Java Reference
In-Depth Information
Software
Software consists of programs written to perform specific tasks. For example, you use
word-processing programs to write letters, papers, and books. The two types of programs
are system programs and application programs.
System programs control the computer. The system program that loads first when
you turn on your PC is called the operating system. Without an operating system,
the computer is useless. The operating system monitors the overall activity of the
computer and provides services, such as memory management, input/output activities,
and storage management. The operating system has a special program that organizes
secondary storage so that you can access information conveniently. The operating
system is the program that runs the application programs. Application programs
perform specific tasks. Word processors, spreadsheets, and games are examples of
application programs. Both operating systems and application programs are written in
programming languages.
When you press A on your keyboard, the computer displays A on the screen, but what is
actually stored inside the computer's main memory? What
is the language of
the
computer? How does it store whatever you type on the keyboard?
Remember that a computer is an electronic device. Electrical signals move along channels
inside the computer. There are two types of electrical signals: analog and digital. Analog
signals are continuous waveforms used to represent things, such as sound. Audio tapes,
for example, store data in analog signals. Digital signals represent information with a
sequence of
0
s and
1
s. A
0
represents a low voltage, and a
1
represents a high voltage.
Digital signals are more reliable carriers of information than analog signals and can be
copied from one device to another with exact precision. You might have noticed that
when you make a copy of an audio tape, the sound quality of the copy is not as good as
that on the original tape. Computers use digital signals.
Because digital signals are processed inside a computer, the language of a computer, called
machine language, is a sequence of
0
s and
1
s. The digit
0
or
1
is called a binary digit,
or bit. Sometimes a sequence of
0
s and
1
s is referred to as a binary code or a binary
number.
Bit:
A binary digit
0
or
1
.
A sequence of eight bits is called a byte. Moreover, 2
10
ΒΌ
1024 bytes and is called a
kilobyte (KB). Table 1-1 summarizes the terms used to describe the various numbers of
bytes.
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