Text 2.

“Hardware”

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.