Meaning of Storing Devices of
Computer -
The
data and instructions that are entered into the computer system through input
units have to be Stored
inside the computer before the actual processing starts. For storing computer
uses its memory.
Computer memory is divided into –
(A) Primary or Main memory
(B) Secondary or Backing storage
1. Meaning of Primary or Main memory -Main memory of a computer is a general term for any type of computer memory
other than backing storage (Secondary Storage devices). It consists of a number
of storage locations, each storage location can hold a quantity of data called
a word. A word is the number of bits that can be processed by the CPU in a
single operation.
Types of main memory -
Main
memory consists of Random Access Memory (RAM) and Read Only Memory (ROM)
1. Random Access Memory
Random
Access Memory is a type of computer memory which holds its data as long as the
computer is switched on. When the computer is switched off, any data held in
RAM is lost. RAM is volatile memory. Data held in RAM can be read or written to
by the CPU in any order. The purpose of random access memory is to hold the
computer’s programs and data while they are being processed.
2. Read only Memory
Read
only memory is a type of computer memory that holds its data permanently. Data
held in ROM Can
be read by the CPU in any order. ROM is therefore another type of direct access
medium. The Contents
of ROM are fixed when the computer is manufactured. In many microcomputer, read
only Memory
is used to hold part of the operating system program, the bootstrap loader,
which is used to Help
start up the computer. ROM has an access time of between 10 and 50 nanoseconds.
Storage Units of storing computer data:
Computers
use the binary system because it is easy to represent the two states ‘on’ and ‘off’
inside a computer.
These two states are represented by voltages. 0 (Zero) volts is used to
represent 0 , and a voltage
usually between 1 and 5 volts depending upon the types of computer , is used to
represent 1. Computer
uses different units to store data. The relationship between the different
units is given below:
4
Bits = 1 Nibble
8
Bits = 1 Byte
1024
Bytes = 1 Kilobyte (KB)
1024
KBs = 1 Megabyte (MB)
1024
MBs = 1 Gigabyte (GB)
1024
GBs = 1 Terabyte (TB)
2. Secondary Memory - Secondary Memory is used as permanent storage for programs
and data in a computer system. Backing storage is also called secondary storage or
mass storage.
The
following backing storage media are included in this unit:
1.
Magnetic
2.
Optical
3.
Magneto-Optical (MO)
4.
Digital Audio Tape (DAT)
Magnetic Hard disks
A
hard disk (commonly known as a HDD (hard disk drive) or hard drive (HD) and
formerly known as a fixed
disk) is a non-volatile storage device which stores digitally encoded data on
rapidly rotating platters
with magnetic surfaces.
Optical
Optical
storage media includes CD-ROM (Compact Disk –Read Only Memory), CD-R (Compact
Disk Recordable),
CD-RW (Compact Disk Rewritable), together with DVD-ROM (Digital Versatile
Disk), DVDVideo, DVD-Audio, DVD-R (Digital Versatile Disk Recordable) and
DVD-RAM.
CD-ROM (Compact Disk- Read Only Memory)
A
CD-ROM is 120 mm in diameter and 1.2 mm thick. When a CD-ROM is made, the data
is moulded into tiny holes called pits on the clear plastic disk. The plastic
disk is then coated with a reflective aluminum Layer
and then a protective lacquer. The pits are arranged in a spiral starting in
the center of the disk. The data is read from a CD-ROM by focusing a laser beam
through the clear plastic on to the tracks.
CD-R (Compact Disk Recordable)
It
is a write once medium. Here after writing data once it cannot be erased and
write further.
CD-RW (Compact Disk – Read /Write)
The
fact that CD-R disks cannot be erased and written has given rise to the
development of CD-RW. CDRW allows a user to record over data or to delete
individual files.
DVD (Digital Video Disk or Digital Versatile Disk)
Like
CD, DVD has many different standards. These include DVD-ROM, DVD-Video,
DVD-Audio, DVD-R, and
DVD-RAM. DVD- ROM is high capacity data storage medium. DVD Video is a digital
storage medium for
movies. DVD-Audio is an audio-only storage format similar to CD. DVD-R is
similar to CD-R and DVD RAM
is a rewriteable form of DVD. With the same overall size as a standard CD, DVD
disks can provide up
to 17 Gigabytes of storage. The data transfer rate of a DVD is higher than a
CD-ROM.
Processing Units
Central Processing unit (CPU) - The
CPU is the brain of a computer and consists of three main parts: the control unit the arithmetic and logic unit (ALU)
and the main memory. However, the processor usually refers to a combination
of the control unit, ALU and associated registers.
The functions of the processor (CPU)
are:
- To control and coordinate the operations for the whole system.
- To manage the main memory.
- To fetch, decode and execute instructions.
- The speed of processors is generally measured in MHz and GHz. For example, a 2 GHZ processor
- Performs 2 X 109 pulses per second. The processor is connected via buses and lines to other part of the system.
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