TEXT 2. “HARDWARE”
( Reading - 2 )
( Reading - 2 )
The functions of the hardware composing a computer can be divided
broadly into the following five categories: Input; Storage; Operation; Control and Output.
The following are the units that implement the above-mentioned
functions:
· Input unit: This unit inputs the data and programs
for computer processing. It is equivalent to the human eyes and ears.
· Storage unit: This unit stores the input data and
programs. It is equivalent to the memory section of the human brain.
· Arithmetic unit: This unit conducts calculation and
decision on the stored data according to the instructions of the program. It is
equivalent to the thinking section of the human brain.
· Control unit: This unit controls the input unit, storage
unit, arithmetic unit and the output unit. It is equivalent to the human
central nervous system.
· Output unit: This unit outputs the results of computer
processing in a format that can be understood by humans. It is equivalent to
the human hands and mouth.
These five units are called the "computer five main units".
Since the control unit and the arithmetic unit are handled as one
unit, they are called the processor (processing unit) or central processing unit (CPU). The general term "peripheral
devices" is used to refer to the input unit, the output unit and the auxiliary storage devices that are outside the
processor and exchange data with the main storage
unit.
Likewise, the storage units
are divided into main storage unit and
auxiliary storage device, depending on their functions.
(1)
RAM (Random Access Memory)
The RAM is
semiconductor memory in which data writing and reading is possible. When the
computer is turned off, the stored data is lost. This property is called volatility. Since most main storage units are
composed of RAMs, the processor can be made to read and write information from
the main storage unit at random by specifying the address.
RAMs are classified
into DRAMs and SRAMs.
1)DRAM
(Dynamic RAM)
A DRAM represents bits, and stores information
depending on whether the part called capacitor is being charged (status
"1") or is not being charged (status "0").
Since the circuits are
simple and small, RAMs of large capacity can be created at low cost. However,
since the charge stored in the capacitor is lost after a lapse of time, the
memory needs to be rewritten (recharged) at regular intervals. This operation
is called refreshing. Once, DRAMs were used in the main storage
unit, but currently they are also used in storage units, etc., contained in the
input/output units of printers and other devices.
2)SDRAM
(Synchronous DRAM)
Due to the progress of IC technology, and the consequent substantial
improvement of the performance of processors, the operating speed of the DRAMs
that composed the storage unit could not keep up with the operating speed of
the processors. For that reason, an external clock signal that indicates the
processor operation timing is now set in the DRAM and through synchronization
with this signal, complicated address specifications are reduced and
simplified, enabling the development of DRAMs that operate at high speeds.
These types of DRAMs are called synchronous DRAMs (SDRAM).
3)SRAM
(Static RAM)
SRAMs are
created with a circuit called the flip-flop. The flip-flop settles the output
according to the previous input and the current input, and can preserve the
status "1" and "0" inside the circuit. Since data is not
lost unless the computer is turned off, memory refreshing is not necessary.
However, since SRAM circuits are complicated, the memory capacity is smaller
than that of DRAMs and the cost is higher. However, since its processing speed
is high, it is used in devices such as the registers contained in main storage
units and processors.
(2)
ROM (Read Only Memory)
The ROM is
semiconductor memory for read use only. Since programs and data are stored in
the ROM from the beginning, the stored information is not lost even if the
computer is turned off. This property is called nonvolatility.
ROMs are classified
into mask ROMs and user programmable ROMs.
1)Mask
ROM
Since programs and data are already written in the Mask ROM before it is shipped by the manufacturer, the
user cannot add any programs or data. Mask ROMs are used in the memories of
game cassettes and IPL (Initial Program Loader), a program used to start the
computer, etc.
2)User
programmable ROM
The user programmable ROM is a
type of ROM, but since at the time it is shipped by the manufacturer it has
nothing stored in it, the user can write data into it once. The following types
of user programmable ROM exist
· PROM (Programmable ROM): Once data has been
written, it cannot be erased.
· EPROM (Erasable PROM): It can be erased with
ultraviolet light and rewritten.
· EEPROM (Electrically Erasable PROM): It can be erased
through the application of electrical voltage and rewritten.
EEPROM is used in a storage medium called flash memory, which is used in the registration of
image data of digital cameras, etc. Likewise, it is also used in the storage
section of IC cards, etc.
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