lecture 1

1
1.1 Introduction
Think about some of the different ways that people use computers. In school, students use computers
for tasks such as writing papers, searching for articles, sending email, and participating in
online classes. At work, people use computers to analyze data, make presentations, conduct business
transactions, communicate with customers and coworkers, control machines in manufacturing
facilities, and do many other things. At home, people use computers for tasks such as paying
bills, shopping online, communicating with friends and family, and playing computer games.
And don’t forget that cell phones, iPods®, BlackBerries®, car navigation systems, and many
other devices are computers too. The uses of computers are almost limitless in our everyday lives.
Computers can do such a wide variety of things because they can be programmed. This means
that computers are not designed to do just one job, but to do any job that their programs tell
them to do. A program is a set of instructions that a computer follows to perform a task. For
example, Figure 1-1 shows screens from two commonly used programs, Microsoft Word and
Adobe Photoshop. Microsoft Word is a word processing program that allows you to create,
edit, and print documents with your computer. Adobe Photoshop is an image editing program
that allows you to work with graphic images, such as photos taken with your digital camera.
Programs are commonly referred to as software. Software is essential to a computer because
it controls everything the computer does. All of the software that we use to make our computers
useful is created by individuals working as programmers or software developers. A
programmer, or software developer, is a person with the training and skills necessary to
design, create, and test computer programs. Computer programming is an exciting and
rewarding career. Today, you will find programmers’ work used in business, medicine, government,
law enforcement, agriculture, academics, entertainment, and many other fields.
Introduction to Computers
and Programming 1
TOPICS
1.1 Introduction
1.2 Hardware and Software
1.3 How Computers Store Data
How a Program Works
L U C T U R E
1.4
2 Chapter 1 Introduction to Computers and Programming
Before we begin exploring those concepts, you need to understand a few basic things about
computers and how they work. This lecture will build a solid foundation of knowledge that
you will continually rely on as you study computer science. First, we will discuss the
physical components that computers are commonly made of. Next, we will look at how
computers store data and execute programs.
1.2 Hardware and Software
CONCEPT: The physical devices that a computer is made of are referred to as the
computer’s hardware. The programs that run on a computer are referred
to as software.
Hardware
The term hardware refers to all of the physical devices, or components, that a computer is made
of. A computer is not one single device, but a system of devices that all work together. Like the
different instruments in a symphony orchestra, each device in a computer plays its own part.
If you have ever shopped for a computer, you’ve probably seen sales literature listing components
such as microprocessors, memory, disk drives, video displays, graphics cards, and
so on. Unless you already know a lot about computers, or at least have a friend that does,
understanding what these different components do might be challenging. As shown in
Figure 1-2, a typical computer system consists of the following major components:
• The central processing unit (CPU)
• Main memory
• Secondary storage devices
• Input devices
• Output devices
Figure 1-1 A word processing program and an image editing program
1.2 Hardware and Software 3
Let’s take a closer look at each of these components.
The CPU
When a computer is performing the tasks that a program tells it to do, we say that the computer
is running or executing the program. The central processing unit, or CPU, is the part
of a computer that actually runs programs. The CPU is the most important component in
a computer because without it, the computer could not run software.
In the earliest computers, CPUs were huge devices made of electrical and mechanical
components such as vacuum tubes and switches. Figure 1-3 shows such a device.
The two women in the photo are working with the historic ENIAC computer. The
ENIAC, which is considered by many to be the world’s first programmable electronic
computer, was built in 1945 to calculate artillery ballistic tables for the U.S. Army. This
machine, which was primarily one big CPU, was 8 feet tall, 100 feet long, and weighed
30 tons.
Today, CPUs are small chips known as microprocessors. Figure 1-4 shows a photo of a lab
technician holding a modern microprocessor. In addition to being much smaller than the
old electromechanical CPUs in early computers, microprocessors are also much more
powerful.
Figure 1-2 Typical components of a computer system
Input
Devices
Output
Devices
Secondary
Storage Devices
Central Processing
Unit
Main Memory
(RAM)
4 Chapter 1 Introduction to Computers and Programming
Figure 1-3 The ENIAC computer (courtesy of U.S. Army Historic Computer Images)
Figure 1-4 A lab technician holds a modern microprocessor (photo courtesy of Intel
Corporation)
Main Memory
You can think of main memory as the computer’s work area. This is where the computer
stores a program while the program is running, as well as the data that the program is
working with. For example, suppose you are using a word processing program to write an
1.2 Hardware and Software 5
essay for one of your classes. While you do this, both the word processing program and the
essay are stored in main memory.
Main memory is commonly known as random-access memory, or RAM. It is called this
because the CPU is able to quickly access data stored at any random location in RAM.
RAM is usually a volatile type of memory that is used only for temporary storage while
a program is running. When the computer is turned off, the contents of RAM are
erased. Inside your computer, RAM is stored in chips, similar to the ones shown in
Figure 1-5.
Figure 1-5 Memory chips (photo courtesy of IBM Corporation)
Secondary Storage Devices
Secondary storage is a type of memory that can hold data for long periods of time, even
when there is no power to the computer. Programs are normally stored in secondary
memory and loaded into main memory as needed. Important data, such as word processing
documents, payroll data, and inventory records, is saved to secondary storage
as well.
The most common type of secondary storage device is the disk drive. A disk drive stores
data by magnetically encoding it onto a circular disk. Most computers have a disk drive
mounted inside their case. External disk drives, which connect to one of the computer’s
communication ports, are also available. External disk drives can be used to create backup
copies of important data or to move data to another computer.
In addition to external disk drives, many types of devices have been created for copying
data, and for moving it to other computers. For many years floppy disk drives were popular.
A floppy disk drive records data onto a small floppy disk, which can be removed from
the drive. Floppy disks have many disadvantages, however. They hold only a small amount
of data, are slow to access data, and can be unreliable. The use of floppy disk drives has
declined dramatically in recent years, in favor of superior devices such as USB drives. USB
drives are small devices that plug into the computer’s USB (universal serial bus) port, and
6 Chapter 1 Introduction to Computers and Programming
appear to the system as a disk drive. These drives do not actually contain a disk, however.
They store data in a special type of memory known as flash memory. USB drives, which are
also known as memory sticks and flash drives, are inexpensive, reliable, and small enough
to be carried in your pocket.
Optical devices such as the CD (compact disc) and the DVD (digital versatile disc) are also
popular for data storage. Data is not recorded magnetically on an optical disc, but is encoded
as a series of pits on the disc surface. CD and DVD drives use a laser to detect the pits and
thus read the encoded data. Optical discs hold large amounts of data, and because recordable
CD and DVD drives are now commonplace, they are good mediums for creating backup
copies of data.
Input Devices
Input is any data the computer collects from people and from other devices. The component
that collects the data and sends it to the computer is called an input device. Common
input devices are the keyboard, mouse, scanner, microphone, and digital camera. Disk
drives and optical drives can also be considered input devices because programs and data
are retrieved from them and loaded into the computer’s memory.
Output Devices
Output is any data the computer produces for people or for other devices. It might be a
sales report, a list of names, or a graphic image. The data is sent to an output device, which
formats and presents it. Common output devices are video displays and printers. Disk
drives and CD recorders can also be considered output devices because the system sends
data to them in order to be saved.
Software
If a computer is to function, software is not optional. Everything that a computer does,
from the time you turn the power switch on until you shut the system down, is under the
control of software. There are two general categories of software: system software and
application software. Most computer programs clearly fit into one of these two categories.
Let’s take a closer look at each.
System Software
The programs that control and manage the basic operations of a computer are generally
referred to as system software. System software typically includes the following types of
programs:
Operating Systems An operating system is the most fundamental set of programs on a
computer. The operating system controls the internal operations of the computer’s
hardware, manages all of the devices connected to the computer, allows data to be saved
to and retrieved from storage devices, and allows other programs to run on the computer.
Figure 1-6 shows screens from three popular operating systems: Windows Vista, Mac OS
X, and Linux.
1.2 Hardware and Software 7
Figure 1-6 Screens from the Windows Vista, Mac OS X, and Fedora Linux operating systems
Windows Vista Mac OS X
Fedora Linux
Utility Programs A utility program performs a specialized task that enhances the computer’s
operation or safeguards data. Examples of utility programs are virus scanners,
file compression programs, and data backup programs.
Software Development Tools Software development tools are the programs that programmers
use to create, modify, and test software. Assemblers, compilers, and interpreters
are examples of programs that fall into this category.
Application Software
Programs that make a computer useful for everyday tasks are known as application software.
These are the programs that people normally spend most of their time running on
their computers. Figure 1-1, at the beginning of this chapter, shows screens from two
commonly used applications: Microsoft Word, a word processing program, and Adobe
Photoshop, an image editing program. Some other examples of application software are
spreadsheet programs, email programs, web browsers, and game programs.
Checkpoint
1.1 What is a program?
1.2 What is hardware?
1.3 List the five major components of a computer system.
1.4 What part of the computer actually runs programs?
8 Chapter 1 Introduction to Computers and Programming
1.5 What part of the computer serves as a work area to store a program and its data
while the program is running?
1.6 What part of the computer holds data for long periods of time, even when there is
no power to the computer?
1.7 What part of the computer collects data from people and from other devices?
1.8 What part of the computer formats and presents data for people or other
devices?
1.9 What fundamental set of programs control the internal operations of the
computer’s hardware?
1.10 What do you call a program that performs a specialized task, such as a virus
scanner, a file compression program, or a data backup program?
1.11 Word processing programs, spreadsheet programs, email programs, web browsers,
and game programs belong to what category of software?
1.3 How Computers Store Data
CONCEPT: All data that is stored in a computer is converted to sequences of 0s
and 1s.
A computer’s memory is divided into tiny storage locations known as bytes. One byte is
only enough memory to store a letter of the alphabet or a small number. In order to do anything
meaningful, a computer has to have lots of bytes. Most computers today have millions,
or even billions, of bytes of memory.
Each byte is divided into eight smaller storage locations known as bits. The term bit stands
for binary digit. Computer scientists usually think of bits as tiny switches that can be either
on or off. Bits aren’t actual “switches,” however, at least not in the conventional sense. In
most computer systems, bits are tiny electrical components that can hold either a positive
or a negative charge. Computer scientists think of a positive charge as a switch in the on
position, and a negative charge as a switch in the off position. Figure 1-7 shows the way
that a computer scientist might think of a byte of memory: as a collection of switches that
are each flipped to either the on or off position.
Figure 1-7 Think of a byte as eight switches
OFF
ON
OFF OFF OFF
ON ON ON
1.3 How Computers Store Data 9
When a piece of data is stored in a byte, the computer sets the eight bits to an on/off pattern
that represents the data. For example, the pattern shown on the left in Figure 1-8
shows how the number 77 would be stored in a byte, and the pattern on the right shows
how the letter A would be stored in a byte. We explain below how these patterns are
determined.
Figure 1-8 Bit patterns for the number 77 and the letter A
The number 77 stored in a byte. The letter A stored in a byte.
OFF
ON
OFF OFF OFF
ON ON ON
OFF
ON
OFF OFF OFF OFF OFF
ON
1 0 0 1 1 1 0 1
20
21
22
23
24
25
26
27
Figure 1-9 The values of binary digits as powers of 2
Storing Numbers
A bit can be used in a very limited way to represent numbers. Depending on whether the
bit is turned on or off, it can represent one of two different values. In computer systems, a
bit that is turned off represents the number 0 and a bit that is turned on represents the number
1. This corresponds perfectly to the binary numbering system. In the binary numbering
system (or binary, as it is usually called) all numeric values are written as sequences of 0s
and 1s. Here is an example of a number that is written in binary:
10011101
The position of each digit in a binary number has a value assigned to it. Starting with the
rightmost digit and moving left, the position values are 20, 21, 22, 23, and so forth, as shown
in Figure 1-9. Figure 1-10 shows the same diagram with the position values calculated.
Starting with the rightmost digit and moving left, the position values are 1, 2, 4, 8, and so
forth.
10 Chapter 1 Introduction to Computers and Programming
128 + 16 + 8 + 4 + 1 = 157
128 64 32 16 8 4 2 1 Position
values
1
0
1
1 1 1
0 0
Figure 1-12 The bit pattern for 157
1 0 0 1 1 1 0 1
1
2
4
8
16
32
64
128
Figure 1-10 The values of binary digits
1 0 0 1 1 1 0 1
1
48
16
128
1 + 4 + 8 + 16 + 128 = 157
Figure 1-11 Determining the value of 10011101
To determine the value of a binary number you simply add up the position values of all the
1s. For example, in the binary number 10011101, the position values of the 1s are 1, 4, 8,
16, and 128. This is shown in Figure 1-11. The sum of all of these position values is 157.
So, the value of the binary number 10011101 is 157.
Figure 1-12 shows how you can picture the number 157 stored in a byte of memory. Each
1 is represented by a bit in the on position, and each 0 is represented by a bit in the off
position.
1.3 How Computers Store Data 11
When all of the bits in a byte are set to 0 (turned off), then the value of the byte is 0. When
all of the bits in a byte are set to 1 (turned on), then the byte holds the largest value that
can be stored in it. The largest value that can be stored in a byte is 1  2  4  8  16 
32  64  128  255. This limit exists because there are only eight bits in a byte.
What if you need to store a number larger than 255? The answer is simple: use more than
one byte. For example, suppose we put two bytes together. That gives us 16 bits. The position
values of those 16 bits would be 20, 21, 22, 23, and so forth, up through 215. As shown
in Figure 1-13, the maximum value that can be stored in two bytes is 65,535. If you need
to store a number larger than this, then more bytes are necessary.
32768 + 16384 + 8192 + 4096 + 2048 + 1024 + 512 + 256 + 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 65535
32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1 Position
values
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Figure 1-13 Two bytes used for a large number
TIP: In case you’re feeling overwhelmed by all this, relax! You will not have to actually
convert numbers to binary while programming. Knowing that this process is taking
place inside the computer will help you as you learn, and in the long term this
knowledge will make you a better programmer.
Storing Characters
Any piece of data that is stored in a computer’s memory must be stored as a binary number.
That includes characters, such as letters and punctuation marks. When a character is
stored in memory, it is first converted to a numeric code. The numeric code is then stored
in memory as a binary number.
Over the years, different coding schemes have been developed to represent characters in
computer memory. Historically, the most important of these coding schemes is ASCII,
which stands for the American Standard Code for Information Interchange. ASCII is a set
of 128 numeric codes that represent the English letters, various punctuation marks, and
other characters. For example, the ASCII code for the uppercase letter A is 65. When you
type an uppercase A on your computer keyboard, the number 65 is stored in memory (as a
binary number, of course). This is shown in Figure 1-14.
A 65 0 0
1
0
1
0 0 0
Figure 1-14 The letter A is stored in memory as the number 65
12 Chapter 1 Introduction to Computers and Programming
10010101110100010101101
Figure 1-15 A digital image is stored in binary format
TIP: The acronym ASCII is pronounced “askee.”
In case you are curious, the ASCII code for uppercase B is 66, for uppercase C is 67,
and so forth. Appendix C shows all of the ASCII codes and the characters they represent.
The ASCII character set was developed in the early 1960s, and was eventually adopted by
most all computer manufacturers. ASCII is limited however, because it defines codes for
only 128 characters. To remedy this, the Unicode character set was developed in the early
1990s. Unicode is an extensive encoding scheme that is compatible with ASCII, but can also
represent characters for many of the languages in the world. Today, Unicode is quickly
becoming the standard character set used in the computer industry.
Advanced Number Storage
Earlier you read about numbers and how they are stored in memory. While reading that
section, perhaps it occurred to you that the binary numbering system can be used to represent
only integer numbers, beginning with 0. Negative numbers and real numbers (such as
3.14159) cannot be represented using the simple binary numbering technique we discussed.
Computers are able to store negative numbers and real numbers in memory, but to do so
they use encoding schemes along with the binary numbering system. Negative numbers are
encoded using a technique known as two’s complement, and real numbers are encoded in
floating-point notation. You don’t need to know how these encoding schemes work, only
that they are used to convert negative numbers and real numbers to binary format.
Other Types of Data
Computers are often referred to as digital devices. The term digital can be used to describe
anything that uses binary numbers. Digital data is data that is stored in binary, and a digital
device is any device that works with binary data. In this section we have discussed how
numbers and characters are stored in binary, but computers also work with many other
types of digital data.
For example, consider the pictures that you take with your digital camera. These images
are composed of tiny dots of color known as pixels. (The term pixel stands for picture
element.) As shown in Figure 1-15, each pixel in an image is converted to a numeric code
that represents the pixel’s color. The numeric code is stored in memory as a binary number.
1.4 How a Program Works 13
The music that you play on your CD player, iPod or MP3 player is also digital. A digital
song is broken into small pieces known as samples. Each sample is converted to a binary
number, which can be stored in memory. The more samples that a song is divided into,
the more it sounds like the original music when it is played back. A CD quality song is
divided into more than 44,000 samples per second!
Checkpoint
1.12 What amount of memory is enough to store a letter of the alphabet or a small number?
1.13 What do you call a tiny “switch” that can be set to either on or off?
1.14 In what numbering system are all numeric values written as sequences of 0s and 1s?
1.15 What is the purpose of ASCII?
1.16 What encoding scheme is extensive enough to represent the characters of many of
the languages in the world?
1.17 What do the terms “digital data” and “digital device” mean?
1.4 How a Program Works
CONCEPT: A computer’s CPU can only understand instructions that are written in
machine language. Because people find it very difficult to write entire
programs in machine language, other programming languages have been
invented.
Earlier, we stated that the CPU is the most important component in a computer because it
is the part of the computer that runs programs. Sometimes the CPU is called the “computer’s
brain,” and is described as being “smart.” Although these are common metaphors, you
should understand that the CPU is not a brain, and it is not smart. The CPU is an electronic
device that is designed to do specific things. In particular, the CPU is designed to perform
operations such as the following:
• Reading a piece of data from main memory
• Adding two numbers
• Subtracting one number from another number
• Multiplying two numbers
• Dividing one number by another number
• Moving a piece of data from one memory location to another
• Determining whether one value is equal to another value
As you can see from this list, the CPU performs simple operations on pieces of data. The
CPU does nothing on its own, however. It has to be told what to do, and that’s the purpose
of a program. A program is nothing more than a list of instructions that cause the CPU to
perform operations.
Each instruction in a program is a command that tells the CPU to perform a specific operation.
Here’s an example of an instruction that might appear in a program:
10110000
14 Chapter 1 Introduction to Computers and Programming
To you and me, this is only a series of 0s and 1s. To a CPU, however, this is an instruction
to perform an operation.1 It is written in 0s and 1s because CPUs only understand instructions
that are written in machine language, and machine language instructions always have
an underlying binary structure.
A machine language instruction exists for each operation that a CPU is capable of performing.
For example, there is an instruction for adding numbers, there is an instruction for subtracting
one number from another, and so forth. The entire set of instructions that a CPU
can execute is known as the CPU’s instruction set.
1 The example shown is an actual instruction for an Intel microprocessor. It tells the microprocessor to move a
value into the CPU.
NOTE: There are several microprocessor companies today that manufacture CPUs.
Some of the more well-known microprocessor companies are Intel, AMD, and
Motorola. If you look carefully at your computer, you might find a tag showing a logo
for its microprocessor.
Each brand of microprocessor has its own unique instruction set, which is typically
understood only by microprocessors of the same brand. For example, Intel microprocessors
understand the same instructions, but they do not understand instructions
for Motorola microprocessors.
The machine language instruction that was previously shown is an example of only one
instruction. It takes a lot more than one instruction, however, for the computer to do
anything meaningful. Because the operations that a CPU knows how to perform are so
basic in nature, a meaningful task can be accomplished only if the CPU performs many
operations. For example, if you want your computer to calculate the amount of interest
that you will earn from your savings account this year, the CPU will have to
perform a large number of instructions, carried out in the proper sequence. It is not
unusual for a program to contain thousands or even millions of machine language
instructions.
Programs are usually stored on a secondary storage device such as a disk drive. When you
install a program on your computer, the program is typically copied to your computer’s disk
drive from a CD-ROM, or perhaps downloaded from a website.
Although a program can be stored on a secondary storage device such as a disk drive,
it has to be copied into main memory, or RAM, each time the CPU executes it. For
example, suppose you have a word processing program on your computer’s disk. To
execute the program you use the mouse to double-click the program’s icon. This causes
the program to be copied from the disk into main memory. Then, the computer’s CPU
executes the copy of the program that is in main memory. This process is illustrated in
Figure 1-16.
1.4 How a Program Works 15
When a CPU executes the instructions in a program, it is engaged in a process that is known
as the fetch-decode-execute cycle. This cycle, which consists of three steps, is repeated for
each instruction in the program. The steps are:
1. Fetch A program is a long sequence of machine language instructions. The first step of the
cycle is to fetch, or read, the next instruction from memory into the CPU.
2. Decode A machine language instruction is a binary number that represents a command
that tells the CPU to perform an operation. In this step the CPU decodes the
instruction that was just fetched from memory, to determine which operation it
should perform.
3. Execute The last step in the cycle is to execute, or perform, the operation.
Figure 1-17 illustrates these steps.
Main memory
(RAM)
Disk drive CPU
The program is copied
from secondary storage
to main memory.
The CPU executes
the program in
main memory.
Figure 1-16 A program is copied into main memory and then executed
CPU
Main memory
(RAM)
10111000
10100001
10011110
00011010
11011100
and so forth...
10100001
1
Fetch the next instruction
in the program.
Decode the instruction
to determine which
operation to perform.
3
Execute the instruction
(perform the operation).
2
Figure 1-17 The fetch-decode-execute cycle
From Machine Language to Assembly Language
Computers can only execute programs that are written in machine language. As previously
mentioned, a program can have thousands or even millions of binary instructions, and writing
such a program would be very tedious and time consuming. Programming in machine language
would also be very difficult because putting a 0 or a 1 in the wrong place will cause an error.
Assembly language programs cannot be executed by the CPU, however. The CPU only
understands machine language, so a special program known as an assembler is used to
translate an assembly language program to a machine language program. This process is
shown in Figure 1-18. The machine language program that is created by the assembler can
then be executed by the CPU.
16 Chapter 1 Introduction to Computers and Programming
Although a computer’s CPU only understands machine language, it is impractical for people
to write programs in machine language. For this reason, assembly language was created in the
early days of computing2 as an alternative to machine language. Instead of using binary numbers
for instructions, assembly language uses short words that are known as mnemonics. For
example, in assembly language, the mnemonic add typically means to add numbers, mul typically
means to multiply numbers, and mov typically means to move a value to a location in
memory. When a programmer uses assembly language to write a program, he or she can write
short mnemonics instead of binary numbers.
2 The first assembly language was most likely that developed in the 1940s at Cambridge University for use with
a historic computer known as the EDSAC.
mov eax, Z
add eax, 2
mov Y, eax
and so forth...
Assembler
10111000
10100001
10011110
and so forth...
Assembly language
program
Machine language
program
Figure 1-18 An assembler translates an assembly language program to a machine
language program
NOTE: There are many different versions of assembly language. It was mentioned
earlier that each brand of CPU has its own machine language instruction set. Each
brand of CPU typically has its own assembly language as well.
High-Level Languages
Although assembly language makes it unnecessary to write binary machine language
instructions, it is not without difficulties. Assembly language is primarily a direct substitute
for machine language, and like machine language, it requires that you know a lot about the
CPU. Assembly language also requires that you write a large number of instructions for
even the simplest program. Because assembly language is so close in nature to machine language,
it is referred to as a low-level language.
In the 1950s, a new generation of programming languages known as high-level languages
began to appear. A high-level language allows you to create powerful and complex programs
without knowing how the CPU works, and without writing large numbers of low-level
instructions. In addition, most high-level languages use words that are easy to understand.
For example, if a programmer were using COBOL (which was one of the early high-level
1.4 How a Program Works 17
languages created in the 1950s), he or she would write the following instruction to display the
message Hello world on the computer screen:
DISPLAY "Hello world"
print Hello world
Doing the same thing in assembly language would require several instructions, and an intimate
knowledge of how the CPU interacts with the computer’s output device. As you can see from this
example, high-level languages allow programmers to concentrate on the tasks they want to perform
with their programs rather than the details of how the CPU will execute those programs.
Since the 1950s, thousands of high-level languages have been created. Table 1-1 lists several
of the more well-known languages.
Table 1-1 Programming languages
Language Description
Ada Ada was created in the 1970s, primarily for applications used by the U.S.
Department of Defense. The language is named in honor of Countess Ada
Lovelace, an influential and historic figure in the field of computing.
BASIC Beginners All-purpose Symbolic Instruction Code is a general-purpose language
that was originally designed in the early 1960s to be simple enough for beginners
to learn. Today, there are many different versions of BASIC.
FORTRAN FORmula TRANslator was the first high-level programming language. It was
designed in the 1950s for performing complex mathematical calculations.
COBOL Common Business-Oriented Language was created in the 1950s, and was
designed for business applications.
Pascal Pascal was created in 1970, and was originally designed for teaching programming.
The language was named in honor of the mathematician, physicist, and
philosopher Blaise Pascal.
C and C++ C and C++ (pronounced “c plus plus”) are powerful, general-purpose languages
developed at Bell Laboratories. The C language was created in 1972
and the C++ language was created in 1983.
C# Pronounced “c sharp.” This language was created by Microsoft around the
year 2000 for developing applications based on the Microsoft .NET platform.
Java Java was created by Sun Microsystems in the early 1990s. It can be used to develop
programs that run on a single computer or over the Internet from a web server.
JavaScript JavaScript, created in the 1990s, can be used in web pages. Despite its name,
JavaScript is not related to Java.
Python Python, the language we use in this book, is a general-purpose language created
in the early 1990s. It has become popular in business and academic applications.
Ruby Ruby is a general-purpose language that was created in the 1990s. It is increasingly
becoming a popular language for programs that run on web servers.
Visual Basic Visual Basic (commonly known as VB) is a Microsoft programming language and
software development environment that allows programmers to create Windowsbased
applications quickly. VB was originally created in the early 1990s.
1.4 How a Program Works 18
A compiler is a program that translates a high-level language program into a separate
machine language program. The machine language program can then be executed any time
it is needed. This is shown in Figure 1-19. As shown in the figure, compiling and executing
are two different processes.
print "Hello
Earthling"
and so forth...
High-level language
program
Machine language
program
Compiler
10111000
10100001
10011110
and so forth...
10111000
10100001
10011110
and so forth...
Machine language
program
CPU
The compiler is used
to translate the high-level
language program to a
machine language program.
1
The machine language
program can be executed
at any time, without using
the compiler.
2
Figure 1-19 Compiling a high-level program and executing it
The Visual BASIC language uses an interpreter, which is a program that both translates
sdand executes the instructions in a high-level language program. As the interpreter rea
each individual instruction in the program, it converts it to machine language
instructions and then immediately executes them. This process repeats for every
instruction in the program. This process is illustrated in Figure 1-20. Because
interpreters combine translation and execution, they typically do not create separate
.machine language programs
Compilers and Interpreters
Because the CPU understands only machine language instructions, programs that are written
in a high-level language must be translated into machine language. Depending on the
language that a program has been written in, the programmer will use either a compiler or
an interpreter to make the translation.
19 Chapter 1 Introduction to Computers and Programming
The statements that a programmer writes in a high-level language are called source code,
or simply code. Typically, the programmer types a program’s code into a text editor and
then saves the code in a file on the computer’s disk. Next, the programmer uses a compiler
to translate the code into a machine language program, or an interpreter to translate and
execute the code. If the code contains a syntax error, however, it cannot be translated. A
syntax error is a mistake such as a misspelled key word, a missing punctuation character,
or the incorrect use of an operator. When this happens the compiler or interpreter displays
an error message indicating that the program contains a syntax error. The programmer corrects
the error and then attempts once again to translate the program.
NOTE: Human languages also have syntax rules. Do you remember when you took
your first English class, and you learned all those rules about commas, apostrophes,
capitalization, and so forth? You were learning the syntax of the English language.
Although people commonly violate the syntax rules of their native language when
speaking and writing, other people usually understand what they mean. Unfortunately,
compilers and interpreters do not have this ability. If even a single syntax error appears
in a program, the program cannot be compiled or executed. When an interpreter
encounters a syntax error, it stops executing the program.
Checkpoint
1.18 A CPU understands instructions that are written only in what language?
1.19 A program has to be copied into what type of memory each time the CPU executes it?
1.20 When a CPU executes the instructions in a program, it is engaged in what process?
1.21 What is assembly language?
1.22 What type of programming language allows you to create powerful and complex
programs without knowing how the CPU works?
1.23 Each language has a set of rules that must be strictly followed when writing a
program. What is this set of rules called?
1.24 What do you call a program that translates a high-level language program into a
separate machine language program?
1.25 What do you call a program that both translates and executes the instructions in a
high-level language program?
1.26 What type of mistake is usually caused by a misspelled key word, a missing
punctuation character, or the incorrect use of an operator?
The interpreter translates each high-level instruction to
its equivalent machine language instructions and
immediately executes them.
This process is repeated for each high-level instruction.
print "Hello
Earthling"
and so forth...
High-level language
program
Interpreter 10100001
Machine language
instruction
CPU
Figure 1-20 Executing a high-level program with an interpreter