banks. These memory banks can hold instructions and
both processed and unprocessed data. Memory access
time and memory capacity are the other main factors
that determine how powerful a computer is.
Externally, computers can store data on magnetic
disks (hard and floppy), magnetic tape, or paper tape.
Disk drive units offer quicker access to the data than
magnetic or paper tape units. On some systems, the
disks can store more data than the internal memory of
a computer. The amounts of each will depend on the
design and requirements of the data system. Some
systems internally store and process the data. Others
depend heavily on the disks to hold and store the data,
bringing the data into memory for processing, and then
storing the results back onto disk.
Disseminate Data
After the computer has processed the data, it can
send it to the I/O section or an I/O unit for immediate
or future dissemination to various equipments. The
data will exit the output section of the computer’s
input/output section. It can be sent to an output device
such as a printer, or to one of many storage devices such
as a magnetic tape or disk unit. It can also be sent to a
subsystem, such as a display system, via its associated
equipment.
Display Data
Computer systems display two general types of
data—data related to the mission of the system and
status information related to operation of the system and
hardware performance.
The computer relies on
peripheral equipment, such as printers and display
units, to display the processed data—the mission related
output of the operational program. Your interest in
output generally relates to whether the data is sent
properly by the computer and is displaying properly. In
other words, you want to know the computer system is
functioning properly. The content of the data is usually
a secondary interest to you and a primary interest to the
user/operator.
The other type of data/information that can be
displayed relates to the operation of the system. This
includes operator information, system error messages,
and indications of system problems. You will be
particularly interested in this information. The
maintenance panels and data terminals can display
real-time data and provide you with current status of the
operational program. For example, the maintenance
panels of some computers have registers where the
presence or absence of indicator lamps can indicate to
the technician if the computer is communicating with a
subsystem such as a display or communication
subsystem. This is a very useful tool when you are
performing maintenance, both preventive and
corrective. Figure 1-2 is an example of a maintenance
console panel. Notice the indicator lights for the I/O
controller, I/O timing, Mode, Central Processor
Register, and soon. These will provide you with status
information. For example, you can monitor the I/O
controller register to see if the computer is interfacing
with a particular subsystem such as display or
communications. Look to see if the indicator of that
channel is illuminated (either flashing or constantly lit).
Or, you could look at the contents of a particular register
in the CPU by selecting that register while installing a
patch to a program using an inspect and change
procedure or utility.
TYPES OF COMPUTERS
In general terms, computers can be classified into
three categories: mainframe computers
minicomputers, and microcomputers. A computer’s
power is determined by the technology it uses, NOT its
physical size.
Greater speed, greater capability and
capacity to store information, and greater facility to
accommodate additional peripheral (external)
equipment will make one computer more powerful than
another regardless of their overall physical sizes. We
do not go into detail on each of the different types of
computers. Rather, we identify examples of each and
point out their physical and internal differences. This
will prove valuable when you are maintaining them.
Let’s take a look at the types of computers you will
maintain in the Navy. Later in this manual, you will
study the internal workings of computers-their basic
functional operation.
Mainframe Computers
Mainframe computers are physically the largest
computers you will maintain. Their ruggedness makes
them better suited than microcomputers and
minicomputers to handle the mechanical shock and
vibration, salt spray, temperature and humidity found
aboard Navy vessels.
The mainframes you will
maintain are general-purpose, digital data computers
with multiprocessing capability.
Mainframe computers are considered the heart of
the afloat and ashore tactical and tactical support data
systems. These mainframe computers are big, fast,
multiprocessor computers with correspondingly large
memories and multiple I/O channel capabilities. They
process large volumes of data and require a lot of
program flexibility. Their operational programs are
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