Introduction - Java Software Solutions: Foundations of Program Design - page 12

The CPU and the main memory make up the core of a computer.
As we mentioned earlier, main memory stores programs and data
that are in active use, and the CPU methodically executes program
instructions one at a time.
Suppose we have a program that computes the average of a list of
numbers. The program and the numbers must reside in main memory
while the program runs. The CPU reads one program instruction from
main memory and executes it. If an instruction needs data, such as a number in the
list, to perform its task, the CPU reads that information as well. This process repeats
until the program ends. The average, when computed, is stored in main memory to
await further processing or long-term storage in secondary memory.
Almost all devices in a computer system other than the CPU and main memory
are called peripherals; they operate at the periphery, or outer edges, of the system
(although they may be in the same box). Users don't interact directly with the
CPU or main memory. Although they form the essence of the machine, the CPU
and main memory would not be useful without peripheral devices.
Controllers are devices that coordinate the activities of specific peripherals. Every
device has its own particular way of formatting and communicating data, and part
of the controller's role is to handle these idiosyncrasies and isolate them from the
rest of the computer hardware. Furthermore, the controller often handles much of
the actual transmission of information, allowing the CPU to focus on other activities.
Input/output (I/O) devices and secondary memory devices are considered periph-
erals. Another category of peripherals consist of data transfer devices, which allow
information to be sent and received between computers. The computer specified in
Figure 1.8 includes a network card, also called a wireless network interface controller
(WNIC), which connects to a radio-based computer network.
In some ways, secondary memory devices and data transfer devices can be
thought of as I/O devices because they represent a source of information (input)
and a place to send information (output). For our discussion, however, we define
I/O devices as those devices that allow the user to interact with the computer.

KEY CONCEPT
The core of a computer is made up
of main memory, which stores pro-
grams and data, and the CPU, which
executes program instructions one
at a time.

Input/Output Devices
Let's examine some I/O devices in more detail. The most common input devices
are the keyboard and the mouse. Others include

bar code readers, such as the ones used at a retail store checkout

■

microphones, used by voice recognition systems that interpret voice commands

■

virtual reality devices, such as handheld devices that interpret the move-
ment of the user's hand

■