age groups.

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Types of Computer Systems

In the past types of computers were classified as microcomputers, minicomputers, or main–frame computers, based on their size. Today these distinctions are rapidly disappearing as the capabilities of even the smallest units rival that of their earlier big brothers.

Today’s microcomputers are faster, more powerful and more versatile than the minicomputers and the main–frame computers were a few years ago. Table 2.X describes the terms used to classify todays of computers and computing devices.

Type of Computer System	Brief Description
Desktop Computer	A PC that is expected to be placed on the top of a computer or other desk. Desktops usually are powerful and have reasonably large secondary storage. Desktops require a number of input and out devices to make them useful
Laptop	Compared to the desktop, laptops are portable computers that have their monitor, keyboard, and other memory and storage devices all included in a package that is up 17 inches in size and is powered by a special battery. Laptops are also called notebooks
Mainframe	Mainly associated with large companies, that process millions of transactions calculations from multiple sources at one time, mainframes previously filled large rooms, but today consist of a number of linked components that allow the different processors to communicate with one another and share their total computing power
Minicomputer	Minicomputers are powerful and can support many users at once. Their size is between microcomputers (PCs) and mainframes.
Palmtop	More commonly called Personal Digital Assistants (PDAs). They are compact computers that often use special secondary flash memory instead of a typical hard drive for storage. Palmtops usually have optional keyboards but mainly use touch screens for input. They are typically, the size of a paperback novel or even smaller and are very lightweight and use battery power.
Personal Computer	Personal Computers (PC) was designed to be used by individuals for general use. The PC is not usually associated with the Apple Mac although it is a PC. Often
Server	A computer that has been optimized to provide services to other computers over a network. Servers usually have powerful processors, lots of memory and large hard drives. They often provide custom services such as an e–mail server, a backup server, a database server, etc.
Super Computer	Supercomputers have the world’s fastest processors and are able to perform millions of computations a minute. Supercomputers are normally comprised of multiple high–performance computers working in parallel as a single system. The best known supercomputers are built by Cray Super Computers
Wearable Computers	This is the latest trend in computing where common computer applications such as electronic mail, are integrated into watches, cell phones, visors and even clothing
Workstation	A workstation is a desktop computer with a more powerful processor, and more memory for performing a special function, such as 3D Graphics

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Computer Hardware and Peripherals

We have briefly looked at the different types of computer systems in Topic 1. We will discuss the different hardware components that make up different computer systems.

The hardware and peripherals can be categorized as either Input, output, process or storage devices.

Input Devices

You use input devices to provide information to a computer, such as typing a letter or giving instructions to a computer to perform a task. Some examples of input devices are described in the following list.

Mouse

A device that you use to interact with items displayed on the computer screen. A standard mouse has a left and a right button.

Trackball

This is an alternative to the traditional mouse and is favored by graphic designers. It gives a much finer control over the movement of items on the screen.

Other screen pointing devices are pointing stick, touch pad, joystick, light pen, digitizing table.

Keyboard

A set of keys that resembles a typewriter keyboard. You use the keyboard to type text, such as letters or numbers into the computer.

Scanner

A device that is similar to a photocopy machine. You can use this device to transfer an exact copy of a photograph or document into a computer. A scanner reads the page and translates it into a digital format, which a computer can read. For example, you can scan photographs of your family using a scanner.

Barcode Readers

When used in a business barcodes provide a lot of information. Made up of columns of thick and thin lines, at the bottom of which a string of numbers is printed.

Multimedia devices

This is the combination of sound and images with text and graphics. To capture sound and image data, special input devices are required.

Microphone

Voice input, for instance, can be recorded via a microphone. A device that you can use to talk to people in different parts of the world. You can record sound into the computer by using a microphone. You can also use a microphone to record your speech and let the computer convert it into text.

Webcam

A device that is similar to a video camera. It allows you to capture and send the live pictures to the other user. For example, a webcam allows your friends and family to see you when communicating with them.

Digital cameras

record photographs in the form of digital data that can be stored on a computer. These are often used to record photographs on identity cards.

Output Device

Output devices in the computer system are the equipment whereby the result of a computer operation can be viewed, heard or printed. You use output devices to get feedback from a computer after it performs a task. Some examples of output devices are described in the following list.

Monitor

A device that is similar to a television. It is used to display information, such as text and graphics, on the computer.

Printer

A device that you use to transfer text and images from a computer to a paper or to another medium, such as a transparency film. You can use a printer to create a paper copy of whatever you see on your monitor.

Plotter

A plotter is an output device similar to a printer, but normally allows you to print larger images. It is used for printing house plans and maps.

Multimedia Output Device

The most common multimedia output is sound including musics,The audio output device on a computer on a speaker headphone can also be used to receive audio output.

Printers

There are various type of printers available and these very considerably in a quality of there production.

Impact Printers

Dot matrix printers are an example of impact printers. They form characters from patterns of dots. They are inexpensive, but the output can be difficult to read.

Non–impact Printers

Non–impact Printers Inkjet printers work by shooting a jet of ink in the shape of the character required they provide god, low–cost color printing.

Laser printers

Laser printers a laser beam is directed at an elector–statically charged surface, creating a template of the page to be printed. This template is then used to transfer the ink to the page. Toner sticks to the light images and is transferred to paper.

PROCESS AND STORAGE DEVICES

The system unit is the name given to the main computer box that houses the various elements as illustrated in the picture below.

How do we get the different computer parts to work together so that your computer can work ? Let’s start of our discussion with the process devices.

How do we get the different computer parts to work together so that your computer can work ? Let’s start of our discussion with the process devices.

Motherboard

Inside the systems unit is a circuit board with tiny electronic circuits and other components which is called a mother board. It is sometimes called a system board. The motherboard connects input (keyboard, mouse and scanner), output (monitor, speakers, and printer), processing (CPU, RAM and ROM) and storage (hard drive, CD–ROM/DVD–ROM and flash drives) components together and tells the CPU how to run (Lubbe and Benson, 2010:18). Other components on the motherboard include the video card, the sound card, and the circuits that allow the computer to communicate with devices like the printer.

Expansion Cards

Yes, you can play music and video files on your computer. But how is it possible ? Inside the computer system box, you also find an expansion card which is another circuit board that can be attached to the motherboard to add features such as video display and audio capability to your computer.

Expansion cards are also called expansion boards that enable your computer COMPUTER HARDWARE 27 to use the multimedia devices. An expansion card either improves the performance of your computer or enhances its features. Examples of expansion cards that can be added include:

Video Card

It is connected to the computer monitor and is used to display information on the monitor.

Network Interface Card (NIC)

The NIC allows the computer to be connected to other computers so that information can be exchanged between them.

Sound Card

It converts audio signals from a microphone, audio tape, or some other source to digital signals, which can be stored as a computer audio file. Sound cards also convert computer audio files to electrical signals, which you can play through a speaker or a headphone. The microphone and the speakers or the headphones connect to the sound card.

STORAGE DEVICES

All computers need to store and retrieve data for processing. The CPU is constantly using memory from the time that it is switched on until the time you shut it down. There are two types of storage devices as illustrated in the flow chart below.

Primary Storage is also called main memory or immediate access store (IMAS). This is necessary since the processing unit can only act on data and instructions that are held in primary storage. Primary storage consists of two types of memory chips :

Random Access Memory (RAM)

Read Only Memory (ROM)

Random Access Memory (RAM)

Random Access Memory (RAM) is the main working memory. RAM is only filled after a computer has been turned on and is given something to do. It holds data and instructions temporarily while processing takes place. RAM is volatile – this means that if the power is turned off or the computer reboots (start up again) all the information held in RAM will be lost.

RAM is measured in MB (megabytes) and most entry level computers will have 1024 MB RAM but you also find some computers having up to 3 GB RAM. RAM chips are expensive and the price of a computer is determined by the amount of RAM space in the chip.

Read Only Memory (ROM)

holds data and instructions necessary for starting up the computer when it is switched on. These instructions are hard–wired at the time of manufacture. ROM is permanent and cannot be deleted but can only be accessed or read, hence the name Read Only.

Memory. Data stored in ROM is non–volatile – meaning that memory will not be lost when power is turned off.

So, we can compare the features of the two memory types as below:

RAM	ROM
Needs power	Does not need power
Data can be changed	Data can’t be changed
Data will be lost if power is turned off	Data will not be lost if power is turned off
Volatile	Non–volatile
Stores data currently being processed	Fixed instructions are stored

Units of Storage

The memory of all digital computers is two–state (bi–stable) devices. Computers operate using a binary number system – and therefore use binary digits (bits). Bits have only two values by 0 and 1. A bit is the smallest unit of storage in a computer.

The amount of data and instructions that can be stored in the memory of a computer or secondary storage is measured in bytes.

A byte is made up of a combination of eight (8) bits and has the storage power to represent one character (a character is a letter or symbol or punctuation mark or blank space).

Units of Storage
1 byte 1 kilobyte (K) 1 megabyte (MB) 1 gigabyte (GB) 1 terabyte (TB)	8 bits 1024 bytes 1000 kilobytes (approx. 1 million bytes) 1000 megabytes (approx. 1 billion bytes) 1000 gigabytes (approx. 1 trillion bytes)

Secondary Storage Devices

PCs use a simple method of designating disk drives to store data. These drives are assigned letters of the alphabet.

A Drive	Floppy drive. Still found in older computers
C Drive	e Internal hard drive (hard disk drive) situated inside the system case.
D Drive	Usually the CD–ROM/DVD–ROM drive although can also be used for another virtual or physical hardware if a second one is deployed.
E Drive or Higher	Usually use for any other disks, such as CD–writer, USB flash drive, external hard drive, etc.

Data and information stored on a permanent basis for later use. Secondary storage is cheaper to purchase and access. Hard disks, Zip drives, Optical disks (CD’s and DVD’s) are all examples of secondary storage.

Internal Hard Disks

internal Hard Disks are rigid inflexible disks made of highly polished metal. Data is stored magnetically. They can contain a single disk or two or disks stacked on a single spindle.

They come in a variety of sizes but all have a very high storage capacity compared to floppy disks. An average computer has a hard disk of about 80 – 250 GB. It provides direct access to information.

External hard Disks/Drive

External hard Disks/Drive – same features as the internal hard disks, but are external to the system unit and therefore can be carried around. USB port is used to connect the external hard drive to the.

Optical Disks are disks that are read by laser beams of lights. The three main types are CD–R, CD–RW and DVD.CD–R or CD–ROM (Compact Disk – Read Only Memory) are so called because you can only red the information on the CD–ROM. They are particularly useful for storing multimedia (texts, graphics, sound and videos), application software packages (encyclopedias, training programs etc.

CD–R or Compact Disk Record able allows you to write information onto the disk only once using a CD record-able burner.

CD–RW or Compact Disk Rewrite able, allows you to write and erase information from the disk many times. They are used to store large volumes of information such as texts, graphics, sound and video.

DVD disks or Digital Versatile Disks are specifically created to store movies. A typical DVD disk can hold between 4.7GB and 17GB of information.

USB flash drive – consists of a flash memory data storage device integrated with a USB (Universal Serial Bus) interface. USB flash drives are typically removable and re writable. They come in a variety of sizes to include 128 MB, 256 MB, 512 MB, 1 G, 2 G, 8 G etc.

Memory Card – Use mainly with digital cameras, cellular phones and music players (MP3, MP4 and iPods). They offer high–re–record ability and fast and power– free storage. Data can be access by linking the card to a computer using a USB cable or a memory card reader.

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Computers and Computing

GENERAL PURPOSE COMPUTER SYSTEMS

When you look at a desktop computer, it comprises a monitor, keyboard, mouse, and a vertical or horizontal box called a system unit. It is acceptable to refer to all of these components as a computer. Technically the systems unit houses the actual ‘computer’ or programmable machine (hardware, firmware and software), which executes (carries out) or responds to a sequence of program instructions.

A computer and Computing system then is a collective term for a computer (housed within the system unit), as well as these other components that are attached to it to allow the user to input data and view output using different devices. Some of these components include a monitor, keyboard, mouse, printer, disk drives, scanners, microphones, and speakers, which are also referred to as peripherals.

The peripherals work together to help the user accomplish a given task effectively. Therefore, as an example, when you perform a calculation, the keyboard – a peripheral device – is used to input the data. It is the system unit that accepts and processes the data, and generates the results. Other peripheral devices such as the hard drive store the data and results, which can be viewed on the monitor.

The peripheral devices mentioned above are collectively called hardware devices, since they can be physically attached to the computer. However, computer systems also include software, or programs that are integral in the functioning of some of these peripheral devices and are necessary to make the computer operate. Every computer system, for example, requires an operating system.

An operating system is a program that establishes communications with the various peripherals and acts as a bridge to other application specific software programs. An operating system is like the command center of the ship. It makes all of the decisions about how the computer will function and work with others.

EMBEDDED COMPUTER SYSTEMS

The typical personal computer or laptop described in the last discussion is categorized as a general–purpose computer system. This section will explore a special type of computer system called an embedded system. Embedded systems are designed to perform a very small number of very specific functions. For example, a GPS computer in a car only provides driving instructions.

It cannot perform any other computing function. A typical embedded system also uses certain types of hardware, and limited resources, such as small keyboards, screen and reduced memory. These systems are also actually built into the device they are controlling. Compared to a typical computer, embedded systems are often smaller in size, tend to be cheaper, and as a result many are mass–produced.

Embedded systems range from small portable devices to large complex systems. The major difference is that they are single purpose devices with only one form of input or output. Some may also have simple user interfaces or displays which include buttons, or touch screens, while others have none at all.

Table – Examples of systems and devices which comprise embedded systems

Portable devices	Large complex systems
1.Digital watches 2.MP3 players 3.• Microwave ovens 4.Washing machines 5.Television sets 6.DVD players 7.Air conditioners 8.Sprinklers 9.Security monitoring system 10.Handheld computers 11.Thermostats	1.Traffic lights 2.Systems controlling nuclear power 3.plants 4.Controllers used in remote 5.machine operation 6.Navigation Systems 7.Military Weapons Systems

Mobile (cellular) phones and handheld computers may be categorized as embedded systems, but since these devices can be enhanced with the addition of other applications and peripherals, they are better suited to the category of general–purpose computer systems and not really embedded systems.

COMPUTER SYSTEM – HOW IT WORKS

The major components of a computer system include input and output devices as well the important processor or central processing unit (CPU). The CPU comprises a set of electronic circuits kept within the system unit. The CPU is the brain of a computer system and is responsible for processing the data that is input from a peripheral device and output to another peripheral device.

Figure 2.2 illustrates the major components of a system unit that is attached to an input and output device. The Central Processing Unit (CPU), also known as the processor or microprocessor has components that control the operating system and other software installed on the computer. The CPU also sends commands and data to the peripherals attached to the computer, such as the monitor or printer.

Therefore, the speed (how fast the electrons move around the chip) at which the CPU executes its tasks (or commands) determines how quickly you can view or hear the output from the command; such as printing a page or allowing you to view a letter on the monitor. This speed is called ‘clock speed’, and is measured in megahertz (MHz). A computer system that responds to commands quicker means that its clock speed is working faster to execute these commands

Table 2 compares some common CPUs and their clock speeds. Different CPUs use different silicon chips to execute the commands and process data. Different types of chips have different clock speeds based on the technology that was used to create them. Some examples are provided below.

Table 2.1 : Typical CPU Speeds Based on Chip Type

Name of CPU	Average CPU Speed
Intel Celeron	500 MHz – 800 MHz
Intel Pentium II	233 MHz – 450 MHz
Intel Pentium III	450 MHz – 1 GHz
Intel Pentium 4	1.4 GHz – 2 GHz
Intel Core i7	3.6 Ghz
AMD Athlon XP	1.4 GHz – 1.8 GHz
Macintosh G4	733 MHz

Now let’s look deeper into the operation of the CPU. All computer systems have a CPU which comprises (1) a control unit and (2) arithmetic and logic unit (ALU).

The Control Unit (CU)

The control unit is similar to an efficient manager. It is responsible for directing other parts of the computer system in order to carry out the instructions for the task required. The control unit (CU) must interact with the arithmetic and logic unit (ALU) and memory to complete its tasks. As instructions are fetched from memory, the CU interprets the instruction.

The CU then sends commands to other components to gather data that may be needed to complete the instruction, even if it involves a transfer of data from other components and devices. The CU than determines where to send the result. This may include activating the printer, showing the result on the monitor or playing a sound on the speakers.

Arithmetic and Logic Unit (ALU)

The Arithmetic and Logic Unit (ALU) is the part of the CPU which performs all arithmetic and logic operations this involves arithmetic calculations including addition, subtraction and multiplication. As calculations are required, the Control Unit sends them to be performed in the ALU which sends the result back to the Control Unit. It also performs logic operations such as comparisons of numbers or letters to test for, such as equal–to (=), less–than (<), greater–than (>) and other combinations, such as less–than or equal–to (<=).

Memory

Primary memory is necessary to store instructions and data for processing, but they are not part of the CPU. Memory are separate chips stored on a mother board which connects all the hardware together. Program instructions or data are kept in primary memory for only as long as the program currently using them. In human terms this would be considered “short term memory”.

The original instructions and data are lost once the computer is turned off; therefore, primary memory is deemed volatile. In later units we discuss those non–volatile devices which allow data and instructions to be permanently stored in a computer system for use later on. Primary memory is also as – memory, main memory, primary storage, main storage, internal storage, computer memory and RAM (Random Access Memory).

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Data Processing

Processing data is the most important activity in the computer, and so the task of the processor is to interpret and execute program instructions, in order to transform data into information. The processor is also responsible for interacting with the input, output and storage devices.

PROCESSING DATA AND COMPUTER MEMORY

Keeping instructions and data in primary memory when a program is not in use is not feasible for three reasons :

• The processed data may be too much to be held in primary memory.
  
• Generally primary memory only stores data and instructions while the computer is turned on, but once the computer is turned off, the data and instructions are lost.
  
• If more than one program or application is opened at the same time, then, a single program should not exclusively use all the memory; some memory should be available for the other programs to use.

The amount of memory available to hold the data and instruction is measured in megabytes or MB. A personal computer with a minimal amount of memory of say less than 512 MB may not be able to perform efficiently and may be rather slow in executing the applications.

Memory is also loosely known as RAM which is an acronym for Random Access Memory. It should be noted there that the word random implies that data can be retrieved quickly, and in the same length of time, regardless of where the data is located (that is, the address of the data).

Read Only Memory (ROM) Read–only memory or ROM is an integrated circuit, programmed with specific data when it is manufactured. It is used in computer systems and other electronic devices. RAM and ROM are different since data stored in ROM cannot be modified easily. In other words, ROM is ideally suitable for storing a set of data for the life of the device, and is therefore nonvolatile.

Modern types of ROM include such as Programmable ROM (PROM), Erasable Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM) and flash memory can be erased and re–programmed many times, although they are still called ROM because the reprogramming process is usually rarely done. Many ROM chips are often identified by the prominent circular ‘window’ which allows ultraviolet (UV) light to enter and erase its contents.

Flash memory (or simply flash) is a modern type of EEPROM which can be erased and rewritten faster than EEPROM. Flash memory is mainly used in memory cards, USB flash drives, memory stick, flash stick and jump drive), which are used for general storage and transfer of data between computers and other digital products.

Function of the Machine Cycle Consider a user who needs to add two numbers, 10 and 15. These numbers may be input at the keyboard and the results shown on a screen. The processing of this simple addition is carried out in the CPU.

Let us examine how a single instruction is executed or carried out in the central processing unit. Of course, computers today generally execute one instruction at a time, although as quickly as a fraction of a second.

In order to carry out an instruction, the program instructions and data must be moved into memory from either an input device or a secondary storage device. Once in memory, the control unit and ALU in the CPU work together to perform the following four steps (fetch, decode, execute, and store) for each instruction. These steps are collectively known as the machine cycle or the processing cycle. This machine cycle has two phases, the instruction cycle (instruction time, or I–Time) and the execution cycle (execution time or E–Time). Let us examine these cycles in more detail.

Phase A : Instruction Cycle Fetch Instruction : The control unit fetches (gets) the program instruction from memory. Decode Instruction : The control unit decodes the instruction (decides what it means) and then moves the necessary data from memory to the ALU. Phase B : Execution Cycle Execute Instruction : The ALU performs the requested action, by executing the arithmetic or logical instructions on the data. Store (write) Result : The ALU writes or stores the result of this operation in memory or in a temporary register

Once these two phases are complete, the control unit instructs memory to send the result to an output device or a secondary storage device. The figure below illustrates how these cycles interact to produce a result.

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