ringkasan alpro: Overview of Computers and Programming

Overview of Computers and Programming

1.1 Electronic Computers Then and Now

The first electronic computer was built in the late 1930s by Dr. John Atanasoff and Clifford Berry at Iowa State University. Atanasoff designed his computer to assist graduate students in nuclear physics with their mathematical computations. The first large-scale, general-purpose electronic digital computer, called the ENIAC, was completed in 1946 at the University of Pennsylvania with funding from the U.S. Army. Weighing 30 tons and occupying a 30-by-50-foot space, the ENIAC was used to compute ballistics tables, predict the weather, and make atomic energy calculations. These early computers used vacuum tubes as their basic electronic component. Technological advances in the design and manufacture of electronic components led to new generations of computers that were considerably smaller, faster, and less expensive than previous ones.

Using today’s technology, the entire circuitry of a computer processor can be packaged in a single electronic component called a computer or microprocessor chip, which is less than one-fourth the size of a standard postage stamp. Their affordability and small size enable computer chips to be installed in watches, cellphones, GPS systems, cameras, home appliances, automobiles, and, of course, computers. Today, a common sight in offices and homes is a personal computer, which can cost less than $1000 and sit on a desk and yet has as much computational power as one that 40 years ago cost more than $100,000 and filled a 9-by-12-foot room. Even smaller computers can fit inside a briefcase or purse or your hand. Modern computers are categorized according to their size and performance. Personal computers  are used by a single person at a time. Large real-time transaction processing systems, such as ATMs and other banking networks, and corporate reservations systems for motels, airlines, and rental cars use mainframes , very powerful and reliable computers. The largest capacity and fastest computers are called supercomputers and are used by research laboratories and in

computationally intensive applications such as weather forecasting.

The elements of a computer system fall into two major categories: hardware and software. Hardware is the equipment used to perform the necessary computations and includes the central processing unit (CPU), monitor, keyboard, mouse, printer, and speakers. Software consists of the programs that enable us to solve problems with a computer by providing it with lists of instructions to perform.

Programming a computer has undergone significant changes over the years. Initially, the task was very difficult, requiring programmers to write their program instructions as long binary numbers (sequences of 0s and 1s). High-level programming languages such as C make programming much easier.

1.2 Computer Hardware

Despite significant variations in cost, size, and capabilities, modern computers resemble one another in many basic ways. Essentially, most consist of the following components:

■   Main memory

■   Secondary memory, which includes storage devices such as hard disks, CDs, DVDs, and flash drives

■   Central processing unit

■   Input devices, such as keyboards, mouses, touch pads, scanners, joysticks

■   Output devices, such as monitors, printers, and speakers

Memory

Memory is an essential component in any computer. Let’s look at what it consists of and how the computer works with it.

Anatomy of Memory Imagine the memory of a computer as an ordered sequence of storage locations called memory. To store and access information, the computer must have some way of identifying the individual memory cells. Therefore, each memory cell has a unique address that indicates its relative position in memory. Figure 1.4 shows a computer memory consisting of 1000 memory cells with addresses 0 through 999. Most computers, however, have millions of individual memory cells, each with its own address. The data stored in a memory cell are called the contents of the cell. Every memory cell always has some contents, although we may have no idea what they are. In a memory cell can also contain a program instruction. The ability to store programs as well as data is called the stored

program concept : A program’s instructions must be stored in main memory before they can be executed. We can change the computer’s operation by storing a different program in memory.

Bytes and Bits A memory cell is actually a grouping of smaller units called bytes.

Storage and Retrieval of Information in Memory Each value in memory is represented by a particular pattern of 0s and 1s. A computer can either store a value or retrieve a value. To store a value, the computer sets each bit of a selected memory cell to either 0 or 1, destroying the previous contents of the cell in the

process. To retrieve a value from a memory cell, the computer copies the pattern of 0s and 1s stored in that cell to another storage area for processing; the copy operation does not destroy the contents of the cell whose value is retrieved. This process is the same regardless of the kind of information—character, number, or

program instruction—to be stored or retrieved.

Main Memory Main memory stores programs, data, and results. Most computers have two types of main memory: random access memory (RAM) , which offers temporary storage of programs and data, and read-only memory (ROM) , which stores programs or data permanently. RAM temporarily stores programs while they are being executed (carried out) by the computer. It also temporarily stores such data as numbers, names, and even pictures while a program is manipulating them.

RAM is usually volatile memory , which means that everything in RAM will be lost when the computer is switched off.

ROM, on the other hand, stores information permanently within the computer.

The computer can retrieve (or read), but cannot store (or write) information in ROM, hence its name, read-only. Because ROM is not volatile, the data stored there do not disappear when the computer is switched off. Start-up instructions and other critical instructions are burned into ROM chips at the factory. When we refer to

main memory in this text, we mean RAM because that is the part of main memory that is normally accessible to the programmer.

Secondary Storage Devices Computer systems provide storage in addition to main memory for two reasons. First, computers need storage that is permanent or semipermanent so that information can be retained during a power loss or when the computer is turned off. Second, systems typically store more information than will

fit in memory.

Secondary storage devices and storage media. Most personal computers use two types of disk drives as their secondary storage devices—hard drives and optical drives. Hard disks are attached to their disk drives and are coated with a magnetic material. Each data bit is a magnetized spot on the disk, and the spots are arranged in concentric circles called tracks. The disk drive read/write head accesses data by moving across the spinning disk to the correct track and then sensing the spots as they move by. The hard disks in personal computers usually hold several hundred gigabytes (GB) of

data, but clusters of hard drives that store data for an entire network may provide as much as several terabytes (TB) of storage (see Table 1.1 ).

Most of today’s personal computers are equipped with optical drives for storing and retrieving data on compact disks (CDs) or digital versatile disks (DVDs) that can be removed from the drive. A CD is a silvery plastic platter on which a laser records data as a sequence of tiny pits in a spiral track on one side of the disk. One CD can hold 680 MB of data. A DVD uses smaller pits packed in a tighter spiral,

allowing storage of 4.7 GB of data on one layer. Some DVDs can hold four layers of data—two on each side—for a total capacity of 17 GB, sufficient storage for as much as nine hours of studio-quality video and multi-channel audio.

Flash drives such as the one pictured in Fig. 1.6 use flash memory packaged in small plastic cases about three inches long that can be plugged into any of a computer’s USB (Universal Serial Bus) ports. Unlike hard drives and optical drives that must spin their disks for access to data, flash drives have no moving parts and all data transfer is by electronic signal only. In flash memory, bits are represented as electrons trapped in microscopic chambers of silicon dioxide. Typical USB flash drives store 1 to several GB of data, but 64-GB drives are also available.

Information stored on a disk is organized into separate collections called files.

One file may contain a C program. Another file may contain the data to be processed by that program (a data file ). A third file could contain the results generated by a program (an output file ). The names of all files stored on a disk are listed in the disk’s directory . This directory may be broken into one or more levels of subdirectories or folders, where each subdirectory stores the names of files that relate to the same general topic. For example, you might have separate subdirectories of files that contain homework assignments and programs for each course you are

taking this semester. The details of how files are named and grouped in directories vary with each computer system. Follow the naming conventions that apply to your system.

Central Processing Unit

The central processing unit (CPU) has two roles: coordinating all computer operations and performing arithmetic and logical operations on data. The CPU follows the instructions contained in a computer program to determine which operations should be carried out and in what order. It then transmits coordinating control signals to the other computer components. For example, if the instruction

requires scanning a data item, the CPU sends the necessary control signals to the input device.

To process a program stored in main memory, the CPU retrieves each instruction in sequence (called fetching an instruction ), interprets the instruction to determine what should be done, and then retrieves any data needed to carry out that instruction. Next, the CPU performs the actual manipulation, or processing, of the data it retrieved. The CPU stores the results in main memory.

The CPU can perform such arithmetic operations as addition, subtraction, multiplication, and division. The CPU can also compare the contents of two memory cells (for example, Which contains the larger value? Are the values equal?) and make decisions based on the results of that comparison. The circuitry of a modern CPU is housed in a single integrated circuit or chip, millions of miniature circuits manufactured in a sliver of silicon. An integrated circuit

(IC) that is a full central processing unit is called a microprocessor. A CPU’s current instruction and data values are stored temporarily inside the CPU in special high-speed memory locations called registers.

Some computers have multiple CPUs ( multiprocessors ) or a multi-core CPU. These computers are capable of faster speeds because they can process different sets of instructions at the same time.

Input/Output Devices

We use input/output (I/O) devices to communicate with the computer. Specifically, they allow us to enter data for a computation and to observe the results of that computation. You will be using a keyboard as an input device and a monitor (display screen) as an

output device. When you press a letter or digit key on a keyboard, that character is sent to main memory and is also displayed on the monitor at the position of the cursor , a moving place marker (often a blinking line or rectangle). A computer keyboard has keys for letters, numbers, and punctuation marks plus some extra keys for performing special functions. The twelve function keys along the top row of the keyboard are labeled F1 through F12. The activity performed when you press a function key depends on the program

currently being executed; that is, pressing F1 in one program will usually not produce the same results as pressing F1 in another program. Other special keys enable you to delete characters, move the cursor, and “enter” a line of data you typed at the keyboard.

Another common input device is a mouse. A mouse is a handheld device used to select an operation. Moving the mouse around on your desktop moves the mouse cursor (normally a small rectangle or an arrow) displayed on the monitor’s screen.

You select an operation by moving the mouse cursor to a word or icon (picture) that represents the computer operation you wish to perform and then pressing a mouse button to activate the operation selected.

A monitor provides a temporary display of the information that appears on its screen. If you want hard copy (a printed version) of some information, you must send that information to an output device called a printer .

Computer Networks

The explosion we are experiencing in worldwide information access is primarily due to the fact that computers are now linked together in networks so they can communicate with one another. In a local area network (LAN), computers and other devices in a building are connected by cables or a wireless network, allowing them to share information and resources such as printers, scanners, and secondary storage devices ( Fig. 1.7 ). A computer that controls access to a secondary storage device such as a large hard disk is called a file server .

Local area networks can be connected to other LANs using the same technology as telephone networks. Communications over intermediate distances use phone lines, fiber-optics cables or wireless technology, and long-range communications use either phone lines or microwave signals that may be relayed by satellite.

A network that links many individual computers and local area networks over a large geographic area is called a wide area network (WAN). The most well-known WAN is the Internet, a network of university, corporate, government, and publicaccess networks. The Internet is a descendant of the computer network designed

by the U.S. Defense Department’s 1969 ARPAnet project. The goal of the project was to create a computer network that could continue to operate even if partially destroyed. The most widely used aspect of the Internet is the World Wide Web (WWW), the universe of Internet-accessible resources that are navigable through

the use of a graphical user interface (GUI) .

If you have a computer with a modem, you can connect to the information superhighway through a telephone line, television or fiber-optic cable, or through wireless or satellite communications. A modem ( mo dulator/ dem odulator) converts binary computer data into audio tones that can be transmitted to another computer

over a normal telephone circuit. At the computer on the receiving end, another modem converts the audio tones back to binary data.

Early modems for telephone lines transmitted at only 300 baud (300 bits per second). Today’s modems transmit over 50,000 bits per second, or if you have a digital subscriber line ( DSL connection ) or fiber-optics telephone line, the associated modem can transmit 1.5 million bits per second (DSL) or a few billion bits per second (fiber optics) while allowing you to use the same line simultaneously for voice calls. Cable Internet access brings Internet data to your computer at speeds of several billion bits per second using the same coaxial cable that carries cable TV.

Wireless and satellite communications provide data speeds comparable to cable.

1.3 Computer Software

Operating System

The collection of computer programs that control the interaction of the user and the computer hardware is called the operating system (OS) . The operating system of a computer is often compared to the conductor of an orchestra, for it is the software that is responsible for directing all computer operations and managing all computer resources. Usually part of the operating system is stored permanently in a read-only memory (ROM) chip so that it is available as soon as the computer is turned on. A computer can look at the values in read-only memory, but cannot write new values to the chip. The ROM-based portion of the OS contains the instructions necessary for loading into memory the rest of the operating system

code, which typically resides on a disk. Loading the operating system into memory is called booting the computer .

Here is a list of some of the operating system’s many responsibilities:

1. Communicating with the computer user: receiving commands and carrying them out or rejecting them with an error message.

2. Managing allocation of memory, of processor time, and of other resources for various tasks.

3. Collecting input from the keyboard, mouse, and other input devices, and providing this data to the currently running program.

4. Conveying program output to the screen, printer, or other output device.

5. Accessing data from secondary storage.

6. Writing data to secondary storage.

In addition to these responsibilities, the operating system of a computer with multiple users must verify each individual’s right to use the computer and must ensure that each user can access only data for which he or she has proper authorization.

Table 1.2 lists some widely used operating systems. An OS that uses a command-line interface displays a brief message, called a prompt, that indicates its readiness to receive input, and the user then types a command at the keyboard.

Application Software

Application programs are developed to assist a computer user in accomplishing specific tasks. For example, a word-processing application such as Microsoft Word or OpenOffice.org Writer helps to create a document, a spreadsheet application such as Microsoft Office Excel helps to automate tedious numerical calculations and

to generate charts that depict data, and a database management application such as Microsoft Office Access or dBASE assists in data storage and quick keyword-based access to large collections of records.

Computer users typically purchase application software on CDs or by downloading files from the Internet and install the software by copying the programs to the hard disk. When buying software, you must always check that the program you are purchasing is compatible with both the operating system and the computer

hardware you plan to use. We have already discussed some of the differences among operating systems; now we will investigate the different languages understood by different processors.

Computer Languages

Developing new software requires writing lists of instructions for a computer to execute. However, software developers rarely write in the language directly understood by a computer, since this machine language is a collection of binary numbers.

Another drawback of machine language is that it is not standardized: There is a different machine language for every type of CPU. This same drawback also applies to the somewhat more readable assembly language , a language in which computer operations are represented by mnemonic codes rather than binary numbers and

variables can be given names rather than binary memory addresses. Table 1.3 shows a tiny machine language program fragment that adds two numbers and the equivalent fragment in assembly language. Notice that each assembly language instruction corresponds to exactly one machine instruction: The assembly language memory cells labeled A and B are space for variables; they are not instructions.

The symbol indicates that we do not know the contents of the memory cells with addresses 00000101 and 00000110.

To write programs that are independent of the CPU on which they will be executed, software designers use high-level languages that combine algebraic expressions and symbols taken from English. For example, the machine/assembly language program fragment shown in Table 1.3 would be a single statement in a high-level language:

a = a + b;

This statement means “add the values of variables a and b , and store the result in variable a (replacing a ’s previous value).”

There are many high-level languages available. Table 1.4 lists some of the most widely used ones along with the origin of their names and the application areas that first popularized them. Although programmers find it far easier to express problem solutions in high-level languages, there remains the problem that computers do NOT understand these languages. Thus, before a high-level language program can be executed, it must first be translated into the target computer’s machine language. The program that does this translation is called a compiler.

Figure 1.11 illustrates the role of the compiler in the process of developing and testing a high-level language program. Both the input to and the output from the compiler (when it is successful) are programs. The input to the compiler is a source file containing the text of a high-level language program. The software developer creates this file by using a word processor or editor. The format of the source file is text, which means that it is a collection of character codes. For example, you might type a program into a file called myprog.c. The compiler will scan this source file, checking the program to see if it follows the high-level language’s syntax (grammar) rules. If the program is syntactically correct, the compiler saves in an object file the machine language instructions that carry out the program’s purpose. For program myprog.c , the object file created might be named myprog. obj . Notice that this file’s format is binary. This means that you should not send

it to a printer, display it on your monitor, or try to work with it in a word processor because it will appear to be meaningless garbage to a word processor, printer, or monitor. If the source program contains syntax errors, the compiler lists these errors but does not create an object file. The developer must return to the word processor, correct the errors, and recompile the program.

Although an object file contains machine instructions, not all of the instructions are complete. High-level languages provide the software developer with many named chunks of code for operations that the developer will likely need. Almost all high-level language programs use at least one of these chunks of code called functions

that reside in other object files available to the system. The linker program combines these prefabricated functions with the object file, creating a complete machine language program that is ready to run. For your sample program, the linker might name the executable file it creates myprog.exe .

As long as myprog.exe is just stored on your disk, it does nothing. To run it, the loader must copy all its instructions into memory and direct the CPU to begin execution with the first instruction. As the program executes, it takes input data from one or more sources and sends results to output and/or secondary storage devices.

Some computer systems require the user to ask the OS to carry out separatelyeach step illustrated in Fig. 1.11 . However, most high-level language compilers are sold as part of an integrated development environment (IDE) , a package that combines a simple word processor with a compiler, linker, and loader. Such environments

give the developer menus from which to select the next step, and if the developer tries a step that is out of sequence, the environment simply fills in the missing steps automatically.

The user of an integrated development environment should be aware that the environment may not automatically save to disk the source, object, and executable files. Rather, it may simply leave these versions of the program in memory. Such an approach saves the expenditure of time and disk space needed to make copies and keeps the code readily available in memory for application of the next step in the translation/execution process. However, the developer

can risk losing the only copy of the source file in the event of a power outage or serious program error. To prevent such a loss when using an IDE, be sure to explicitly save the source file to disk after every modification before attempting to run the program.

Executing a Program

To execute a machine language program, the CPU must examine each program instruction in memory and send out the command signals required to carry out the instruction. Although the instructions normally are executed in sequence, as we will discuss later, it is possible to have the CPU skip over some instructions or execute

some instructions more than once.

During execution, data can be entered into memory and manipulated in some specified way. Special program instructions are used for entering or scanning a program’s data (called input data ) into memory. After the input data have been processed, instructions for displaying or printing values in memory can be executed

to display the program results. The lines displayed by a program are called the program output .

1.4 The Software Development Method

Software Development Method

1. Specify the problem requirements.

2. Analyze the problem.

3. Design the algorithm to solve the problem.

4. Implement the algorithm.

5. Test and verify the completed program.

6. Maintain and update the program.

PROBLEM

Specifying the problem requirements forces you to state the problem clearly and unambiguously and to gain a clear understanding of what is required for its solution.

Your objective is to eliminate unimportant aspects and zero in on the root problem.

This goal may not be as easy to achieve as it sounds. You may find you need more information from the person who posed the problem.

ANALYSIS

Analyzing the problem involves identifying the problem (a) inputs, that is, the data you have to work with; (b) outputs , that is, the desired results; and (c) any additional requirements or constraints on the solution. At this stage, you should also determine the required format in which the results should be displayed (for example, as a table with specific column headings) and develop a list of problem variables and their relationships. These relationships may be expressed as formulas.

If steps 1 and 2 are not done properly, you will solve the wrong problem. Read the problem statement carefully, first, to obtain a clear idea of the problem and second, to determine the inputs and outputs. You may find it helpful to underline phrases in the problem statement that identify the inputs and outputs, as in the

problem statement below.

Computer and display the total cost of apples given the number of pounds of apples purchased and the cost per pound of apples .

Next, summarize the information contained in the underlined phrases:

Problem Inputs

quantity of apples purchased (in pounds)

cost per pound of apples (in dollars per pound)

Problem Output

total cost of apples (in dollars)

Once you know the problem inputs and outputs, develop a list of formulas that specify relationships between them. The general formula

Total cost _ Unit cost _ Number of units

computes the total cost of any item purchased. Substituting the variables for our particular problem yields the formula

Total cost of apples _ Cost per pound _ Pounds of apples

In some situations, you may need to make certain assumptions or simplifications to derive these relationships. This process of modeling a problem by extracting the essential variables and their relationships is called abstraction .

DESIGN

Designing the algorithm to solve the problem requires you to develop a list of steps called an algorithm to solve the problem and to then verify that the algorithm solves the problem as intended. Writing the algorithm is often the most difficult part of the problem-solving process. Don’t attempt to solve every detail of the problem at the beginning; instead, discipline yourself to use top-down design. In

top-down design (also called divide and conquer ), you first list the major steps, or subproblems, that need to be solved. Then you solve the original problem by solving each of its subproblems. Most computer algorithms consist of at least the following subproblems.

ALGORITHM FOR A PROGRAMMING PROBLEM

1. Get the data.

2. Perform the computations.

3. Display the results.

Once you know the subproblems, you can attack each one individually. For example, the perform-the-computations step may need to be broken down into a more detailed list of steps through a process called stepwise refinement .

You may be familiar with top-down design if you use an outline when writing a term paper. Your first step is to create an outline of the major topics, which you then refine by filling in subtopics for each major topic. Once the outline is complete, you begin writing the text for each subtopic. Desk checking is an important part of algorithm design that is often overlooked.

To desk check an algorithm, you must carefully perform each algorithm step (or its refinements) just as a computer would and verify that the algorithm works as intended. You’ll save time and effort if you locate algorithm errors early in the problem-solving process.

IMPLEMENTATION

Implementing the algorithm (step 4 in the software development method) involves writing it as a program. You must convert each algorithm step into one or more statements in a programming language.

TESTING

Testing and verifying the program requires testing the completed program to verify that it works as desired. Don’t rely on just one test case. Run the program several times using different sets of data to make sure that it works correctly for every situation

provided for in the algorithm.

MAINTENANCE

Maintaining and updating the program involves modifying a program to remove previously undetected errors and to keep it up-to-date as government regulations or company policies change. Many organizations maintain a program for five years or more, often after the programmers who originally coded it have left or moved on

to other positions.

A disciplined approach is essential if you want to create programs that are easy to read, understand, and maintain. You must follow accepted program style guidelines (which will be stressed in this book) and avoid tricks and programming shortcuts.

1.5 Applying the Software Development Method

We walk you through a sample case study next. This example includes a running commentary on the process, which you can use as a model in solving other problems.

CASE STUDY

Converting Miles to Kilometers

PROBLEM

Your summer surveying job requires you to study some maps that give distances in kilometers and some that use miles. You and your coworkers prefer to deal in metric measurements. Write a program that performs the necessary conversion.

ANALYSIS

The first step in solving this problem is to determine what you are asked to do. You must convert from one system of measurement to another, but are you supposed to convert from kilometers to miles, or vice versa? The problem states that you prefer

to deal in metric measurements, so you must convert distance measurements in miles to kilometers. Therefore, the problem input is distance in miles and the problem output is distance in kilometers . To write the program, you need to know the relationship

between miles and kilometers. Consulting a metric table shows that one mile equals 1.609 kilometers.

The data requirements and relevant formulas are listed below. miles identifies the memory cell that will contain the problem input and kms identifies the memory cell that will contain the problem input and kms identifies the memory cell that will contain the program result, or the problem output.

DATA REQUIREMENTS

Problem Input

miles /* the distance in miles*/

Problem Output

kms /* the distance in kilometers */

Relevant Formula

1 mile = 1.609 kilometers

DESIGN

Next, formulate the algorithm that solves the problem. Begin by listing the three major steps, or subproblems, of the algorithm.

ALGORITHM

1. Get the distance in miles.

2. Convert the distance to kilometers.

3. Display the distance in kilometers.

Now decide whether any steps of the algorithm need further refinement or whether they are perfectly clear as stated. Step 1 (getting the data) and step 3 (displaying a value) are basic steps and require no further refinement. Step 2 is fairly straightforward,

but some detail might help:

Step 2 Refinement

2.1 The distance in kilometers is 1.609 times the distance in miles.

We list the complete algorithm with refinements below to show you how it all fits together. The algorithm resembles an outline for a term paper. The refinement of step 2 is numbered as step 2.1 and is indented under step 2.

ALGORITHM WITH REFINEMENTS

1. Get the distance in miles.

2. Convert the distance to kilometers.

2.1 The distance in kilometers is 1.609 times the distance in miles.

3. Display the distance in kilometers.

Let’s desk check the algorithm before going further. If step 1 gets a distance of 10.0 miles, step 2.1 would convert it to 1.609 _ 10.00 or 16.09 kilometers. This correct result would be displayed by step 3.

IMPLEMENTATION

To implement the solution, you must write the algorithm as a C program. To do this, you must first tell the C compiler about the problem data requirements—that is, what memory cell names you are using and what kind of data will be stored in each memory cell. Next, convert each algorithm step into one or more C statements.

If an algorithm step has been refined, you must convert the refinements, not the original step, into C statements.

Figure 1.13 shows the C program along with a sample execution or run. For easy identification, the program statements corresponding to algorithm steps are in color as is the input data typed in by the program user. Don’t worry about understanding the details of this program yet. We explain the program in the next chapter.

TESTING

How do you know the sample run is correct? You should always examine program results carefully to make sure that they make sense. In this run, a distance of 10.0 miles is converted to 16.09 kilometers, as it should be. To verify that the program

works properly, enter a few more test values of miles. You don’t need to try more than a few test cases to verify that a simple program like this is correct.

1.6 Professional Ethics for Computer Programmers

Privacy and Misuse of Data

As part of their jobs, programmers may have access to large data banks or databases containing sensitive information on financial transactions or personnel, or information that is classified as “secret” or “top secret.” Programmers should always behave

in a socially responsible manner and not retrieve information that they are not entitled to see. They should not use information to which they are given access for their own personal gain, or do anything that would be considered illegal, unethical, or harmful to others. Just as doctors and lawyers must keep patient information confidential, programmers must respect an individual’s rights to privacy.

A programmer who changes information in a database containing financial records for his or her own personal gain—for example, changes the amount of money in a bank account—is guilty of computer theft or computer fraud . This is a felony that can lead to fines and imprisonment.

Computer Hacking

You may have heard about “computer hackers” who break into secure data banks by using their own computer to call the computer that controls access to the data bank. Classified or confidential information retrieved in this way has been sold to intelligence agencies of other countries. Other hackers have tried to break into computers to retrieve information for their own amusement or as

a prank, or just to demonstrate that they can do it. Regardless of the intent, this activity is illegal, and the government will prosecute anyone who does it. Your university probably addresses this kind of activity in your student handbook.

The punishment is likely similar to that for other criminal activity, because that is exactly what it is.

Another illegal activity sometimes practiced by hackers is attaching harmful code, called a virus , to another program so that the virus code copies itself throughout a computer’s disk memory. A virus can cause sporadic activities to disrupt the operation of the host computer—for example, unusual messages may appear on the

screen at certain times—or cause the host computer to erase portions of its own disk memory, destroying valuable information and programs. Viruses are spread from one computer to another in various ways—for example, if you copy a file that originated

on another computer that has a virus, or if you open an e-mail message that is sent from an infected computer. A computer worm is a virus that can replicate itself on other network computers, causing these computers to send multiple messages over the network to disrupt its operation or shut it down. Certainly, data theft

and virus propagation should not be considered harmless pranks; they are illegal and carry serious penalties.

Plagiarism and Software Piracy

Using someone else’s programs without permission is also unprofessional behavior.

Although it is certainly permissible to use modules from libraries that have been developed for reuse by their own company’s programmers, you cannot use another programmer’s personal programs or programs from another company without getting

permission beforehand. Doing so could lead to a lawsuit, with you or your company having to pay damages.

Modifying another student’s code and submitting it as your own is a fraudulent practice—specifically, plagiarism—and is no different than copying paragraphs of information from a book or journal article and calling it your own. Most universities have severe penalties for plagiarism that may include failing the course and/or being dismissed from the university. Be aware that even if you modify the code slightly or substitute your own comments or different variable names, you are still guilty of plagiarism if you are using another person’s ideas and code. To avoid any question of

plagiarism, find out beforehand your instructor’s rules about working with others on a project. If group efforts are not allowed, make sure that you work independently and submit only your own code.

Many commercial software packages are protected by copyright laws against software piracy —the practice of illegally copying software for use on another computer. If you violate this law, your company or university can be fined heavily for allowing this activity to occur. Besides the fact that software piracy is against the

law, using software copied from another computer increases the possibility that your computer will receive a virus. For all these reasons, you should read the copyright restrictions on each software package and adhere to them.

Misuse of a Computer Resource

Computer system access privileges or user account codes are private property.

These privileges are usually granted for a specific purpose—for example, for work to be done in a particular course or for work to be done during the time you are a student at your university. The privilege should be protected; it should not be loaned to

or shared with anyone else and should not be used for any purpose for which it was not intended. When you leave the institution, this privilege is normally terminated and any accounts associated with the privilege will be closed.

Computers, computer programs, data, and access (account) codes are like any other property. If they belong to someone else and you are not explicitly given permission to use them, then do not use them. If you are granted a use privilege for a specific purpose, do not abuse the privilege or it will be taken away.

Legal issues aside, it is important that we apply the same principles of right and wrong to computerized property and access rights as to all other property rights and privileges. If you are not sure about the propriety of something you want to do, ask first. As students and professionals in computing, we set an example for others. If

we set a bad example, others are sure to follow.

Chapter Review

1. The basic components of a computer are main memory and secondary storage, the CPU, and input and output devices.

2. All data manipulated by a computer are represented digitally, as base 2 numbers composed of strings of the digits 0 and 1.
3. Main memory is organized into individual storage locations called memory cells.

■   Each memory cell has a unique address.

■   A memory cell is a collection of bytes; a byte is a collection of 8 bits.

■   A memory cell is never empty, but its initial contents may be meaningless to your program.

■   The current contents of a memory cell are destroyed whenever new information is stored in that cell.

■   Programs must be loaded into the memory of the computer before they can be executed.

■   Data cannot be manipulated by the computer until they are first stored in memory.

4. Information in secondary storage is organized into files: program files and data files. Secondary storage provides a low-cost means of storing large quantities of information in semipermanent form.

5. A CPU runs a computer program by repeatedly fetching and executing simple machine-code instructions.

6. Connecting computers in networks allows sharing of resources—local resources on LANs and worldwide resources on a WAN such as the Internet.

7. Programming languages range from machine language (meaningful to a computer) to high-level language (meaningful to a programmer).

8. Several system programs are used to prepare a high-level language program for execution. An editor enters a high-level language program into a file. A compiler translates a high-level language program (the source program) into machine language (the object program). The linker links this object program to other object files, creating an executable file, and the loader loads the executable file into memory. All of these programs are combined in an integrated development environment (IDE).

9. Through the operating system, you can issue commands to the computer and manage files.

10. Follow the first five steps of the software development method to solve programming problems: (1) specify the problem, (2) analyze the problem, (3) design the algorithm, (4) implement the algorithm, and (5) test and verify the completed program. Write programs in a consistent style that is easy to read, understand, and maintain.

11. Follow ethical standards of conduct in everything you do pertaining to computers. This means do not copy software that is copyright protected, do not hack into someone else’s computer, do not send files that may be infected to others,

and do not submit someone else’s work as your own or lend your work to another student.