Computer Systems

Computer Systems

Trends in Computer SystemsNowadays, computer systems come in a variety of sizes, shapes and computing capabilities, from the smallest handheld personal digital assistant (PDA) to the largest multiple-CPU mainframe for the enterprise.

The technology in microprocessors continues to advance rapidly. Today, every computer, no matter how large or small, uses one or more microprocessors as its CPU.

The game of putting more electronic components into a single chip will go on. Computers will continue to become smaller, faster, more reliable, less costly to purchase and maintain, and more interconnected within computer networks.

Computer CategoriesWe shall classify computers based on their applications, into handheld computers, personal computers, network servers, mainframes and supercomputers.(1) Handheld computer

A handheld computer, also called palm or personal digital assistant (PDA), is typically used as an electronic appointment book, address book, calculator and notepad. It is a pocket-sized computer that runs on batteries and is used while you are holding it. A handheld computer is designed to be an accessory of a computer. It cannot replace a personal computer because of its slow processing speed and small screen.

(2) Personal computerA personal computer (PC) is a single-user computer, providing access to a wide variety of computing applications, such as word processing, photo-editing, e-mail and Internet access. Personal computers used as a client on a network are called workstations.There are two major categories of personal computers: desktop computers and notebook computers.A notebook computer, also called laptop computer, is a portable personal computer that encases the basic input devices (like keyboard and touch pad/pointing stick), output devices (like the visual display unit and speaker), the processing unit and other electronic components inside a plastic or metal case.A desktop computer is a personal computer with a system unit and other peripheral devices. The entire system can be placed on a desk.The system unit of a desktop personal computer exists in the form of a metal or plastic case, which houses the motherboard, CPU, main memory, interface cards, power supply and storage devices. The peripheral devices include all the input, output and storage devices. They are connected to the ports or interface of the system unit through cables or radio waves.

(3) Network serverOn a client/server network, a server is a computer that responds to the requests from other computers, called clients. For example, a database server stores the database of an organization and responds to a client's request for certain database records; A file server stores data files centrally and allows authorized users to access their own set of data files; A printer server allows clients to access a centralized printer; A Web server is connected to the Internet and responds to requests for Web pages from clients of other networks.

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A server does not require a specific type of hardware. Any personal computer, workstation-class computer, mainframe or supercomputer can be configured to perform the work of a server. It is the software installed in the computer that matters.

(4) MainframeMainframes are large, fast, powerful and expensive computers that are capable of supporting hundreds or thousands of users simultaneously. A typical mainframe works in time-sharing mode which requires a huge main memory to store all the programs of individual users at the same time.

System UnitA computer system consists of both hardware and software. Hardware refers to the actual machinery, for instance, the microprocessor, keyboard, mouse and monitor etc. Software refers to programs or data.

The hardware components of a basic computer system consists of Central processing unit (CPU) Main memory Input devices Output devices Secondary storage Communication devices.Components other than the CPU and main memory are collectively known as peripheral devices.

A personal computer can be viewed as made up of a system unit and some peripheral devices. For a desktop computer, the system unit appears in the form of a metal or plastic case which houses the motherboard, CPU, main memory, interface cards, power supply and storage devices. For a notebook computer, the system unit encases almost all its electronic components in a metal or plastic case. Note that storage devices are regarded as peripheral devices.

(1) MotherboardThe motherboard is the main circuit board that includes all essential chips, connecting circuitry and interfacing facilities.A chip is an integrated circuit that contains many transistors on a small piece of semi-conducting material. On the motherboard, there are sockets which allow certain chips to be removed for repairs or upgrades. Both the microprocessor and main memory are chips that can be plugged into the motherboard.The connecting circuitry links between the chips on the motherboard. For example, the connecting circuitry provides channels for data transfer between the CPU and main memory. Each channel is called a bus and is made up of a number of conducting wires. Each bus is a composition of address bus, data bus and control lines.The interface facilities of a motherboard appear in the form of ports and expansion slots. A port is built on the motherboard and is designed to connect to a peripheral device or another computer.

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An expansion slot allows an interface card to be inserted into the motherboard. An interface card is a separate circuit board which is an add-on component to increase the function of a computer. Examples are the network interface card (NIC), video card, sound card and modem card. Some interface cards come with their own ports, allowing connection to other peripheral devices.

(2) Central Processing Unit (CPU)A central processing unit (CPU) is designed to process instructions that operate a computer. The CPU of a personal computer consists of two parts: microprocessor (e.g. Intel Pentium IV) and the main memory.A CPU consists of two major components: the CU and ALU. It also includes circuitry for devices such as registers, cache memory and various execution units.

(a) Control Unit (CU)The control unit controls the overall operations of the computer. It repetitively interprets an instruction from a program and do appropriate action. It also controls the operations of the peripheral devices.

(b) Arithmetic Logic Unit (ALU)The arithmetic logic unit performs arithmetic operations, such as addition and subtraction of integers. It also performs logic operations, such as comparing two numbers to see whether they are equal or not.

(c) RegistersRegisters are special memory locations inside a CPU, allowing fast access. Some of them are needed by the ALU in computation to hold data to be processed. Others are needed by the CU to control the execution of instructions. The accessing speeds of registers are extremely high.

(d) Cache memoryData between the processor and main memory is slow. Very often, the processor has to wait for memory access and become idle in order to reduce the computer efficiency. Cache memory is a high-speed memory that holds data that duplicate some data stored in the main memory. When the data needed by the processor are stored in the cache memory, the access time will be much smaller fetching the same data from the main memory. When the data needed by the processor are not stored in the cache memory, the processor will switch the required data from the main memory to cache memory to replace any oldest pieces of data.

The performance of a CPU is affected by several factors: clock rate, word length, cache size, and processing techniques.

(a) Clock rateA CPU works with a timing device, called clock, which generates ticks regularly. The clock rate is the number of ticks produced per second. It determines the pace for executing instructions. The unit of clock rate is hertz (Hz). Mega (M) is a prefix representing one million (106). Giga (G) represents one billion (109). For example, an 800 MHz microprocessor produces 800 million ticks in one second.It is important to understand that the clock rate of a CPU is not equal to the number of instructions that the CPU can execute in one second. The reason is that most instructions take several ticks to complete.

(b) Word lengthWord length refers to the number of bits that a microprocessor can manipulate at one time. Word length is the size of the registers in the CPU. In general, a longer word length will give better performance. Suppose we wish to perform an addition of two 32-bit numbers. If the registers are all 16 bits, each value will have to be stored in two registers and the calculation will involve two separate additions. If the registers were all 32 bits, the amount of processing would be halved. Today's personal computers typically contain 32-bit.

(c) Cache memoryData transfer between the CPU and main memory is slow. Very often, the CPU has to wait for memory access and becomes idle. Cache memory is a high-speed memory that holds data which duplicate some data stored in the main memory. When the data needed by the CPU are stored in the cache memory, the access time will be much smaller than fetching the same data from the main memory.

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(3) Main memoryMain memory is the working memory of a computer system. It is used to store data and instructions that fetched into processor for execution. There are three types of main memory: RAM, ROM and CMOS. Main memory is classified into: volatile and non-volatile. The contents of the volatile memory will be lost when the computer power is turned off. On the other hand, non-volatile memory will retain the contents after the power is turned off.

(a) Random Access Memory (RAM)RAM is a temporary holding area for data, application programs and the operating system, as long as the computer is running. All instructions or programs must be pre-loaded into RAM before they can be executed by the processor. Sometimes, RAM is called “user memory”. RAM is volatile such that its content will be lost when the computer power is turned off.

(b) Read Only Memory (ROM)Since RAM is volatile but there must have some fundamental operations e.g. BIOS needed to be executed when the computer is switched on, so that a non-volatile memory called ROM must be used. The programs stored in ROM can be read only but cannot be overwritten or erased. Sometimes we call such operations as firmware since they are stored onto the ROM by the firm manufacturer. There are several type of ROMs: programmable read only memory (PROM), erasable programmable read only memory (EPROM) etc.Difference between RAM and ROM:

Random Access Memory (RAM) Read Only Memory (ROM)Data can be read from and written onto RAM.

Data can only be read from ROM. The contents cannot be overwritten or erased.

RAM is volatile. When the computer is switched off, all the data held in the RAM are lost.

ROM is non-volatile. When the computer is switched off, the contents of the ROM remain unchanged.

RAM is used to store temporary data. ROM is used to store permanent data.RAM stores user data. ROM stores manufacturer's fundamental

operations.Size of RAM is relatively large and can be expanded.

Size of ROM is small and fixed.

(c) Complementary Metal-oxide Semiconductor Memory (CMOS)CMOS is a type of memory on the motherboard but not within the CPU. It stores the configuration information about a computer. It includes information like current time and date, capacity of hard disk, types of monitor and so on. Unlike ROM, the computer can change the information in CMOS. Unlike RAM, the contents in CMOS are retained even the main power supply is turned off. This is because the contents in CMOS are kept by an independent battery.

Memory size of main memory is measured in byte. A byte consists of eight bits.Manufacturers always produce memory in multiples of 210 = 1,024 bytes abbreviated as KB. For example, a memory of 128 KB is equal to 128 × 1,024 = 131,072 bytes.

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In recent years, memory size increases drastically. Memory chips are produced to hold megabyte (MB). 1 MB is equal to 1,024 × 1,024 bytes = 1,048,576 bytes.Similarly, larger memory size is measured in kilo of megabyte, called gigabyte (GB). 1 GB is equal to 1,024 × 1,024 × 1,024 = 1,073,741,824 bytes.However in other measuring, e.g. data transmission rate, kilo, mega and giga represent 1,000, 1,000,000 and 1,000,000,000 respectively.The size of main memory affects the efficiency of the computer. A computer is usually upgraded by increasing the memory size.The main memory can be thought of as a series of locations each of length one byte. Each location has a unique address, counting from zero onwards. The address is called memory address. For example, for a computer with a memory size of 1 K, the memory address ranges from 0 to 1,023.Clearly, the size of a memory is limited by the maximum address. The maximum address is determined by the number of bits used in each address. For example, if 8 bits are used as address, the address space is from 000000002 to 111111112 which is equivalent to 0 to 127. Therefore, the maximum size of memory is 128 bytes,In general, if n bits are used to represent an address, then the maximum size of memory = 2n bytes.

Peripheral D evices

Any computer devices outside the CPU are called peripheral devices, e.g. keyboard, mouse, printer, modem, scanner, hard disk, memory stick, CD ROM etc. Peripherals can be classified into three main categories:

(1) Input DevicesThe function of an input device is to convert the input data into machine-readable form and place it into the main memory. There are several types of input devices:

Type Input devices Remarks

KeyboardStandard keyboard Typing character resembles a typewriter.MIDI keyboard Enter electronic music into a computer.

Pointing

Mechanical mouse / Optical mouse

Select an icon by the point-and-click principle.

Trackball Work like an upside-down mechanical mouse.Joystick Consists of a vertical lever and some buttons.

Often used for playing video games.Graphic tablet A flat rectangular board connects with a pen.

Often used together with handwriting recognition software.

Touch screen A pressure-sensitive display that allows you to select option with a finger.

Touch pad Built-in pointing device of some notebook computers.

Scanning

Image scanner A light-sensitive input device that reads printed text and photos. Optical character recognition (OCR) is a technology that analyses the digital image and converts the results into text.

OMR reader Detect a pencil mark in a MC paper by optical mark recognition.

Barcode reader Barcode reader is fast and accurate.Magnetic ink character recognition

Often used to read text printed with magnetized ink (e.g. bank cheque).

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Voice InputMicrophone Often used in enter spoken words.

Words can be converted to text by voice recognition software.

Authentication

Magnetic card reader Read the magnetic stripe on the back of a card.Smart card reader A smart card contains a chip with a built-in

microprocessor and memory. Octopus and ID card are examples.

Fingerprint scanner It captures curves and indentations of a fingerprint.Face recognition reader It captures the live face image. It is a kind of biometric

authentication devices.

(2) Output DevicesThe function of an output device is to display or print the results of processing. The output devices convert result stored in the computer to human readable form.A display device is an output device that visually presents text, graphics and video information on a screen. Information on a display device is called softcopy which will be lost when the power is turned off.

(a) Visual display unit (VDU)Visual display unit is the most common form of computer softcopy. There are two major technologies for VDU: C athode R ay T ube (CRT)

A CRT monitor consists of a glass fluorescent screen which produces images as an electron beam inside the CRT strikes on the screen. A CRT operates at a very high voltage inside and requires a large amount of electronic components. This explains why a CRT monitor is hot, heavy and occupies a lot of desk space.

L iquid Crystal D isplay (LCD) LCD displays are used in notebook computers, tablet PC and handheld computers. You can also find LCD displays in many other devices, like digital cameras and mobile phones. LCD monitors are thinner in size and lighter in weight than CRTs. They are known as flat-panel display. As the price of LCD monitor continues to drop, CRT monitor will eventually disappear from the desktop of common users.

(b) Data projectorA data projector uses strong light beams to project images on a larger screen so that a group of people can see the image clearly. Data projectors are commonly used in classrooms to facilitate presentation.

A printer is an output device that prints texts and graphics information on a piece of paper or other physical medium, like transparency film. Printed information is called hardcopy since it exists physically and is more permanent than softcopy. Printers can be divided into two types: impact printers and non-impact printers.

(a) Impact printersAn impact printer physically strikes against an inked ribbon using a "hammer" so that text or graphics are formed on the paper. Dot-matrix printer

Dot-matrix printer is a typical impact printer. It uses a matrix of pins contained in the print head to strike an inked ribbon against the paper. Characters are formed by dots as the print head moves horizontally across the page.

Since dot-matrix printers can print through several layers of paper, they are used for printing multiple forms such as 3-ply invoices. Most dot-matrix printers use continuous-form paper which has holes (called perforations) in the margins on both sides. The disadvantages of dot-matrix printer are slow and noisy.

(b) Non-impact printersA non-impact printer forms characters and graphics on a paper without physically striking the paper. In general, non-impact printers are much quieter and faster than impact printers. Ink j et printer

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An inkjet printer sprays tiny ink drops onto a page. Old inkjet printers used electrically charged ink droplets which were deflected into their proper positions on the paper as the print head moved horizontally. Nowadays, most inkjet printers use a series of nozzles that eject droplets of ink straight onto the paper, rather than deflecting the droplets by electrical means.

Inkjet printers are very popular because of their high quality, high speed and low price. However, there are some disadvantages:(1) The capacity of the ink cartridge is small, making frequent replacement necessary.(2) The ink used in some inkjet printer is soluble. The image may be blurred by

water.(3) The printout cannot be used immediately since time is required for drying.(4) The color of the printout is not permanent.

Laser printer A laser printer is a high-speed and high-quality non-impact printer. It uses a laser beam to create images onto a drum. The images are electronically charged to attract toner powders, which are then transferred from the drum onto a paper. A heating process follows which fixes the toner powders onto the paper. This technology is similar to that used in a photocopying machine.

The laser printer produces the best text printout quality among the three printer technologies mentioned. While a laser printer may cost more than an inkjet printer, the printing cost per page is usually less than an inkjet. For the same volume of printing, replacement of toners is less frequent than ink cartridges. Laser printers are widely used in business where economical and high-speed output is required.

Thermal printer Thermal printers are of three types: direct thermal printer, thermal wax transfer printer and dye sublimation printer.(1) Direct thermal printer produces images as a heat-sensitive paper is heated. An

example is the barcode printer in supermarkets. The main advantage is that it will never run out of ink.

(2) Thermal wax transfer printer adheres a wax-based ink onto paper. Wax is melted and transferred to the paper. The wax-based ink will adhere to almost any kind of stock, from ordinary paper to complex synthetics and film.

(3) Dye sublimation printer uses heat to transfer coloured dye to paper. As a dyed ribbon is heated, the dye vaporizes and adheres to the paper.

Since the last two thermal printers are not very popular, the consumables (the wax-cartridges and the dyed ribbons) are quite expensive.

Plotter A pen plotter consists of several colour pens which move back and forth across the page while the paper is moved up and down by a roller. Unlike most other printouts in which the images are made up of many small dots, continuous lines and curves are

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drawn by a pen plotter on a paper. It is mainly used by engineers in drawing blueprints, maps and circuit diagrams.

Besides printers, audio output devices, e.g. speakers and headsets are other common output devices. Usually they are connected to the audio-out port of a computer.

(3) Secondary storageSecondary storage, also called backing store, holds data and instructions for future use. Examples of storage media are floppy disks, hard disks, optical discs, tapes and flash memory cards. Since a CPU can access data only from the main memory, the file must be loaded to the main memory.Secondary storage is needed in addition to the main memory for the following reasons: RAM is volatile Data stored in the RAM of

the main memory are lost when the machine is powered off.

The memory size is limited It may not be possible to hold all the data in main memory at the same time because the size of a file might be greater than the memory size.

Main memory is more expensive.

Common storage medic can be classified into magnetic, optical and semi-conducting.(a) Magnetic media

A magnetic medium stores data by magnetizing microscopic particles on the disk or tape surface. The particles retain their orientation until they are changed by a magnetic field. This makes disks and tapes fairly permanent but modifiable storage media. F loppy disk

A floppy disk is an inexpensive storage medium that consists of a thin plastic disk which is coated with magnetic powder. A floppy disk is portable. Therefore, you can easily transfer data by moving a floppy disk from one computer to another. A disk drive which consists of a read-write head is needed to access data on a floppy disk.

Hard disk A hard disk provides huge storage. It consists of a set of circular plates coated with magnetic powder. The plates and read/write heads are sealed in a metal case.Hard disks are fast storage because the plates rotate at a high speed, e.g. 7,200 rpm (revolutions per minute). However, a hard disk is subjected to head crash when a read/write head touches a disk plate. This may lead to data loss. Therefore, frequent backup by other storage media, such as tapes or optical discs, should be done.Current personal computer hard disk can store several hundred GB of data.

Magnetic Tape A magnetic tape, or tape, is a magnetically coated plastic ribbon capable of storing large amount of data at a low cost. Current tape mainly appears in the form of tape cartridge.

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A tape is sequential access. It means that you have to forward or rewind the tape to a point to access a specific piece of data.Sequential access is slow and is not suitable for routine applications. Tape is almost exclusively used for backup.

(b)Optical mediaAn optical disc, also called a compact disc (CD), exists in the form of a plastic disc with a thin metal coating. An optical disc has large storage capacity. It is portable. The storage cost is low and the access time is short.Data are stored by using microscopic pits and lands on the metal coating. A laser beam is used to read the patterns of pits and lands. A land reflects the light, whereas a pit does not. An optical disc stores data in single track that spirals from the centre of the disc to the edge of the disc. This single track is divided into evenly sized sectors.

There are many variations of optical discs: CD-ROM (CD read only memory)

A CD-ROM can normally store data up to a capacity of 650 MB. Its contents can only be read but cannot be changed or overwritten. CD-ROM is commonly used in distributing software.

CD-R (CD recordable)A CD-R is an optical disc onto which data can be recorded. Once recorded, the contents can be read many times by an ordinary CD-ROM drive. However, each disc area can be written once. New contents are recorded in areas which have not been used for recording before.

CD-RW (CD rewritable)A CD-RW is an optical disc on which every disc area can be overwritten again and again similar to a hard disc. A CD-RW writer is needed to record data on a CD-RW.

DVD-ROM (digital versatile disc)A DVD-ROM is similar to a CD-ROM, except that it has much higher capacity.

DVD R (DVD recordable)A DVD+R or DVD-R is similar to a CD-R, except that it can record items up to a much higher volume.

DVD RW (DVD rewritable)(c) Semi-conducting Media

A flash memory card uses semi-conductor to record data. Data can be transferred at a high speed because there is no mechanical movement. As it is small in size, it is the most portable storage medium. Flash memory cards are also common storage medium for digital cameras. Since the sizes and shapes are not standardized, there are several types of flash memory cards that require different readers. As the capacity of flash memory cards continues to increase and their price continues to decrease, flash memory cards will become a popular secondary storage medium.

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Comparison between main memory and backing store:main memory backing store

Function provides a working area holds data or programs permanentlyCapacity limited very large

Cost expensive cheapPermanence volatile non-volatileAccess time very fast slow

Comparison between different types of backing storage devices:Magnetic tape Hard disk Floppy disk Optical disk

Cost of drive high high low highCost of media relatively low high low mediumAccess method sequential only direct direct directAccess time slow fast slow medium

Classification of Software

Software can be categorized into system software or application software. System software is a collection of programs normally supplied by computer manufacturer. It enables the application software to interact with the computer and helps the computer manage its peripherals.

Application software is a program designed to solve particular problems for users. It enables users to perform work on specific tasks and facilitate communications.

System Software

System software is divided into three categories:(1) Operating System

An operating system (OS) is a set of programs that controls the operation of computer system, manages resources and provides an environment for users to access the computer system. The functions of OS include:

(a) Recognizing input from keyboard(b) Sending output to monitor

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(c) Keep track of files and directories on the disk(d) Managing the use of main memory(e) Scheduling the programs run by CPU(f) Maintaining security(g) Monitoring system performance(h) Provide communications services that allow computers to share data, program and

peripheral devices(i) Providing a user interface for user to control the computer

There are two types of user interface: graphical user interface (GUI) and command line interface.

Graphical User Interface Command Line Interface

A graphical user interface (GUI) provides a way to point and click a pointing device (such as a mouse) to select menu options and manipulate graphical objects that are displayed on the screen. An icon is a small picture that represents a command, program file or folder.A command line interface requires users to memorize and type commands. This interface can be very efficient for skilled users, whereas a beginner often finds it difficult to get started.The advantages of GUI are: It is intuitive so that it is easy to use. Users need not learn a set of instructions to operate the system.

The disadvantages of GUI are: It is resource demanding because it uses a lot of processing power, memory,

and disk space, and requires a good graphical display device. It may not be as fast as command line interface for the experienced users.

In nearly all computers, the OS is quite large so that most of it is stored on a hard disk. In that case, the ROM just stores a small bootstrap program called BIOS. When the system boots, the BIOS will locate the program files on the hard disk that include the core part of the OS, and then load the core part into the RAM.

The core part of the OS is called a kernel, which provides the most essential system services, such as memory management and file access. The kernel always stays in the RAM until the computer is turned off. While the other parts of the OS are loaded into the RAM as they are needed.

Popular operating system includes: DOS (Disk Operating System) MS Windows XP Mac OS Unix, Linux Novell Netware

(2) Utility programsA utility program is a type of system software that allows a user to manage a computer, including its devices and programs. Utility programs should be run regularly to ensure smooth

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running of a computer system. Some utility programs come with the OS, like those managing the hard disks, while some are provided by third parties, like those detecting virus.

Examples of utility programs are:(a) File viewer(b) File compression utility(c) Diagnostic utility(d) Disk scanner(e) Program uninstaller(f) File defragmenter(g) Backup utility(h) Screen saver(i) Anti-virus program(j) Firewall

(3) Driver programsA driver program or device driver, is a small piece of add-on codes designed to provide information about a device to the operating system. Once installed, the driver program will be become part of the OS. The OS controls the device by using the specialized commands provided by the device driver. Each device, such as printer, disk drive, or interface card must have a driver program which may exist in the OS.

Applications Software

Application software is a program designed to solve particular problems for users. It enables users to perform work on specific tasks and facilitate communications. In general, application software can be classified into productivity software, graphic and multimedia software, software for home, personal and educational use, and communication software.(1) Productivity Software

Productivity software, also called business software, help people increase effectiveness and efficiency of routine clerical work like typing, calculation, charting, record handling, presentation and scheduling. Productivity software includes the software for word processing, spreadsheet, accounting, database, presentation and desktop publishing etc.

(2) Graphic and Multimedia softwareGraphics and multimedia software are designed to assist people in creating and editing graphics, animation, sound and video. This type of software includes computer-aided design, desktop publishing, vector/bitmap graphics editing, audio/video editing, multimedia authoring and web authoring etc.

(3) Home, Personal and Educational SoftwareSoftware for home users includes home design, tax preparation and entertainment etc. Software for personal use includes personal finance, legal, and personal information manager etc. Software for educational use include CAL (computer-aided learning), CBT (computer-based training), clip art/image gallery and encyclopedia etc.

(4) Communication softwareCommunication software enables people to communicate and share information with others. This type of software includes e-mail client, web browsers, instant messaging software and FTP programs etc.

Custom-made application software is a program written by users themselves or the software-houses. Advantage of writing custom-made software: Programming time can be saved Software is tailor-made and easy to modifyDisadvantage of custom-made software: Higher building cost

Software Licenses

A software license is a document that permits authorized users to use the software. It does not mean selling the copyright to you. A license for individuals allows you to install the software on one

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computer only. A site license means that the piece of software is accessible from all the computers within the organization.(1) Commercial software

All paid commercial software package, like word processing and graphics applications, come with a license. You should read and observe the terms of the license, which normally allows installing the package on only one computer. The license and the associated license number should be kept safely for future reference.

(2) SharewareA piece of shareware is software distributed free on a trial basis, but you need to pay for it later. Some shareware has a built-in expiration date, while others are limited version with certain capabilities disabled. For instance, it does not allow you to save your work.

(3) FreewareA piece of freeware is software distributed without charge. However, it is copyrighted meaning that the ownership is still retained by the author. Also, you cannot modify the program or develop a new program by including it as a component.

(4) Open source code SoftwareOpen source code software refers to a program in which the source code is available to the general public for use and/or modification from its original design without any charge. A piece of such software is uncopyrighted because its authors intend to share it with everyone else. You can reuse such code in developing new programs. Examples are the software in the Linux community.

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