electrical, electronic, and magnetic devices within the
computer itself and all related PERIPHERAL devices.
The peripheral devices consist of items, such as a
keyboard, magnetic tape unit, mouse, scanner, printer,
and so on. The software items are programs and
operating aids written so the computer can process data.
The manufacturer supplies much of the initial software
for a particular computer. This software is known as
SYSTEMS SOFTWARE. Systems software is
designed for broad general use. Examples of systems
software are DOS (disk operating system) for IBM
compatible computers and System 7 for Apple
computers. Software designed to meet a specific
requirement or purpose is called APPLICATION
SOFTWARE. Examples of application software are
WordPerfect, Lotus 1-2-3, and Adobe Photoshop.
FUNCTIONAL UNITS OF A
COMPUTER SYSTEM
Digital computer systems consist of three distinct
units. These units are as follows:
Input unit
Central Processing unit
Output unit
These units are interconnected by electrical cables
to permit communication between them. This allows
the computer to function as a system.
Input Unit
A computer must receive both data and program
statements to function properly and be able to solve
problems. The method of feeding data and programs to
a computer is accomplished by an input device.
Computer input devices read data from a source, such
as magnetic disks, and translate that data into electronic
impulses for transfer into the CPU. Some typical input
devices are a keyboard, a mouse, or a scanner.
Central Processing Unit
The brain of a computer system is the central
processing unit (CPU). The CPU processes data
transferred to it from one of the various input devices.
It then transfers either an intermediate or final result of
the CPU to one or more output devices. A central
control section and work areas are required to perform
calculations or manipulate data. The CPU is the
computing center of the system. It consists of a control
internal storage section (main memory). Each section
within the CPU serves a specific function and has a
particular relationship with the other sections within the
CPU.
CONTROL SECTION.--The control section
directs the flow of traffic (operations) and data. It also
maintains order within the computer. The flow of
control is indicated by dotted arrows in figure 3-1. The
control section selects one program statement at a time
from the program storage area, interprets the statement,
and sends the appropriate electronic impulses to the
arithmetic-logic and storage sections so they can carry
out the instructions. The control section does not
perform actual processing operations on the data. The
control section instructs the input device on when to
start and stop transferring data to the input storage area.
It also tells the output device when to start and stop
receiving data from the output storage area.
ARITHMETIC-LOGIC SECTION.--The
arithmetic-logic section performs arithmetic
operations, such as addition, subtraction,
multiplication, and division. Through internal logic
capability, it tests various conditions encountered
during processing and takes action based on the result.
between the arithmetic-logic section and the internal
storage section during processing. Specifically, data is
transferred as needed from the storage section to the
arithmetic-logic section, processed, and returned to
internal storage. At no time does processing take place
in the storage section. Data maybe transferred back and
forth between these two sections several times before
processing is completed. The results are then
transferred from internal storage to an output device, as
indicated by the solid arrow in figure 3-1.
INTERNAL STORAGE SECTION.--The internal
storage section is sometimes called primary storage,
main storage, or main memory, because this section
functions similar to our own human memory.
The storage section serves four purposes; three
relate to retention (holding) of data during processing.
First, as indicated by the solid arrow (fig. 3-1), data is
transferred from an input device to the INPUT
STORAGE AREA where it remains until the computer
is ready to process it. Second, a WORKING
STORAGE AREA ("scratch pad" memory) within the
storage section holds both the data being processed and
the intermediate results of the arithmetic-logic
operations. Third, the storage section retains the
3-2

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